Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Edward on December 18, 2007, 01:15:22 PM
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A yellow powder, when dissolved in water, the pH is 12.9. Could anyone please enlighten me on how to analyze an unknown sample (aqueous or solid) for its chemical composition (cation and anion) with special reference to an alkali or acid?
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There is a procedure of inorganic salt analysis, should be described in any decent analytical chemistry book. Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on. Same holds for anions. High pH may suggest salt of weak acid.
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A yellow powder, when dissolved in water, the pH is 12.9. Could anyone please enlighten me on how to analyze an unknown sample (aqueous or solid) for its chemical composition (cation and anion) with special reference to an alkali or acid?
Such high pH suggests oxides, hydroxides (or cyanides) of alkali metals.
Only some oxides, peroxides and superoxides of these metals can be yellow.
Search: www.webelements.com
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There is the EDTA titration technique to help identify cation.
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There is the EDTA titration technique to help identify cation.
As far as I know EDTA titration is used for quantitative determination, I am not aware of qualitative applications. Can you elaborate?
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Using a colorimetric indicator, you can use the pH at which the hydroxide precipitates to give you an idea of what the cation is. There are plenty of tables that you can use to narrow down possible cations in a range of pH. using EDTA would help narrow the cation down to about 3 from which you could determine use other methods to determine the exact cation.
If you have access to an atomic absorption specrophotometer you would be able to determine what the cation is by the wavelength of light that is emitted in the flame.
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Using a colorimetric indicator, you can use the pH at which the hydroxide precipitates to give you an idea of what the cation is. There are plenty of tables that you can use to narrow down possible cations in a range of pH. using EDTA would help narrow the cation down to about 3 from which you could determine use other methods to determine the exact cation.
Somehow it doesn't translate to a convenient procedure to me. Too many caveats and assumptions to make. Besides, it won't help if the hydroxide is soluble. Systematic cation analysis (or some spectroscopic approach) is IMHO a much better approach.
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Actually, my problem is a systematic analysis of an acid or alkali. There are so many acids and bases (organic and inorganic) in this world. I found some logical scheme for cation and anion analyses (including Ag, Hg, Pb, Fe, Al, Ni, Mg, Zn, Ba, K, Na, Sr, Ca; OH, PO4, CO3, HCO3, SO4, Cl, I), but can I simply apply to acid and bases identification. Is the logical cation analysis which involves different solubility of salts in acid, bases, applicable to an acid or alkali medium? I have problem in identifying the hydrogen ion moiety in an acid (e.g, sulfuric acid, nitric acid, etc), in identifying acetic acid, carbonic acid, dihydrogen-phosphate ion, phosphoric acid. Likewise, I don't know how to approach the identification of an oxide, acetate ion, monohydrogen-phosphate ion. Adding to the complexity is other organic acids such as carboxylic acids and phenols. I don't know how many acids and bases there are in this world. Moreover, I simply don't have an systematic way to identify an unknown powder that is acidic or alkaline.
Could any experts please help me out? Many thanks!
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Systematic inorganic analysis will work no matter whether you start with salt, acid or base. If you have an acid, you will just find out that there no cations, if you start with base - you will find no anion.
However, full analysis is not possible without spectroscopic methods.
Number of organic compounds (including bases/acids) is unlimited.
Please give some more information about the substance and circumstances - methods and tools to use depend on whether you are Dow Chemical Company empolyee or High School student :)
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XRF, AA or ICP are your best bets....were you looking ffor a spot test? Spot tests will give you alot of 411 but not the exact id.....for that you must use elemental analysis: XRF, XRD, ICP, AA.
If your material is in a powder form XRF is the fastest and easiet route. If you use AA or ICP you will have to subject that sample to digestion....you will be changing the matrix....
If you use XRF....you can get very fast qual ID....then for quant you can do a matrix match....sample prep is so easy aqnd so reliable....
Best of Luck!!!!!
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Thank you so much, Borek and Alpha-Omega! If systematic inorganic analysis can't identify the cation in acid and anion in base, for base, can I use simple, separate tests to identify CO3 (CO2 gas on HNO4 addition), HCO3 (Add MgSO4 and heat --> white MgCO3 ppt), OH (Add aq. NH4Cl to give NH3 gas)? For other bases and all other acids I have no ways to identify the anion and cation respectively. I work in a Clinical Toxicology Reference Lab and sometimes we receive unknown powder and liquid that when ingested, cause mouth swelling and bleeding in patients. We therefore need to identify unknown substances received with certainty. There are limitless possibilities of powder or liquid but most probably they are found in household products such as detergent, soap, drain cleaner, etc; which can be organic or inorganic. I don't have much knowledge in analytical chemistry, could Alpha-Omega please tell me what XRF, XRD are? We don't have an Inductively coupled plasma analyzer. But we have an atomic absorption spectrophotometer. We use it for Pb, Cu quantitation in blood. How can it be applied to acid and base id in macro quantity? You mentioned XRF, is this piece of instrument expensive and difficult to use? I can report results based on systematic analysis and give comments that we only cover certain common household products such as detergent, caustic drain cleaner, HCl, soap, washing powder. Is it possible to id these products by simple tests? Thank you very much for your *delete me*
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If systematic inorganic analysis can't identify the cation in acid and anion in base, for base, can I use simple, separate tests to identify CO3 (CO2 gas on HNO4 addition), HCO3 (Add MgSO4 and heat --> white MgCO3 ppt), OH (Add aq. NH4Cl to give NH3 gas)?
These will be easily identified with the systematic approach, besides, your approach will give you false positives - systematic analysis is designed in such a way, that you remove all possible interfering cations/anions first.
I work in a Clinical Toxicology Reference Lab and sometimes we receive unknown powder and liquid that when ingested, cause mouth swelling and bleeding in patients.
I wonder - perhaps forensic labs have an established procedure for determining such substances?
Perhaps you can start with the list of the most probable substances and then look for the way of determining them assuming (at first) that the substance is on the list?
It is a very delicate thing, as false ID can be harmfull.
could Alpha-Omega please tell me what XRF, XRD are?
Google is your friend:
http://www.learnxrf.com/
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A website (http://chemlab.truman.edu/CHEM121Labs/QualAnalysis.htm) describes a systematic analytical approach which does not follow the procedures: Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on. Is that approach free from false positives? Besides, the approach doesn't indicate how to identify H, HCO3, OH, etc. That is why I think of using separate tests for HCO3, OH.
I don't have access to forensic standard procedures. Do you have any references or sources that I can refer to?
Thanks a lot!
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systematic analytical approach which does not follow the procedures: Group I are cations that precipitate in HCl, group II those precipitated in the presence of sulfides and so on. Is that approach free from false positives? Besides, the approach doesn't indicate how to identify H, HCO3, OH, etc. That is why I think of using separate tests for HCO3, OH.
If I read it correctly it is designed to work only for a given subset of cations/anions.
I don't have access to forensic standard procedures. Do you have any references or sources that I can refer to?
Google may help, I suppose US based users will have more to add.
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Do you have references for a more comprehensive systematic analysis of cations and anions? For acids identification, is it possible to use titration method to determine their identity? From titration with NaOH, is it possible to obtain molar mass and pKa? From these data, can I identify all existing acids? Are there any good references on this?
Thanks!
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Can you tell me exactly what you are looking for? If you want an exact ID you need to use elemental analysis. Thhat would be ICP...ok not an option...AA is a good second....that will give you any metals in there...XRF is x-ray fluorescence. It is used for qualitative and quantitative id of any lemental substance from Na (Z=11) to PU (Z=92) in the periodic table.
There are two versions of XRF...WDXRF and EDXRF...wavelength dispersive and energy dispersive. WDXRD relies on optics and not the detector. EDXRD relies heavily on the detector and all spectra can be displed in terms of wavelength as well as in terms of energy. See www.panalytical.com....or www.bruker.co....they give all the theory and many applications...it is used extensively in forensics...which is where I got my taste..and in pharm...
I can certainly tell you ablout detection of cations and anions...IC Ion Chromatography is your #1 bet...and I work for the leader in IC analysis....
Go to www.dionex.com....on the homepage go to upper right hand corner go to search...type in cations and many hits for literature will come up.....do the same for cations....
Now it is impt regarding matrix...what you are going to want to look at....for common anions EPA uses AS14 and AS14A columns...or AS4A, AS9-HC and sometimes AS11 and AS11-HC.
Cations they use CS12, CS12A, CS5..
You can also check out the Metrhome site...
Borek is correct spot tests are just that an initial ID used them all the time at US Customs....was a Forensic Chemist there for 5 years....BUT you must validate...with an absolute qual/quant method void of false pos/neg results....and that would require instrumental analysis.
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Thanks Alpha-Omega. I suppose my lab doesn't have the equipment you mentioned except Atomic absorption. We are looking for a protocol to detect any unknown substances that come to us and that is an acid/ base measured by a pH meter. My lab is a clinical toxicology reference lab and we usually receive samples from patients with clinical problems usually after ingestion or contact with household products. Spot test and AA are probably what we have got. From your expert forensic knowledge, could you pls suggest any references or protocol ?
Grateful for your *delete me*
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Metrohm has some titrimetric apps for cations: phosphate, chloride, flouride, sulfate....not sure if you are using an autotitrator:
http://www.analysis-food.com/beverage/beer.html
This is a good pdf for titrimetric analysis of a few differnt types of species.....you can download the pdf
http://oncampus.richmond.edu/~rdominey/301/local/Titrimetry_Methods.pdf
You can do very good elemental analysis via AA....what OEM is the AA?
If you go to their website and are an owner...you should be able to register and get access to their applications...
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Don't know if my concept is right or not. Is that titration kit fit for determination of any acids and bases known? Is it expensive to buy one (we don't have autotitrator)?
We've got a Varian SpectrAA800. I just have a quick glimpse of the company website and have not found any application related to id of macro quantity of acid /base/ cation and anion. Afraid that will damage the analyzer. Maybe I can ask the company for more information. Is a preliminary titration method and AA confirmation of possible cations a sufficient approach? How about the species other than metal which are ?not detectable by AA?
Thanks!
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Hang on...let me do a few things...are you with Truman College downtown Chicago??
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No, I work in a Hospital of Hong Kong.
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Please take your time and really appreciate your great *delete me* Don't know if I can leave email here?!
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If I go to http://www.varianinc.com/cgi-bin/nav?applications/aa&cid=JMJMQHLNFM all of the Application Notes Varian offers are on the left.
Click on AANow you have to choose your search criteria
By Martix/Application
By Element
By Title
In this case I would choose Element....maybe matrix...do you know the matrix of your sample(s)? Wher do they come from? What is their origin? For example, if you had groundwater samples I would imagine they would be tainted with mineral deposits and trace metals...like iron....this will definitly produce interferences in a method like IC. The sample matrix has to be accounted for....
Try this...these are Applications....they give you a guideline to follow....basically it is an application designed for a specific analysis by Varian known to work on thier AA system....
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The specimen types can be diverse. But most frequently, some drain cleaner (acids or caustics), soap powder, detergent.
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Away for an hour or so. Please keep sending messages here Thanks!
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Basic and contain surfactants....I would choose elements...go thru those steps and check those notes...
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Sure!
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Do you have references for a more comprehensive systematic analysis of cations and anions?
Only in Polish :)
For acids identification, is it possible to use titration method to determine their identity? From titration with NaOH, is it possible to obtain molar mass and pKa? From these data, can I identify all existing acids? Are there any good references on this?
Not molar mass but equivalence mass, and pKa only for acids with pKa > 3 (approximately).
Vogel's Quantitative Chemical Analysis comes to mind.
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I am thinking to perform an acid base titration, then use systematic inorganic qualitative analysis by different solubilities in acids, H2S, NaOH, then Na2CO3 to detect cations. But I don't know if this approach covers all acids or bases. I don't know know if this approach covers all organic acids, either.
For titration experiment, is there a complete list of acid and base identification by equivalence mass and pKa/pKb?
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If pKa or pKb is unique, together with equivalence mass, is it possible to deduce the identity of an acid or base respectively? If I don't know whether the substance is inorganic or organic, can I approach it by assuming it is inorganic, i.e. use systematic inorganic qualitative analysis for cation and anion. Then perform acid base titration to confirm. If I can't identify any known inorganic ions or inorganic base/acid (?at least some organic acids may also be identified by acid base titration)by these methods, I can treat the sample as an organic substance. But there are limitless possibilities of organic substances in this world!? Maybe I can find ways to identify some common household products such as detergent, soap, washing powder, which I don't have any idea yet.
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No such thing as unique pKa/pKb, when you take finite accuracy of the determination into account there is simply no place for them all in the range of values pKa and pKb can take. Titration will be of no use here, too many factors for reliable results.
Organic compounds will be either easily melted, or evaporated, or decomposed when heating, while inorganics will usually stay unchanged on heating (that's not 100% correct, they can change form loosing crystalline water, sometimes they also can decompose loosing ammonia or carbon dixode, but the final result will differ from messy carbon leftowers you may expect after heating many organic compounds).
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I hate to say this; but, if your lab-supervisor is interested in generating resls that are undisputable...he/she will have to invest in some instrumentation that will resolve this issue. Especially if this is a forensic lab.
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I am very puzzled now. You both think that my approach of titration + systematic qualitative analysis is not applicable, right? Case 1, if I only need a preliminary id (not 100% confirmed) do you have any suggestions? Case 2, if I want to confirm the presumptive results, any suggestions? Given that I only have an AA spectrophotometer, HPLC, GC/MS, LC/MS/MS. Case 3, if my boss is willing to invest, what is the most cost-effective and easy to develp instrument to acquire?
Thanks!
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ICP is around 60K...will usually pay for itself in 1-2 years depending on sample load..EDXRF..nice portable models...can take it with you...a bit more pricey...and WDXRF..higher due to the optics....but ICP good choice...why not use that AA...you have it...and I showed you the Application Notes on the Perkin Elmer site...
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Is AA only for metal ions id?
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The list of commonly encountered acids are: acetic acid, carbolic acid, formic acid, HCl, monochoroacetic acid, nitric acid, oxalic acid, phosphoric acid, sulfuric acid, oxalic acid, chromic acid, selenious acid, formic acid, hydrofluoric acid. Alkali: ammonia, CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl.
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Well, if you are looking for all those acids IC is capable of detecting them all. So if you used IC with AA you would be in business....you can detect Na but not OH-, carbonate is a go..but not HCO3 (to unstable).
If you look into IC columns Check out the IonPac AS11-HC....great for organic acids in complex matrices...
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For OH-, add NH4Cl to give ammonia gas. For HCO3, add MgSO4 --> Mg(HCO3)2 which on heating gives white ppt MgCO3. Is this ok? Do you know if forensic labs have any standard procedures for acid/ base? There must be some homicidal cases or attack by corrosive liquids. What is the algorithm ? Do they use IC, AA straight away? I can't find any protocols after exhaustive search in internet.
Thanks!
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Yes...they are OK...but they are not 100%. A spot test or qual flash collormetric test is just that....it is an initial ID...it is meant to give you direction to do deeper more comprehensive analysis.
I cannot begin to tell you how valuable those tests can be at times...I had one for stainless steel. If immersed in HNO3 it will NOT oxidize....OK that is fine and did you test the HNO3???? And how do you validate all this...these are just flash teast...
Whenever a major piece of equipment went down spot est saved the day momentarilty they allow or cataloguing of the samples....but the results of those tests are in conclusive.
So I have a piece of Aluminum and some sodium hydroxide....well I think I have Al because the label says so....so I toss it in the 50% NaOH...and what happens...I get this violent reaction indicating Al...but then I subjected that solution to FTIR as a confirmation. Even that is not enough in forensics....every single material you use for an anaysis must be validated.
Titrate acids and bases against an NIST Std...and you are good to go....you run the same exact test using primary stds then you run your samples....if they fall on your cal curve witin allowed RSD you are good to go...
Homicidal?? Not in customs we dealt with the US Tariff and classifying materials that enter the USA...No dead floating bodies for me...that would be one reason I stay away from FBI....that is one of the questions on theoir application...."So how would you feel if you had to fish a dead body out of a body of water."
Well, so if you have never done that before how would you respond???? LOL
You can contact any Forensic lab: There is an entire socity dedicated to this here is the link:
www.ascld.org/accreditation.html
they use AA, IC, FTIR, SEM, Microscopy, HPLC, IC, GC, UV/Vis, XRF, XRD, Polarimetry, autotitrators, Karl Fisher, NMR, MS...and a few more I cannot think of right now...ASE-Accelerated Solvent Extractors...
There is one old timey wet test for NaOH people use begins with a B....and people in IC constantly try to compare their reults...and wonder why they are different...HELLO...2 different methods...two different protocols...cannot compare apples and oranges....
Check this; http://www.clarksonlab.com/caps.pdf
That is a pdf and I attached a copy...if you cannot download it download it from the link
Here are a ew more:
http://www.atsdr.cdc.gov/toxprofiles/tp7-c6.pdf
http://scholar.lib.vt.edu/theses/available/etd-07242002-111511/unrestricted/ThesisDraft.pdf
http://www.lib.utexas.edu/chem/info/analytical.html
Inorganic Qual Analyses:
http://chemlab.truman.edu/CHEM121Labs/QualAnalysis.htm
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Thanks for your reply. I just sent an email to the association of forensic lab directors requesting for standard protocol. And now starting to read your references. Thanks! My boss just wants some simple preliminary tests, not confirmatory tests. I am thinking in this direction.
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I read some textbook. I don't know if my understanding of the way to derive the pKa is right or not:-- Measure the pH of the acid in question. If it is a strong acid (? don't know the definition of strong acid), use a weak base (? what weak base is suitable and ? conc. usually used) to titrate. Plot pH vs volume of weak base used. Plot the first derivative and second derivative to help find the equivalence point(s). Note the volume of base used to reach equivalence point(s). Divide this volume by 2. Read the corresponding pH value at this volume from the titration curve. This pH corresponds to pKa value of the unknown. Match the pKa value with table and find out the unknown acid.
For unknown weak acid (? definition of weak acid), use strong base e.g. NaOH to titrate.
Is this the correct way of finding pKa? I don't have a complete list of pKa for inorganic acids. Please comment.
Thanks!
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When titrating weak acid with strong base pKa = pH at 50% neutralization. It applies to acids with pKa in the 3-11 range, outside errors are getting large. But as I have alredy told you it is of no use - there are too many acids that can fit in the - say - 3.5+/-0.1 range (and don't expect better accuracy). When you have only a few with well separated pKa values it can work, when you can have anything this approach is IMHO a waste of time.
Using weak base for strong acid titration is completely off. This way you can determine pKb of the base, but only if you will add 50% excess of titrant.
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If I have a list of acids we are interested in : acetic acid, carbolic acid, formic acid, HCl, monochoroacetic acid, nitric acid, oxalic acid, phosphoric acid, sulfuric acid, oxalic acid, chromic acid, selenious acid, formic acid, hydrofluoric acid. Alkali: ammonia, CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl. I have found out pKa of some but not carbolic, HCl, nitric, selenious acids and not CaCO3, CaHCO3, Ca(OH)2, KOH, NaOH, NaOCl. Can I check the pKa the identity by titration? I have no way to confirm.
Is there any method you know of that can detect these acids and bases?
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OK more 411:
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Go to www.dionex.com and download AN93. I cannot upload it due to size restrictions.
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Many thanks! Frankly speaking, I am desparate to find some simple methods to detect unknown acids and bases. The information you gave here is very useful for anions and organic acids determination. Do you know if the columns for anions and organic acids can be bought separately from ion chromatography analyzer system. Can it be used in our HPLC system (which is an Agilent 1100 model) which uses a UV detector? ? an ECD is necessary for this purpose. I am still stuck with acid and base determination. Borek said that titration applies to pH 3-11 range, so in my list at least monochloroacetic, nitric and sulphuric acid are out. If the unknown's pKa does not match those of other acids in my list, I can rule those out. My titration method is for ruling out of acids that I am interested in.
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HCl is out also. My problem is how to find the identity of monochloroacetic, nitric, sulfuric acids.
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Borek said that titration applies to pH 3-11 range
pKa not pH, and I have not told "titration doesn't apply" but "error in the pKa determination using titrimetry gets high outside pKa 3-11 range".
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<Using weak base for strong acid titration is completely off. This way you can determine pKb of the base, but only if you will add 50% excess of titrant.>
Thanks Borek. What does the above mean? Do you mean determining pKa of strong acid is impossible? Even after adding 50% more titrant: weak base? Can I dilute the acid in question, say ten thousand fold to get a a lower pH before titration?
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Using weak base for strong acid titration is completely off. This way you can determine pKb of the base, but only if you will add 50% excess of titrant.
Thanks Borek. What does the above mean? Do you mean determining pKa of strong acid is impossible?
Possible, but not by titration. Strong acids have pKa well below 3 (limit I have signalled earlier).
Even after adding 50% more titrant: weak base?
You are so confused I have no idea what to start with...
pH = -log([H+)
Strong acid - full dissociation, so if you have solution of strong monoprotic acid concentration of H+ is identical with acid concentration, and pH = -log(acid concentration).
During titration you are neutralizing H+ and what you are left with is a mixture of salt and fully dissociated strong acid. pH = -log(concentration of strong acid left). No place (or need) for pKa. After all acid has been titrated, pH depends only on the added excess of base.
Weak acid - partial dissociation, described by the acid dissociation constant Ka. pKa = -log(Ka). When you add strong (fully dissociated) base you neutralize acid - and you are left with salt and not yet neutralized acid in the solution. It happens that if the acid is weak enough (pKa > 3) pH can be calculated using so called Henderson-Hessalbalch equation (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch):
pH = pKa + log(concentration of neutralized acid/concentration of not neutralized acid)
When exactly half of the acid is neutralized pH = pKa. It works reasonably correctly only for acids with pKa within the 3-11 range.
After full neutralization, pH of the solution once again depends only on the concentration of excess base.
This picture is slightly simplified, but it shpould be enough to understand what's going on.
For bases replace Ka with Kb and pH with pOH (pOH+pH=14), otherwise they behave identical.
Can I dilute the acid in question, say ten thousand fold to get a a lower pH before titration?
If you dilute acid pH will go up, not down.
BTW: I was wrong, it should be 100% excess of titrant, not 50%. My mistake. But it still doesn't apply to your problem. You can't easily determine pKa of strong acid with titration, no matter if you use strong or weak base.
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IC requires a pump, a column (your separator) , a suppressor (to suppress the ions in your eluent, and a detector...a conductivity detector.
UV can be impemented with post column derivitization systems. Hex chrome analysis is an example..
You Agilent HPLC is for HPLC. There are columns you can use...but these columns are made of PEEK and may not be compatible with a SS system. But that is a DIFFERNET ANIMAL.
If you are really interested in IC and what you can do; again, I suggest, www.dionex.com and you will have to read some of the literature on the method and its capabilities...
I can send you a power point my friend AVM just developed for teaching IC Applications on the ICS-3000 RFIC system. Cannot upload here too big...If there is a place on this site I can send it to I would be more than glad to post it for you...
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Powerpoint received. Really appreciate your great *delete me* Do you know of any ways to id strong acids?
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What *delete me*?
Here is a GC Method using that ECD/also requires Liquid Solid Extraction- you say you have:
DETERMINATION OF CHLORINATED ACIDS IN WATER USING LIQUID-SOLID
EXTRACTION AND GAS CHROMATOGRAPHY WITH AN ELECTRON
CAPTURE DETECTOR
http://www.accustandard.com/asi/pdfs/epa_methods/515_2.pdf
Your best bet is to go to the EPA site and do a search of all their approved methods for determining you species of interest...you can gdo a serch under GC methods on their site...tyer is a way of limiting your criteria...
You can determine if a method is availble with what equipment you have.
EPA SITE INDEX: http://www.epa.gov/epahome/index/
Here is a PDF with all their approved test methods INDEXED: http://www.ch2m.com/webuploads/newsgenerator/Ext_Environment/Env_ACCS_AboutUs/EPAMETH.pdf
there is this site also/National Environmental Methods Index: http://www.nemi.gov/
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hi, Alpha-Omega. I read through the whole EPA list and found some useful target acids for my purpose: chlorinated acids, monochloroacetic acids, phenol. And I sent an enquiry email to EPA, asking for further information about these acids and others, as well as bases. Then I found that you have already given me the chlorinated acids manual. Marvellous! Thanks for your very useful information. I would like to find similar manuals of standard methods in forensic practice. To my disappointment, I couldn't find any after a day's search thro' google. I am very impressed by your knowledge in this area.
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Borek, forgive me of my ignorance. I have to admit that my chemistry is poor. Thanks for explaining the essential basics. I understand now why strong acids can't be determined by titration. For the latter question, sorry diluting strong acid should raise the pH. By the same principle, is it still not possible to use titration because the strong acid is still fully dissciated?
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Borek, forgive me of my ignorance. I have to admit that my chemistry is poor. Thanks for explaining the essential basics. I understand now why strong acids can't be determined by titration.
Two separate things, which you are mixing now - perhaps your thnking is OK, but wording is wrong. You can use acid/base titration to determine AMOUNT of strong acid, you just can't use titration to determine its pKa.
For the latter question, sorry diluting strong acid should raise the pH. By the same principle, is it still not possible to use titration because the strong acid is still fully dissciated?
See above - you can determine amount (although if the concentration is too low titration is of no use). You can't determine pKa, reason (full dissociation) remains the same regardless of the concentration.
Note: reality is a little bit more complicated, but it doesn't change the outcome - you can't use titration for pKa determination for strong acids.
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Analytical Chemistry is analytical chemistry ...the way Forensic analysis differs from the usual is the CHAIN OF CUSTODY.
The Chain of Custody in Forensic distinguishes what happens during the analysis process....Every thing done to sample must be cataloged and documented...that includes a signature for every finger that touches a sample...
Let me see what I can find for you....
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AHHH Seek and ye shall find. Here you go: http://www.forensicpage.com/new25.htm
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http://www.forensicpage.com/new25.htm: No test method in this homepage. The Reference Manual on Scientific Evidence
link there doesn't work. What is AHHH? Thanks!
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http://www.forensicpage.com/new25.htm
Go to Trace Evidence Recovery Guidelines
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http://www.fjc.gov/public/pdf.nsf/lookup/sciman00.pdf/$file/sciman00.pdf
here is the PDF Reference Manual on Scientific Evidence...
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Seems there are no standard methods for acid base identification in the website?!
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I would not think there would be...they would use methods from ASTM or EPA or USP/EP/JP/ACS...and they have to be documented to work...then they apply their chain of custody to those methods....
The species you are looking at...they would apply a quantitative intrumentation method to...like IC...something that has documentation behind it to show it is a validated method.
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You showed me ASTM, EPA. what is USP/EP/JP/ACS?
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USP = United States Pharmacopia
EP = European Pharmacopia
JP = Japanese Pharmacopia
ACS = American Chemical Society
BP = British Pharmacopia
These books contain methods which are GLOBALLY accepted in the Pharmaceutical industry and the methods are also used in other industries.
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USP: http://www.usp.org/
JP: http://jpdb.nihs.go.jp/jp14e/
EP: http://www.edqm.eu/site/Homepage-628.html
http://www.rt-corp.com/products/c238.aspx
BP: http://www.pharmacopoeia.org.uk/
http://www.pharmacopoeia.co.uk/
ACS: http://portal.acs.org/portal/acs/corg/content
http://pubs.acs.org/reagents/comminfo/purpose.html
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Found some standard methods for flame test, titrimetric analysis in JP. None in others. Thanks for sharing these useful websites to me.
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Borek, could you please tell me the source of reference for the statement: titrimetric analysis for pKa is suitable for pH3-11?
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No reference, but you may calculate it by yourself (although you will be probably forced to use some software, like my BATE, as these calculations can be challenging). The whole idea of pKa determination by titration is based on the Henderson-Hasselbalch equation (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch) and fact that pH of weak acid titrated 50% equals its pH. But this approach is based on simplyfying assumption that concentrations of HA (acid) and A- (conjuagted base from neutralized acid) are defined by the neutralization stoichiometry. In the case of stronger acids HA will tend to dissociate "on its own", in the case of weaker acids A- will hydrolize "on its own" and the difference between pH at 50% and pKa will be larger. Perhaps 3-11 is slightly conservative, perhaps you can extend it to 2.5-11.5 - it all depends on the error you are ready to accept.
| pKa | pH at 50% |
| 0.00 | 1.50 |
| 1.00 | 1.67 |
| 2.00 | 2.18 |
| 3.00 | 3.02 |
| 4.00 | 4.00 |
| 5.00 | 5.00 |
| 6.00 | 6.00 |
| 7.00 | 7.00 |
| 8.00 | 8.00 |
| 9.00 | 9.00 |
| 10.00 | 10.00 |
| 11.00 | 10.97 |
| 12.00 | 11.82 |
| 13.00 | 12.32 |
| 14.00 | 12.48 |
You may try to use some correction tables - like pH of 2.18 at 50% means pKa of 2.00 - but still, the stronger the acid the larger the error, as the dependence becomes more and more flat (ie differences in acid strength give smaller differences in pH).
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Do you know how much an autotitrator for acid base titration is? What brand do you recommend? Thanks!
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Only need to do pH titration automatically and for recording volume of base added and pH for equivalence point and pKa determination. Preferably can plot pH titration curve and/or first or second derivative inflection point determination. Metrohm has some sophisticated equipment, but no price listed.
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Methrom and all other Instrumentation companies WILL NOT list prices. You have to call them and give them your 411 and they will put your regional sales rep in touch with you.
They do not list prices....That rep will make sure that you purchase the most appropriate system for your purposes. Explain the application and assay....they will hook you up.
I had a temp assignment where they were trying to use an autotitrator for an assay...there were two chemists....I could clearly see the instrument was inappropriate for the assay....now way was it going to detect those levels...in fact after a few runs...I could see their procedures for the 2 asays were reversed...No documentation trail....so the process engineer and I ran them together to prove the point....PROBLEM SOLVED!!! But the down time was significant...
You should talk to a sales rep....
These people sell used surplus autotitrators-they take bids-just like on ebay: http://www.labx.com/v2/newad.cfm?catID=46
Be very careful buying from a reseller. Most times you will do better buying from the provider/manufacturer-all around.
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Thanks! you have got so many valuable experiences and useful tips. I left a message on the Methrom website asking for an autotitrator suitable for my purpose and waiting for their reply.
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Keep us posted, I wonder what they will tell you once they will learn you plan to use pKa for acid identification.
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They will tell him you cannot ID an acid by pKa....that his application will require further analysis and then they will try and sell him an IC system....
This link takes you to a number of downloadable applications for potentiometric analysis:
http://www.brinkmann.com/literature/default_B.asp?GRP=apps
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No reply yet. But I wonder what the people in the old days use when they did not have the sophisticated instruments such as IC, etc. Did they not do any prelim. id?
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No reply yet. But I wonder what the people in the old days use when they did not have the sophisticated instruments such as IC, etc. Did they not do any prelim. id?
these preliminary tests are called qualitative analysis and eventually chemical-technical analysis if samples are not pure. Did you read any texbook on this subjest.
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I did read some textbooks. Qualitative analysis is for id of cations and anions. Seems not comprehensive enough for acids and some bases. But I haven't seen a term chemical-technical analysis. Could you suggest any references pls?
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I found a really old book, dated 1898, An introduction to Chemical Technical analysis.
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I did read some textbooks. Qualitative analysis is for id of cations and anions. Seems not comprehensive enough for acids and some bases. But I haven't seen a term chemical-technical analysis. Could you suggest any references pls?
Acids contains anions, bases contains cations you can find in your samples. Qualitative analysis works this way over 150 years and is sufficient to identify also acids and bases. Simply if you cannot find any cation, and solution is sufficiently acidic, then you have the acid containing this anion. Of course this is valid for individual substances.
In the case of mixtures, inorganic qualitative analysis detects only cations and anions (also a few organic anions).
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Do you mean if I find Cl- ion by qualitative analysis and the pH is low (? how low), then I can say it is HCl? In the same sense, if I find Na in a pH 13 liq that gives NH3 gas when NH4Cl is added, I can say this is NaOH?
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I am intending to use acid base titration for weak acid to decide its pKa and check from a table for the corresponding acid's ID. This doesn't work for strong acid as Borek stated. If I can find the cation, how can I say the anion is H+? Also, I want to use qualitative analysis to test for the cation and anion in a base. what do you think of this approach?
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sorry, should be "If I can find the anion, how can I say the cation is H+?
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I am intending to use acid base titration for weak acid to decide its pKa and check from a table for the corresponding acid's ID. This doesn't work for strong acid as Borek stated.
I have also stated it will not work for weak acids, as the accuracy of the pKa determination is low and there are too many acids with very similar pKa values. Alpha-Omega told you the same thing.
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If I can find the anion, how can I say the cation is H+?
If you have eliminated all other possible cations you are left with H+ only.
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My boss doesn't want to invest on IC, etc. So the only thing I can count on is simple titration for acids like acetic, carbolic, chromic, formic, monochloroacetic, oxalic, phosphoric acids. For HCl, HF, nitric, sulfuric acid I may use qualitative analysis for anions, and exclusion of other cations for H+.
I'd love to use IC if I had this instrument. Many constraints when cost is concerned.
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Instead of titration you may as well try card reading - much cheaper and similar accuracy.
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What is this please: card reading?
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What is this please: card reading?
Fortune telling with deck of cards, like palm reading.
We told you several times that titration will not work as ID method yet you still cling to it.
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Should I rather tell my boss we can do nothing except buy an IC?
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He probably won't believe there is no simple preliminary, not asking for a confirmative, test for acids. I told him once I could not find any standard method from forensic labs and he thought that there must be some. And it is my job to find the solution, not to create problem. I am really worried that I will be out of my job.
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OK...as I see this...if you use a pH meter you can thell him the material is ACIDIC or BASIC....you cannot tell him what it is....
If you use colormetric IDs you can determine this or that is there....but how the this or that is put together is a different ball game....
IC by itself is not a 100% either it will tell you what anions and or cations are in there....just look at those chromatograms I sent you...what do you see?
Most analyses for a material ....LET ME BE VERY CAREFULE HERE...require more than one test to give ALL the information one requires....then you put all those pieces together and explain why something is what it is....
There are cases where one analysis will yield enough information to give you a 100% accurate positive picture/ID of what you are looking for.
XRD would be an example of this....it depends on what you are looking for and are required to report.
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I see lactate instead of lactic acid in the chromatograph, that's the anion, not the exact identity of the acid.
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Imagine mixture containing 0.25M H+, 0.25 M Na+, 0.25 M NO3- and 0.25 M Cl-. Can you tell what acid and what salt does it contain?
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Really appreciate your explanation!
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"Imagine mixture containing 0.25M H+, 0.25 M Na+, 0.25 M NO3- and 0.25 M Cl-. Can you tell what acid and what salt does it contain?"
Can I say I can't tell the answer with any method I use?
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Yep. All you can say is what ions (or molecules) at what concentrations are present.
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With IC you will see the lactate anion. That is correct. The software will monitor the pH of your system during the analysis. You can also do a pressure trace...good for diagnostic purposes...But NO it does not tell you it is lactic acid.
In IC there are so many things going on in that suppressor...difficult to say exactly what you have going on in there....you can infer...you cannot ... and these are UNKNOWNS you will be analyzing....this is just a piece of an unknown...not the whole picture...
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I think I am going to present to my boss what methods are there and the limitations. My understanding now from our discussion is no technology can tell the exact id of an unknown acid or base.
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I know I sent this earlier...OK WEIRD....VERY WEIRD
That is correct-to see lactate...not lactic acid....and you will not see lactic acid with IC....you can analyze a lactic acid std...but you will see the lactate anion...the only way you know you have lactic acid is because you are using a certified std..
The softaware for most IC systems will allow you to monitor pH while you are acquiring.Still, that does NOT tell you you have lactic acid...and there is so much activity going on inside a suppressor...hard to say what the pH is at any instant in time in there...
There is a USP method for sodium lactate assays by HPLC-the sodium salt of lactic acid....because I analyzed it myself for 6 months.....that will give you a lactic acid determination...
Here is a list of hits: http://search.yahoo.com/search;_ylt=A0geu7DV7I5HFGYAcxtXNyoA?p=HPLC+method+lactic+acid&fr=&ei=UTF-8
Specifically check this list: http://www.ttb.gov/ssd/methods_list_composite1-06.pdf
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I just took a Method Development Course...came across some information that might solve many of your problems. The Acclaim OA HPLC Column will do all organic acids from C1 - C7...and alot more...
Check the following links:
http://www1.dionex.com/en-us/webdocs/41786_31996-01_AcclaimOA_V21.pdf
http://www1.dionex.com/en-us/webdocs/25925_Acclaim_OA_V22.pdf
http://www1.dionex.com/en-us/lp41188.html
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Really very grateful for your message. It's so kind of you to share with me the valuable information and always put my problem in your mind. Thank you so much!
I don't understand why this time the HPLC can identify exactly the organic acid itself, not its salt e.g. lactic acid instead of lactate.
After all the discussion before, I abandoned the pKa titration approach. And I have drafted a method outline using chemical spot tests.
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OK...this is how you do it:
1) You have to research the column.....I sent you the links so you can see what is does...
2) You ahve toi purchase the column and the standards for the acids you want to analyze....
3) You run the standards...you have HPLC with UV (I remember you said that)
4) Create a standard curve
5) Run your unknowns against the standard curve..
6) That and the retention times...will give you information regarding what you have...
This manual even tells you how to VALIDATE: Look at the example applications chromatograms...
http://www1.dionex.com/en-us/webdocs/41786_31996-01_AcclaimOA_V21.pdf
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Seems to be a good method for organic acids. We've got an Agilent 1100 and 1200 HPLC. Don't know if the Dionex column is suitable for use in Agilent equipment.
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Yes they are used in Agilent HPLC Systems all the time...definitly compatible....