Can sodium carbonate-sodium bicarbonate buffer be used for EDTA titration of calcium? 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!
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.