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Name: Francois Status: other Grade: other Country: South Africa Date: Spring 2013

Question:
I have seen the question about the link (or lack thereof) between conductivity and pH. I was wondering however whether one could say there is a link between conductivity and a change in pH? My thoughts around this considered the situation where you dose with either an acid or base, thereby introducing new ions into the solution which would increase the conductivity. Is this correct or have I missed something? The two reactions I had in mind are:

H3O(+) + NH3 <=> H2O + NH4(+) H2SO4 + 2H2O <=> 2H3O(+) + 2SO4(2-)

In the first reaction, oxonium is being replaced by ammonium - would this affect conductivity? In the second reaction it definitely looks as though more ions have been introduced!

Replies:
Francois,

As the previous response ( http://www.newton.dep.anl.gov/askasci/chem99/chem99571.htm) suggested there is no general trend linking conductivity and pH. conductivity depends on the number of charge carriers in solution, whereas pH only describes the concentration of H(+). Your example already points out that there are too many different situations where the number of ions (and hence, the concentration of charge carriers and conductivity) vary -cannot be trended- from the concentration of H(+). Any relationship of pH to conductivity would depend on each particular reaction or solution system, each case being a little different.

Greg (Roberto Gregorius) Canisius College


The two reactions I had in mind are:

H3O(+) + NH3 <=> H2O + NH4(+)

H2SO4 + 2H2O <=> 2H3O(+) + 2SO4(2-)



In the second reaction it definitely looks as though more ions have been introduced!

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pH = log(1/H+) Now the H+ ion has the largest conductivity of all the ions, and the conductivity is exponential. So any change that decreases the pH (higher concentration of H+) increases the conductivity greatly. You can find a lot of info by searching the word string: ?electrical conductivity of water at various pH values?.

Vince Calder


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