[ArturoTanus]

QUESTION

I am testing calcium in meat, and I need to prepare reference standards to validate my test results… So I want to prepare 100 ml solutions of various concentrations (10ppm, 50ppm, 100ppm, and so on), using stock solutions of  5N HCl and 0.02M CaCO₃ that I have in the lab so they are equivalent to the real test procedure. Thus when I run my test, if there is any questions, I can just titrate a couple of my standards and validate my result.

I have found very little out there on how to prepare solutions by mixing the two solutions. But this is what I have …

So,

CaCO₃ has a molecular weight of:                        100.08747
   Ca:   40.0784    x 1 = 40.07840
         C:   12.01078  x 1 = 12.01078
    O:   15.99943  x 3 = 47.99829

So in 1 L of a 1M CaCO₃ solution I should have approximately 40.08g of Calcium therefore… In 1 L of a 0.02M CaCO₃ solution I should have approximately 0.80 g of Calcium, or 800 ppm.

Now, my 100 ml test aliquot comes from a mix of 25ml of 5N HCl and 475 ml of deionized H₂O (according to procedure below), so in 100 ml I should have 5 ml of 5N HCl and 95ml of deionized H₂O.

Therefore, If I dilute 10 ml of 0.02M CaCO₃ with 5 ml of 5N HCL, and 85 ml of DI-H₂O, then I should have 80 ppm in a solution equivalent to the one I usually get from the procedure, right?

Well… when I titrated this solution I had to use 15 ml of the hidroxy naphtol bluesolution (containing 3 ml of the EDTA – which is odd, because we normally it takes 5 ml when following the normal procedure, not 15), and used 4.3 ml of CaCO3 during the titration, so using the formula (below) I subtracted the 4.3 ml of CACO₃ from 15 [(3 ml of the EDTA) * (the actual molarity 0.1/0.02=5)]  and multiplied by 80 resulting in 856ppm !!!

Could anyone please tell me if I am going in the right direction? I need to learn this…

I included the procedure for titration below for clarification.

Million Thanks!!!

ARTURO TANUS

PROCEDURE
The procedure is to weight 5g (±0.100g) of sample into a 40 mL crucible, place the sample in a muffle furnace set at 600°C, and heat for 2 hours or until sample is white ash. Then, to remove samples, allow them to cool down, add 25mL 5N HCL to the crucible. Set temperature to medium heat (300°C) and allow contents of crucible to fume. Then the contents of crucible are rinsed into a 500mL volumetric flask and brought to volume with deionized water. We weigh 100 ml of the sample solution and adjust pH to 12.5 ± 0.1 using 5N KOH. Then 5.0 mL of pre-mixed Hydroxy Naphthol Blue Solution is added to the sample and it is titrated with .02M CaCO3. The pre-mixed Hydroxy Naphthol Blue Solution is a mixture of 100 mL of EDTA 0.1M with 400 mL of deionized water and 20.0g of Hydroxy Napthol Blue. To calculate calcium I use a formula as follows:

Ca (mg/100g) = [A(M)-(BxR)] * (0.08) * 1000
   Where:
   A= mL of EDTA (3 ml added with the 15 ml of hidroxy naphtol blue)
   B= mL of CaCO3 (4.7 measured from burete or titrator)
   R= EDTA to CaCO3 ratio (1 from standardization)
   M= actual molarity of EDTA divided by 0.0200 = [0.1/0.02] = 5
So we subtract the ml of CACO3 from 15 and multiply by 80 to obtain ppm.

[Borek]

using stock solutions of  5N HCl and 0.02M CaCO₃ that I have in the lab

0.02 M solution of CaCO₃? With K_sp around 5×10^-9?

[Arkcon]

There is much that I don't understand in what you've written, and Borek: was kind enough to pick out one bit -- I don't understand your choice of source of calcium.

OK, so I gather that you're doing a titration of the residue left from an HCl digestion of organic material.  You need calcium standards.  You should use a soluble calcium salt of known concentration, I'd suggest you purchase calcium chloride, of an analytical grade, possibly of a known concentration, or the solid and you make the concentration you need.

We have more to talk about, but you really do need to start with that, and then move forward.

[Corribus]

Depending on your needs for precision, you can buy standards for just about every element at elemental analysis suppliers like SCPScience and Inorganic Ventures, and they're not unreasonably expensive. You can also purchase trace metal grade reagents and acids for your digestions - I highly suggest the latter, especially for a rather common element like calcium that could have high backgrounds.

Preparing your own standards from calcium salts is an option, but if they're hydroscopic at all, it's going to be hard to get a very accurate concentrations.

[Borek]

Actually calcium carbonate is one of the standard substances, so it can be used to prepare standard solutions, after being dried in 110°C.

But we are talking about solid dissolved in hydrochloric acid, not about 0.02 M solution.

[ArturoTanus]

Gentlemen, Thanks for your replies...

Borek, I really do not know what is the K_SP of CaCO₃. Long time ago, we use to make the 0.02M solution of CaCO₃ from scratch in the lab, but now we purchase it already prepared.

I thought of purchasing some standards as Arkcon and Corribus suggest, however I thought it may be easier just using the solutions of 0.02M of CaCO₃ and 5N HCl since those are used in the original procedure and I already had them available. Also, using this solutions to make my standards would be equivalent to digesting the meat samples and titrating them per procedure.

Sorry guys, I wish I could be more specific, or clear, but I am not a chemist, so please be patient with me. I just need to know where in my thought process is that I am missing something. My question still remains... I managed to make a standard that I thought it would be around 80 ppm and after titrating it turned out to be around 800 ppm. What did I miss?

[Borek]

Borek, I really do not know what is the K_SP of CaCO₃. Long time ago, we use to make the 0.02M solution of CaCO₃ from scratch in the lab, but now we purchase it already prepared.

K_sp is even listed on the wikipedia page for CaCO₃, so it is not that hard to find. And it clearly shows solubility of CaCO₃ is in the range of 7×10^-5 M, so it is not possible to prepare 0.02 M solution.

I managed to make a standard that I thought it would be around 80 ppm and after titrating it turned out to be around 800 ppm. What did I miss?

As explained above, hard to say what you are doing, when you claim to use solution that can't exist.

[ArturoTanus]

Borek... I purchased a 0.02M Solution of CaCO₃ and I wish I knew hot to post images to show you the label!!!! The Catalog# is CX-925 from Red Bird Services... check it out and try to help, not to criticize... Nice forum this is....

[Borek]

Yes, I was able to google it, no, I have no idea what they are selling. There is no way to dissolve calcium carbonate to get that kind of concentration, period, so the solution they are selling is not 0.02 M calcium carbonate.

I am not criticizing, I am just stating the fact. I would like to help, but I can't as long as I don't understand what is going on. And the description of the procedure doesn't make chemical sense.

Judging from the CAS numbers listed here it can be a solution prepared by dissolving CaCO₃ (471-34-1) in hydrochloric acid (7647-01-0). If so, they sell CaCl₂ solution, not calcium carbonate solution.

[ArturoTanus]

Alright!!! So now I am totally confused and lost! The procedure I am following was implemented a long time ago (before my time) and it is supposed to be a modification of AOAC procedure 983.19 (which I have that as pdf, but how to get it to you?).

anyway...

All I am trying to make sure is that if I buy a standard of a certain ppm, and I titrate it per procedure, I get the right ppm. As a form of validating my procedure... Does it make sense?

[Borek]

Let's assume it is in fact 0.02 M solution in Ca²⁺, that's what matters here.

Therefore, If I dilute 10 ml of 0.02M CaCO₃ with 5 ml of 5N HCL, and 85 ml of DI-H₂O, then I should have 80 ppm in a solution equivalent to the one I usually get from the procedure, right?

80 ppm it is.

Well… when I titrated this solution I had to use 15 ml of the hidroxy naphtol bluesolution (containing 3 ml of the EDTA

I don't follow. I thought amount of naphtol blue solution is always the same (looks like what you are doing is a back titration, so it makes sense), but you varied it here.

[ArturoTanus]

Sorry it took me this long to reply, but I had to spend Christmas and a couple of more days in bed because of a miserable flu virus not included in the vaccination package

Yes Borek, it is supposed to be always the same. When we perform the procedure, we always use 5 ml (and solution turns to blue), when I did the dilution I referred to, it took 15 ml to get to the blue color (so a total of 3 ml of EDTA).

Now that you agreed that the solution its 80, why do I get a titration result of ±800?