How to increase pH of lime water


by Caccioppoli
Tags: increase, lime, water
Caccioppoli
Caccioppoli is offline
#1
Oct2-11, 04:45 AM
P: 10
Hi to all,
I'm in this problem:
I've got to prepare a solution of Ca(OH)2 with pH=13.2 .
On Perry I read that the solubility value of Ca(OH)2 at T=25C is 1.65 g/L, so a solution with water and Calcium Hydroxide cannot reach pH=13.2.
So I was suggested to prepare the mixture in the following manner:
distilled water + 4g Ca(Oh)2 + 1g Na2SO4 + 1g NaCl + 1g NaNO3 .

I'd like to know if this solution is correct and why.

Thank you very much,
Caccioppoli
Phys.Org News Partner Engineering news on Phys.org
Lifting the brakes on fuel efficiency
PsiKick's batteryless sensors poised for coming 'Internet of things'
Researcher launches successful tech start-up to help the blind
Borek
Borek is offline
#2
Oct2-11, 05:22 AM
Admin
Borek's Avatar
P: 22,673
I don't see how sodium sulfate, chloride and nitrate can help in changing pH (other than increasing ionic strength of the solution, but it won't change the result much).

Perhaps start with saturated Ca(OH)2 and add NaOH till pH goes up to 13.2? That will mean precipitating part of the calcium, but you can't cheat on Ksp.
Caccioppoli
Caccioppoli is offline
#3
Oct2-11, 05:41 AM
P: 10
Thank you very much for the answer.
An european guide line (etag) says that I have to prepare a solution with pH 13.2 having Ca(OH)2 powder and distillated water.
Since theoretically and experimentally I have verified that it is impossible to reach that pH, I asked for information.
They said that I have to add those substances in these quantities to obtain pH 13.2.
I have not yet prepared the mixture as they said, but for me is really strange, and I have thought about adding NaOH too.
By the way I think I should find an explanation for their suggestion.

Borek
Borek is offline
#4
Oct2-11, 12:56 PM
Admin
Borek's Avatar
P: 22,673

How to increase pH of lime water


If they say that's the standard procedure, you have not much choice but to try. I don't see how it can work, but can be I am missing something.
uby
uby is offline
#5
Oct3-11, 11:53 AM
P: 176
two issues at play that should not be conflated. first is the solubility of the species in water and second is the dissociation fraction.

with regards to the additions of sodium sulfate and nitrate ... it may have to do with the difference in dissociation fraction between sodium and calcium salts. the sodium salts, dissolved in water, may react with undissolved Ca(OH)2 to produce NaOH (which will dissociate to a large degree and increase pH) and soluble CaSO4/Ca(NO3)2.
Borek
Borek is offline
#6
Oct3-11, 12:08 PM
Admin
Borek's Avatar
P: 22,673
Quote Quote by uby View Post
the sodium salts, dissolved in water, may react with undissolved Ca(OH)2 to produce NaOH
I don't see how. You can't cheat on Ca(OH)2 Ksp. Introducing Na+ doesn't change OH- concentration, so it can't shift Ca(OH)2 dissolution equilibrium.

That's all not entirely true, as both Na+ and Ca2+ get complexed by OH- to some extent, so it is not only about Ksp - but these complexation reactions don't increase OH- concentration, so pH is not going up.
chemisttree
chemisttree is offline
#7
Oct6-11, 06:28 PM
Sci Advisor
HW Helper
PF Gold
chemisttree's Avatar
P: 3,725
It can if calcium sulfate precipitates or if sulfate complexes the calcium. Complexing agents like sucrose can change the solubility of CaOH2. Ammonia can change the solubility of silver chloride, etc...
Borek
Borek is offline
#8
Oct7-11, 01:59 AM
Admin
Borek's Avatar
P: 22,673
Quote Quote by chemisttree View Post
It can if calcium sulfate precipitates or if sulfate complexes the calcium.
Right - but I have problems calling it the lime water after that.


Register to reply

Related Discussions
How much lime juice to be used iso. citric acid?? Chemistry 4
simulation lime kiln Mechanical Engineering 1
Why does water volume increase when heated? Biology, Chemistry & Other Homework 3
Eating up all lime's slices Biology 12
Store energy in lime CaO? Chemistry 2