Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: angelku on January 15, 2008, 01:41:19 AM
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A Known excess amount of sodium hydroxide is mixed with weighed mass of aspirin, the amount of hydoxide can be determined by a back titration with H2SO4.The mass of aspirin can be determined by calculations.
If a base CaCO3 present in the tablet,what is the difference betwwen the calculated value with the actual value?
I have no idea about that.Would CaCO3 present in the tablet can allow smaller amount of NaOH to hydrolysis the ester?So that more amount of H2SO4 needed to used to reach the end point.The lower mass of aspirin may be resulted.
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H2SO4 reacts with sodium acetylsalicylate, sodium hydroxide and calcium carbonate. You should specify more details of your method. (If you use phenolpthalein in titration then only excess of NaOH will react and question on CaCO3 is out of science assuming no reaction between aspirin and calcium carbonate took place during storage of aspirin.)
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Dear Angelku;
Yes!, ─ Please, explain more about your “methodic”.
It looks like a good method for training Titration, but also as a bad method for Quality Control.
Good Luck!
ARGOS++
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1.Initiate the hydrolysis of the aspirin by adding NaOH .
2. Titrate the solution mixture (aspirin and NaOH)diluted by deionized water with H2SO4 using phenol red as indicator.
I also have question why the active ingredient in a tablet produced by drug manufacturer seldom with higher than 90%?Why they dilute the active ingredient?
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Dear Angelku;
Thank you very much for the short description!
I will not comment it, because you may have already realised it by yourself, that it is a very unspecific method!
The content of a Tablet can’t be very much above 90% (Tablet weight), because other Ingredients are required, at least one for “blasting” the Tablet (for a higher release rate [DR]), and one for compacting/formulating the Tablet (= for Galenics).
But in case you find equal or less than 90% of the “Nominal Content”, then it will be very bad.
At least in such situations an alternative method like UV/Vis-Spectrometry is indicated, which by correct handling is much more specific and can by used for identification too!
I hope it gives you at least an idea.
Good Luck!
ARGOS++
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1.Initiate the hydrolysis of the aspirin by adding NaOH .
2. Titrate the solution mixture (aspirin and NaOH)diluted by deionized water with H2SO4 using phenol red as indicator.
I also have question why the active ingredient in a tablet produced by drug manufacturer seldom with higher than 90%?Why they dilute the active ingredient?
What do you mean under hydrolysis?
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2-ethanoyloxybenxoic acid which is an active ingredient of aspirin.It can be hydrolysed by using excess sodium hydroxide into the sodium salts of two weak acids.
After the hydrolysis of aspirin,the reaction mixture was diluted to 250 ml.25ml was titrated against H2SO4 by using phenol red as indicator.
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Hydrolysic of acetylsalicylic acid is not quite easy. You should warm solution at lest a part of hour. Then excess of NaOH will catch carbon dioxide from air and your mixture for titration will be qute complex.
Can you write down chemical reaxtions?
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C9H8O4+2NaOH-->CH3CO2Na+C7H5O3Na +H20
Does a aspirin tablet contain CaCO3 will affect the calculated value ?
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Dear Angelku;
It should nearly not influence, because with using Phenol Red (pH: 8.0 – 6.6) as Indicator most of the carbonate effect is already “eliminated”.
A little better could be the use of Litmus (pH: 8.0 – 5.0) or Methyl Red (pH: 6.2 – 4.4).
Apropos: I found the identical Method, except Backtitration with HCl instead of H2SO4, in a German Chemistry Forum, ─ with similar problems.
Good Luck!
ARGOS++
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Why the presence of CaCO3 have no influence when using phenol red as an indicator?Do you mean the calculated value have no difference between the actual value. ???How the amount of reactants used change?
The drug manufacturer dilute the active ingredient because it require the presence of ingredients that formulating the tablet.Besides,are there any reasons account for this action? ???
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Carbonates start decomposing when pH is lower than 7, and you finish titration at pH 8 in your case
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Dear Angelku;
Sorry!, ─ that my statement is giving a cause for misinterpretation!
(When I answered you I hade (only) the carbonate statement from Mr. AWK in mind, sorry!)
Of course you are right! In case a manufacturer adds “any” Carbonate to the Tablet you will titrate the contained base too, because at the end point of your Titration, around or below pH 5.5, most or all Carbonate will be “formed” to CO2.
Such an addition will require more H2SO4 and simulate a lower Content.
But that’s also only one indication of an unspecific method.
(To stop Titration on higher pH may also have disadvantages, as collected CO2.)
Yes! ─ There are “several” reasons, like Stability (phys. & chem.) of the Formulation for different “Climates” on Earth, Hardness of Tablet, DR (Disolution Ratio), etc.
(Galenics is quite a broad field for R & D.)
Sorry to have given you a wrong cause.
Good Luck!
ARGOS++
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Why more H2SO4 needed to be used when carbonate decompose into CO2?
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First of all you should obtain H2CO3 to decompose it. Decomposition of carbonic acid is rather complex problem from scientific point of view (but not practical one).
Concentration of carbonic acid in solution (when formed) depends on concentration of dissolved carbon dioxide in solution together with partial pressure of carbon dioxide over solution.
You can change the concentration of CO2 in the solution by
lowering the pressure of CO2 over solution what cause
lowering the concentration of soluble CO2 in the solution.
Moreover lowering of CO2 concentration in solution is possible by adding strong acid or by heating the solution.
Closely to pH 7 eventually hydrogen carbonates may exist in solution. They also may be decomposed by addition of strong acids or by heating the solution.
In my opinion that rather reaction CO2 from air with an excess of NaOH may be a source of an rror in you titration since reaction:
2NaOH + CO2 = Na2CO3 + H2O
During titration using phenyl red as an indicator you can titrate only half NaOH involved in the above reaction because reaction
CO32- + H+ = NaHCO3
takes place.
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Dear Angelku;
Maybe? a little bit simpler:
That’s because for CaCO3 as an Example, and/at the given End Point:
CaCO3 + H2SO4 ---> CaSO4 + H2O + CO2(g) .
And that would be quasi equivalent to:
Ca(OH)2 + H2SO4 ---> CaSO4 + 2H2O
(because CO2(g) has left the System.)
I hope it may be of help.
Good Luck!
ARGOS++
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At ph 8 CO2 do not left the system!
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Dear Mr. AWK;
That’s precisely why I was also talking about Indicators and/at the given End Point: Not pH 8!
(Please see the previous posts.)
Good Luck!
ARGOS++
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But Angelku use the phenol red indicator, and just its using practically prevents decomposition of CaCO3.
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Dear Mr. AWK;
I’m surprised!, ─ Something below pH 6.6?: "Phenol Red (http://en.wikipedia.org/wiki/Phenol_red)”
Good Luck!
ARGOS++
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When we titrate from basic side?
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Dear Mr. AWK;
Sorry!, ─ Exactly in this case Phenol Red changes from Red to Yellow and so our Endpoint will lie at, or below pH 6.6!
(Not only Wiki is telling that.)
Good Luck!
ARGOS++
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How about using phenolphthalein as an indicator?Its pH is 8.2-10.0.
Is is calcium carbonate in the solution have already completely decompose into CO2 which left to the system at pH below 7?Do you mean H2SO4 needed to be use for neutralising the additional amount of CO2 presence in the solution no matter the CO2 left or not?
I
Although the CO2 leave the system at pH below 7,more amount of H2SO4 still needed to be used even the end point is at pH above 8.Am I right?
???
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Dear Angelku;
Cheer!, ─ You are very Solomonic, and you learn quite fast/well, but how is it called in English (American): “TANSTAAFL”.
If the CO2(g) from CaCO3 doesn’t leave the System, then also all the CO2(g) you picked up during hydrolysis, during dilution, and during titration will not leave the System either.
In such case you will find a too small value of H2SO4, now simulating a too high content of Aspirin!
Quintessence:
You can’t make a method more specific as it is by its nature, ─ You have to live with.
(But you can combine/change methods!)
Good Luck!
ARGOS++
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How about using phenolphthalein as an indicator?Its pH is 8.2-10.0.
Is is calcium carbonate in the solution have already completely decompose into CO2 which left to the system at pH below 7?Do you mean H2SO4 needed to be use for neutralising the additional amount of CO2 presence in the solution no matter the CO2 left or not?
I
Although the CO2 leave the system at pH below 7,more amount of H2SO4 still needed to be used even the end point is at pH above 8.Am I right?
???
I think phenolphtalein is a good idea. It change color (become colorlees) just 0.1 pH unit over pH of NaHCO3 hydrolysis
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Dear Mr. AWK;
I don’t understand: ─ A good Idea? ─ For what?
─ For the evidence that the CO2 will not leave the system and we titrate a more or less erroneous actual content?
─ Or that we Chemists are so genial that we are able to titrate with a visual precision of better then 0.1 pH Units with the help of an indicator with its well known problems at acidic end of its indication range?
I don’t believe that continuing such discussions are doing any good job/help for anybody who tries to learn, ─ Sorry!
Good Luck!
ARGOS++