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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jackokildangan on November 06, 2012, 05:25:54 PM

Title: pH -Reduction of high ph using different compounds
Post by: jackokildangan on November 06, 2012, 05:25:54 PM
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!!!
Title: Re: pH -Reduction of high ph using different compounds
Post by: Hunter2 on November 07, 2012, 01:11:15 AM
How this should work. You need some acid to decrease the pH. Calcium sulfate has bad solubility and calcium chloride is a neutral salt.
Title: Re: pH -Reduction of high ph using different compounds
Post by: jackokildangan on November 07, 2012, 09:38:52 AM
Could you expand on this in more detail please?
Thanks
Title: Re: pH -Reduction of high ph using different compounds
Post by: Hunter2 on November 07, 2012, 11:30:07 AM
You have to add some Hydrochloric or sulfuric acid. Thesalts dont have an effect.
Title: Re: pH -Reduction of high ph using different compounds
Post by: curiouscat on November 07, 2012, 02:51:09 PM
Quote from: Hunter2 on November 07, 2012, 01:11:15 AM
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.
Title: Re: pH -Reduction of high ph using different compounds
Post by: jackokildangan on November 07, 2012, 04:44:04 PM
Salts definetly do lower this pH. Iv completed the experiments for my final year project in university. No acid needed.
Title: Re: pH -Reduction of high ph using different compounds
Post by: Borek on November 07, 2012, 05:26:40 PM
Please elaborate on the experimental procedure.
Title: Re: pH -Reduction of high ph using different compounds
Post by: jackokildangan on November 08, 2012, 08:46:18 AM
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.
Title: Re: pH -Reduction of high ph using different compounds
Post by: Borek on November 08, 2012, 09:48:06 AM
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.