[Simon22]

Hi Everyone,
I have been trying desperately to figure this out!. I work for a company who sells calium chloride solution. Usually the concentrations are around 30% and sometimes as high as 38%. Typically we test the product before it goes out using a hydrometer and convert  the °Baume result into percent using a chart. Any other complicated testing is complete at an outside lab.
We now have a customer who wants us to titrate the samples and provide a certificate of analysis based on the titration results.
I am not a chemist and have been trying to decipher the method that the customer provided us with. It is horribly incomplete and is supposed to be a simplified procedure for determining calcium content. I do not want to resort to asking the customer how to titrate the product that we are selling them.
The method uses an EDTA solution that is to be mixed and standardized. My frustration begins here as the EDTA crystals do not dissolve in the water without heat and as soon as the solution cools the EDTA crystalizes again. After spending quite a bit of time on the internet I found that the EDTA will dissolve at a higher pH so the addition of sodium hydroxide is necessary. I found this to be true. 
      *Will the addition of the sodium hydroxide in the EDTA sol alter my end point when I perform the titration?
      *Does anyone have any experience titrating strong calcium chloride solutions?
      *The only other detailed method that I have is the ASTM standard and it is more complicated and requires potassium cyanide ( I am not very comfortable using this   chemical)

Here is the method described on the customers SOP.

Pipet and aliquot into a 250ml flask and dilute to 100ml (stir with magnetic stirrer)
Add 10 ml of pH 12 buffer solution ( 1N sodium hydroxide sol) and 100 to 300mg hydroxy naphthol blue
Make sure the pH of the solution is at 12
Titrate the solution with standardized EDTA to blue endpoint
Calculation= A x F x B / C x D x 10 = % Ca
Where A= Titration volume in ml
          F= Factor in mg/ml (from EDTA standardization)
          B= Dilution volume in ml
          C= Aliquot size in ml
          D= Sample weight in grams

When looking at the calculation, first does it make sense and second where do the B and D numbers come from?

I would appreciate any help you can give.
Thanks in advance
Simon

[Arkcon]

Well, one thing you can do is purchase standardized EDTA titrant.  It will come in a jug, and will have the concentration typed on the label.  Its more expensive than bulk EDTA, but you won't have to make and titrate the EDTA.  The number on the bottle will go in as F.  B and D aren't perfectly clear to me, it looks like you mass out some dry powder D, dilute it in some amount B, take an aliquot C.  But I might be missing it.

[Simon22]

Well, one thing you can do is purchase standardized EDTA titrant.  It will come in a jug, and will have the concentration typed on the label.  Its more expensive than bulk EDTA, but you won't have to make and titrate the EDTA.  The number on the bottle will go in as F.  B and D aren't perfectly clear to me, it looks like you mass out some dry powder D, dilute it in some amount B, take an aliquot C.  But I might be missing it.

Thanks for the quick response.
I didn't order standardized EDTA because the instructsions had me making the EDTA using the following
 288g EDTA Disodium salt, 0.4g magnesium chloride hexahydrate and water.

The magnesium chloride threw me off and I was not sure if that is in the standardized solutions or if I even need it.
I would perfer to purchase standardized solutions if I can. I do with the other titrations that we perform here.
I suppose I could just order some and try it but I was trying to avoid wasted money.

Thanks
Simon

[Borek]

You will find some additional information about EDTA titration here:

http://www.titrations.info/EDTA-titration

Are you using pure EDTA or disodium salt? If memory serves me well disodium salt is not that difficult to solve (that is - it dissolves slowly, but it does dissolve).

288g - what concentration should it yield? I am getting some strange values (using concentration calculator), no matter if I try EDTA, disodium EDTA, or disodium EDTA dihydrate (that is, I assume 1L of the solution - but it doesn't matter much, mass of the EDTA should be some nice number of moles, not some random number).

Could be magnesium chloride is added to help detect end point using Eriochrome Black T (I believe near the endpoint Ca displaces Mg in the complex).

But it is 0:40 AM here and I am not going to check details now, sorry.

[Simon22]

Thanks for the response at 0:40AM! So sorry about the insomnia.

I double checked and the EDTA that I am using is the disodium salt (ethylenediamine tetraacetic acid disodium salt dihydrate). I had the solution mixing for most of the day yesterday and it does not seem to be dissolving.

The instructions say that once the EDTA solution (EDTA Magnesium and water) are mixed it must be standardized with calcium solution.
Using and aliquot of 5 ml of 1mg/ml standard solution of calcium carbonate and them calculating
A x B / C =F
Where:A= Aliquot size of calcium chloride
         B= mg of calcium per ml of calcium standard solution
         C= Volume of EDTA in ml used for titration
         F=mg of calcium per ml of EDTA
F would be the factor for the calculation in the final titration that I described previously.

Nowhere in the instructions does it say to use Eriochrome Black T. I have seem referrences to the use of that while I was trying to understand all of this.

Thanks for ths link. I stumbled accross that one earlier. It is a bit over my head but I will look at it closer and see if I can understand it better.

Any comments and input is appreciated!

Thanks
Simon
 

[Arkcon]

You will need an indicator like Eriochrome Black T.  The whole point of a titration is that you will add EDTA solution, one drop at a time, to the calcium solution.  Once you have added just enough EDTA solution to react with and consume all of the calcium in solution, you will get a permanent color change in the indicator.  That's how you'll know to stop adding EDTA, and to use the volume in your calculation.

[Simon22]

You will need an indicator like Eriochrome Black T.  The whole point of a titration is that you will add EDTA solution, one drop at a time, to the calcium solution.  Once you have added just enough EDTA solution to react with and consume all of the calcium in solution, you will get a permanent color change in the indicator.  That's how you'll know to stop adding EDTA, and to use the volume in your calculation.

Thanks, I understand that I need an indicator. I am using hydroxy naphthol blue. Is this indicator suffucient for the EDTA used in calcium analysis?

Thanks
Simon

[Arkcon]

If that's the way the customer wants it done, you should do it that way.  Innovate as little as possible -- is one corollary to Murphy's Law applicable to science in general.

[Simon22]

If that's the way the customer wants it done, you should do it that way.  Innovate as little as possible -- is one corollary to Murphy's Law applicable to science in general.

True... I don't want to deviate from their instructions. I am just having a hard time understnding them.
Thanks