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Topic: pH of chlorine water  (Read 10479 times)

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Offline Caustikola

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pH of chlorine water
« on: April 29, 2012, 05:58:56 PM »
I've read and found that the solubility of chlorine in water is 3.26g/L
 I tried to use this value to calculate the pH of a saturated solution of chlorine water.
Here's what i did:
Cl2+H2O<--->HClO+HCl
chlorine dissolves according to the above reaction until the solution reaches maximum solubility at 3.26g/L
then calculating the molarity, i did 3.26/71, ie mass concentration/Molar mass of Cl2 which gave~0.0459M
since the saturated solution is almost an equimolar mixture of HCl &HClO, from the reaction 0.0459M of Cl2 will give 0.0459M of HCl, giving 0.0459M of H+
then considering the HClO,
HClO<---->H+ + ClO-
k=[H+][ClO-]/[HClO]
k~3*10^-8
solving for [H+] gives [H+]~ 3.71*10^-5

solving for the pH, i'm getting about 1.34. I feel this value is too low. Is there something i'm missing? Pls i need clarification
 
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Offline cheese (MSW)

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Re: pH of chlorine water
« Reply #1 on: April 29, 2012, 07:36:37 PM »
You have failed to take into account that your first rxn is an equilibrium:
Cl2  +  H2O  ⇋  HOCl  +  H^+  +  Cl^- 
Ka = [HOCl][H^+][Cl^-]/[Cl2]  = 4.2×10^-4   [1]
As you have shown the contribution of the weak acid HClO to the pH can be ignored.
[1] N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. (1997) p 856
(So yes - your pH is a bit low. ;))


Offline Caustikola

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Re:pH of chlorine water
« Reply #2 on: April 29, 2012, 08:52:55 PM »
@cheese.
Thanks a lot:)
although i worked with that equilibrium and i got 1.66 which still appears low to me :(
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Offline cheese (MSW)

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Re: pH of chlorine water
« Reply #3 on: April 30, 2012, 08:21:45 PM »
When I looked at this the first time round I was thinking weak acid, but this equilibrium involves x^3 where x is [H^+].  Solving the equation (successive iterations) does indeed give [H^+] = 0.021 and pH 1.66.  This appears reasonable to me given the concs and that the strong acid HCl is formed.  You are not confusing with “Cl2” in tapwater ~1.0 ppm max conc?  (Incidentally, there is essentially no Cl2 present at the pH of drinking water 6.5-8.5.)
At RT, a saturated solution of chlorine in water contains about two-thirds hydrated dichlorine molecules and one third of the acid mixture. [1] So pH ~1.82 using this different approach.
However: the solubility of Cl2 you’re using appears to be that from Wikipedia, but several other more reliable sources have the solubility at ~7 g L^-1 (~0.1 M) [2] so your pH value will be even lower using this solubility!
[1] G. Raynor-Canham, Descriptive Inorganic Chemistry 2nd ed (2000) p 386
[2]  CAS; 2012 CRC Handbook 5-151 (need to do a conversion);
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html

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