[jackokildangan]

Im doing a very important project which involves the reduction of pH 11 Sodium hydroxide down to around pH 9. The chemicals i am using are gypsum(CaSO4.2H2O) and calcium chloride, i know both these chemicals can reduce pH from lab tests but i dont understand the chemistry involved in lowering pH.....can someone please help me!!!

[Hunter2]

How this should work. You need some acid to decrease the pH. Calcium sulfate has bad solubility and calcium chloride is a neutral salt.

[jackokildangan]

Could you expand on this in more detail please?
Thanks

[Hunter2]

You have to add some Hydrochloric or sulfuric acid. Thesalts dont have an effect.

[curiouscat]

How this should work. You need some acid to decrease the pH. Calcium sulfate has bad solubility and calcium chloride is a neutral salt.

Can't CaCl2 precipatate out Ca(OH)2?

 CaCl2 + 2NaOH > Ca(OH)2 + 2NaCl

That would reduce pH won't it? Not sure where equilibrium lies.

[jackokildangan]

Salts definetly do lower this pH. Iv completed the experiments for my final year project in university. No acid needed.

[Borek]

Please elaborate on the experimental procedure.

[jackokildangan]

Taking pH 11 NaoH (0.001M) and adding 0.01g of gypsum to 100ml reduces pH to 9 over a six hour period (constant mixing).
Simillary adding 0.1g of CaCl2 to 100ml of NaOH (ph 11) reduces the pH to 8.

[Borek]

I don't see how. There are two reactions that can remove some OH^- from the solution containing Ca²⁺ and OH^-. One is production of CaOH⁺, the other precipitation of Ca(OH)₂. In the solution you described there is no way to see precipitate (we are below solubility product), and CaOH⁺ is present only in a minute quantities (8*10^-5M) - that means pH can be lowered by about 0.1 pH unit, to 10.9.

I suspect some problems with the pH measurement.