I am planning to measure general/total hardness (calcium and magnesium) of aquarium water using EDTA titration with Eriochrome Black T indicator. From what I understand, the unknown solution must be kept basic, at pH 10, for the EDTA to work in chelating the calcium ions off of the dye indicator complex. The procedures I can find online all use ammonia-ammonium chloride buffer. I would prefer using another buffer if possible as I find ammoniacal solutions difficult to handle and relatively hard to source.

Can I use a sodium carbonate-sodium bicarbonate buffer? Will it interfere with the titrant: will the EDTA form chelates with sodium ions?

From a similar titration procedure for exclusively determining calcium ion concentration using EDTA but with a Patton-Reeder indicator (calconcarboxylic acid), pH was raised to 12.5 using sodium hydroxide to precipitate magnesium off the solution. I suppose sodium will not interfere then?

Thank you!

  • $\begingroup$ Did you check the $K_{sp}$ of $\ce{CaCO3}$? $\endgroup$ – MaxW Apr 21 '19 at 19:04
  • $\begingroup$ That made me thinking. I think that's already the answer... sodium carbonate will precipitate the calcium being measured via the common-ion effect? @MaxW care to post that as an answer? $\endgroup$ – shimofuri Apr 21 '19 at 19:17
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    $\begingroup$ I'm not sure what you mean by common ion effect, but the precipitation of calcium carbonate and magnesium carbonate would interfere with the EDTA titration. $\endgroup$ – MaxW Apr 21 '19 at 19:22
  • $\begingroup$ @MaxW I remembered something about sodium soaps not working in saltwater because the equilibrium is shifted to prevent the dissociation of sodium from the soap in the presence of excess sodium chloride. In contrast potassium soap will work. From that I drifted into Wikipedia and under the "common-ion effect", they have listed an example of precipitating calcium carbonate off solution by adding sodium carbonate. $\endgroup$ – shimofuri Apr 21 '19 at 19:32
  • $\begingroup$ @shimofuri Rather, soap does not work in saltwater(=seawater) due content of calcium and magnesium, forming insoluble salts of fatty acids, like in very hard fresh water. $\endgroup$ – Poutnik Apr 22 '19 at 6:07

Sodium hydroxide $(\ce{KOH})$ works as $\mathrm{pH}$ modifier (measure the $\mathrm{pH}$ by adding drop by drop of hydroxide until $\mathrm{pH}$ is approx. $10$ to $12).$

Carbonate ion will be combined with calcium(2+) cation to form calcium carbonate. You can measure the calcium content by precipitating the calcium using soda ash light solution. Good luck.

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