Problem with an experiment with buffer solutions

[frank633]

Hello everyone, after struggling for a good hour to find an answer without success, I decided to come here and ask my question hoping that someone would be able to help me.

 

So, in my biochemistry lab, we had to measure the effect of different factors, such as a change of temperature, ionic strenght and the effect of dilution on a Trizma base buffer adjusted to a pH of 7.5.

 

Now, my problem concerns the dilution. I know that usually, dilution should not affect the pH of a buffer solution because the ratio [A-]/[AH] doesn't change. For the experience, we had to do 3 dilution: 1/2, 1/10 and 1/100.

 

I measured the pH of my deiinized water and it is approximately 5.7 (I then learned that this was caused by dissolved carbon dioxyde in water). As expected, the pH didn't change significantly for the 1/2 and 1/10, but to my surprise, i raised up to 8.04 from 7.47 when measured in my 1/100 dilution and I don't understand why.

 

If my water is acid, I would have expected my pH to drop somewhere between 7.47 and 5.7 if this were to happen.

 

So to put my question in simpler terms, how could the pH of my buffer solution (pH 7.47) raise by 0.57 units of pH when diluted in an acid deionized water?

 

 

Thank you

 

frank633

 

 

[John Cuthber]

That's very odd.

I suspect it is due to some contamination of the glassware somewhere.

One important thing about buffers is that they work less well the more you dilute them.

[frank633]

Thank you, in the meanwhile I spoke with other people who did the same experiment and it happened to some of them too so you are probably right.

[John Cuthber]

A lot of glassware washing detergents are strongly alkaline. If you don't wash them out with lots of water then the traces left behind can upset your results.

The more dilute buffer will be less able to overcome any impurities.

[frank633]

thank you very much