Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Jenn478 on October 31, 2005, 08:46:10 PM
-
Please help me!! Can you explain how increasing the amount of salt in a solution of water decreases the pH? I added 0.67g of NaCl to 100g of tap water and had a pH of 7.53, I added 0.90g of NaCl to 100g tap water and then got a pH of 7.33. What is the mechanism behind the salt lowering the pH? Thank you so much!!!
-
This is an effect of activity coefficients and ionic strength. Generally, at modestly increasing salt concentrations the activity coefficients decrease. I say modestly because this does not generally hold in brine solutions. By addiing additional salt to your solution you have increased the ionic strength. The pH is formally defined as the negative log of the hydrogen ion activity. The activity is calculated by mulitplying the activity coefficient "gamma" times the hydrogen ion concentration. Therefore increasing salt, increased the ionic strength, decreased the activity coefficient of the proton and your measured pH is therefore less. Of course you also run into other problems if you were measuring with a glass electrode, things like junction potentials etc but this is way beyond the scope of your question. For a further discussion of activity coefficients, ionic strength, I would suggest looking up the Davies equation for activity coefficients. These concepts are all easily google-able and available in Harris' text on Quantitative Chemical Analysis. Hope this was helpful.
-
http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients (http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients)
Also note that while NaOH is a strong base, Na+ ions get complexed by OH- which gives the same effect as hydrolysis of weak base (pKb=0.2), shifting pH slightly dwon. Read FAQ at chembuddy site.
-
Contrary to some of these posts , I believe your observations have more to due to electrical conductivity than the actual pH as determined using a standard hydrogen electrode. Water is actually not a very good conductor, so simply putting an electrode into the water will give erroneous readings. Adding enough salt, NaCl, will alleviate this problem. I'm traveling an don't have texts at hand, but in searching the internet I've not found any credible information to the contrary. I am a PhD chemist. Could be wrong, but the affect would have to be miniscule to unmeasurable. IMHO.
-
I believe your observations have more to due to electrical conductivity than the actual pH as determined using a standard hydrogen electrode
Would be definitely a good point if it wasn't a tap water we are talking about. In purified water lack of conductivity can be a problem during measurements, but tap water already contains enough ions.
-
I hope Jenn478 has solved the problem by now (2018).
How stable is a pH around 7? How sensitive would it be to any impurity in NaCl or elsewhere?
We're speaking of an acidity change like 10-8 mol/L after adding 10-1 mol/L of salt.
-
We're speaking of an acidity change like 10-8 mol/L after adding 10-1 mol/L of salt.
Tap water is typically buffered a bit (at least by carbonates).
-
In purified water lack of conductivity can be a problem during measurements,
Manifested as what?
-
In purified water lack of conductivity can be a problem during measurements,
Manifested as what?
To be honest I don't know, as I never tried it in person. But, many years ago a friend of mine had to work with 18 MΩ water and he told me he was unable to measure pH. He consulted the supplier and was told it is completely normal with UPW. Potential measurements depend on some minute charge transfers and if you put a huge resistance in series things get awry. I can try to ask him what was exactly happening.
-
To be honest I don't know, as I never tried it in person. But, many years ago a friend of mine had to work with 18 MΩ water and he told me he was unable to measure pH. He consulted the supplier and was told it is completely normal with UPW. Potential measurements depend on some minute charge transfers and if you put a huge resistance in series things get awry. I can try to ask him what was exactly happening.
Just was curious. Recently I've been doing some pH measurements and using regular deionized water without too much difficulty. Occasionally the pH meter does some wonky things (takes a long time to reach equilibrium, etc.) but nothing too bad. Ofc, there is CO2 dissolved, and thus formation of carbonic acid, so maybe that's enough conductivity. I haven't tried using the ultrapurified stuff.