[Alberto_Kravina]

Does anyone of you guys know how I could perform such an analysis?
I thought about gravimetry and/or Argentometry, but I think that the matrix interfears in this case...

Any other ideas

[Borek]

Does anyone of you guys know how I could perform such an analysis?
I thought about gravimetry and/or Argentometry, but I think that the matrix interfears in this case...

Any other ideas

Argentometry will give - at best - total chlorides, not NaCl.

[Alberto_Kravina]

What other chlorides would you expect to find? Other alkali chlorides (such as KCl)...?

[Borek]

Problem is more general. Doesn't matter what salts are dissolved - what matters are concentrations of individual ions.

You dissolve 10g of NaNO₃, 10 g of NaI, 10 g of KCl and 10 g of CaCl₂. How much NaCl does the solution contain?

[lemonoman]

My guess Alberto doubts there are other chlorides in the ketchup.

And I'm inclined to agree...I'm no ketchup buff, and I wouldn't count on NaCl being the only source of chlorides...but I would count on it being the PRIMARY source of chlorides.  Depending on accurate you want this analysis to be...that may be good enough (of course, check to make sure there aren't other hidden sources...my word isn't THAT trustworthy lol)

If you find another method or other information, keep us posted!

[Borek]

Tomatoes are considered to be high on potassium.

But that's not the main point - chlorides content is not the same as NaCl content. At best you should determina both Na⁺ and Cl^- and give lower of both concentrations as NaCl content - which doesn't make sense. Much better is to give inofrmation in the form
Na⁺ - 1%
Cl^- - 1.3%
K⁺ - 0.2%
...

and so on, listing all ions determined.

[Alberto_Kravina]

Much better is to give inofrmation in the form
Na+ - 1%
Cl- - 1.3%
K+ - 0.2%

That's a smart idea!
I think that I'll determine the total content of chlorides (argentometry, Mohr) and then the % of potassium (gravimetry).
....How could I determine the amount of sodium? I don't think that gravimetry is a solution in this case...

[Borek]

Some spectrophotometric method will do. No idea how to determine sodium with volumetric or gravimetric methods. Well, my sources list precipitation of Na as NaZn(UO₂)₃^.(CH3COO)₉, but no details are given.

[Alberto_Kravina]

I thought about Magnesium or Zinc uranyl acetate/nitrate and hexahydroxoantimonate as well, but I think that a spectroscopic method would be better...

[Albert]

What about an ion-selective electrode for sodium?

[lemonoman]

This source agrees that spectroscopic methods are doable.  As long as you have a Sodium Source (HCL maybe) and a take a few precautions...routine ones in spectroscopy...like adding Cs to ensure full determination by Easily Ionizable Elements....and adding La to get rid of a bunch of matrix interferences....

http://www.nilu.no/projects/ccc/manual/documents/04_6-Determination%20of%20sodium,%20potassium,%20magnesium,%20and%20calcium%20in%20precipitation.htm

[Alberto_Kravina]

I'll think about how to analyse the sodium content later, but now, more important: How can I get rid of the matrix so that I can titrate the sample to determine the content of Cl^-?

[Borek]

Boiling to dryness with concentrated nitric acid comes to mind. But you should check details in some handbook.

[Dude]

You might also want to try heatig with sulfuric acid.  A common laboratory technique to generate HCl is to heat an aqueous solution of sodium chloride with sulfuric acid.  I believe I remember seeing a reference of the procedure recently on this site.  The strong(er) acid sulfuric acid displaces the Cl to generate HCl as a vapor.  Route the vapors through some tubing into a solution containing an excess of base to trap the HCl.  Reacidify with an acid other than HCl and then titrate with silver nitrate.

Alternatively, see if someone has an ECS-1200 coulometer from ThermoNoran to measure the chlorine content.

[Alberto_Kravina]

Thanks for the advice Borek and Dude. I'll try both methods (I'll also search for other methods on the web/school library). I should have a result on Wednesday (only total chlorides I think). Does anyone have an idea of what kind of result I should expect? I think something like 5-10%...