[MattyB]

I am currently undergoing a titration in which sodium hydroxide is used to precipitate magnesium out of a solution before a complexometric titration to find the calcium content. However, I would like to use a substitute chemical for the sodium hydroxide which will follow a similar role. I thought of these, though they may not be suitable:

• Sodium (or potassium) Carbonate (make Magnesium Carbonate) - this is highly insoluble so will precipitate out
• Sodium Bicarbonate (make Magnesium Bicarbonate)

[Borek]

What are you titrating? Mg is usually precipitated as Mg(OH)₂ during determination of Ca/Mg.

[MattyB]

What are you titrating? Mg is usually precipitated as Mg(OH)₂ during determination of Ca/Mg.

Ah yes. I am titrating with EDTA, finding the concentration of Ca, using the hydroxynaphthol blue indicator. Just wondering if there's a potential replacement for Sodium Hydroxide, which is normally used when removing Mg ions from solution to form Mg(OH)₂.

[Borek]

I am not aware of any reasonable replacement, although I guess KOH would work as well. Note that this titration is notoriously difficult and the procedure with NaOH is probably selected as the one giving the best results.

[MattyB]

I'll try some of the chemicals I had theorized, to see if they will do the job - if not, it's something to talk about in the write up. I am suspecting you are right though. Anyway, thanks for the reply.

[DrCMS]

You have a sample with a mixture of Mg²⁺ and Ca²⁺ in solution and you add NaOH to precipitate Mg(OH)₂ while leaving Ca(OH)₂ in solution so you can quantify the Ca²⁺ right.

What are the relative solubilities of magnesium and calcium hydroxide?

Have you checked the solubilities of magnesium and calcium carbonates or the bicarbonates?  Do you think you could precipitate only the Mg and not also the Ca salt of them?