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Mercurometric titration (chloride determination)

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Taninn:
Hello,

I have an issue with titration chloride ions by mercury nitrate in the presence of diphenylcarbazide.

I noted that there is no linearity respect to concentration of chloride.

I.e. i have prepared natrium chloride solution with 300 mg/L of chloride and second solution with 150 mg/L chloride.

When i prepare solution of mercyru nitrate which shows 300 mg/L in first solutution, then there is problem with the second one - it shows contentration about 170-180 mg/l insteat 150 mg/L.

Conversely when i prepare solution of mercury nitrate, which shows 150 mg/L on second solution, then it shows concentration 270-290 mg/L of first solution instead 300 mg/L.

I have bought standard solution, but titration shows as this solution would have concentration about 25% highter than it shoud have.

Calculations and preparations are correct. So i dont really understand what is wrong. With another titrations methods i have no problems just like this.

P.S. sorry for my english, i'm from Poland, and there is no chemical forum.

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