I did a grade 12 high school experiment on the effect of pH/concentration of HCl on the rate of hydrolysis/ rate constant of aspirin hydrolysis, my results showed that for 0.1M HCl (corresponding to a pH of 1.7 at 37oC) the rate of hydrolysis was slower than in a 0.01M (corresponding to a pH 2.5 at 37oC) HCl solution. I thought that the more acidic/lower the pH the faster the rate of hydrolysis, I am not sure why I got these results. Any explanation or good resources to look at to understand this would be greatly appreciated.
Thank you
First reproduce the experiment at 25 oC (see the link below). The pH of maximum stability is ~pH2:
https://www.bing.com/images/search?view=detailV2&ccid=%2bMxhbjLQ&id=2B3FDE4B34BEEF9CA0F0DE0B6544A54456133339&thid=OIP.-MxhbjLQoROfZnkvEUwS2gHaLH&mediaurl=https%3a%2f%2fimage.slidesharecdn.com%2fdrugstabilityforpharmaceuticalscientists-170317070449%2f95%2fdrug-stability-for-pharmaceutical-scientists-68-638.jpg%3fcb%3d1489734480&exph=957&expw=638&q=images+aspirin+ph-rate+profile&simid=608005344600591544&selectedIndex=8&ajaxhist=0
Recalibrate the pH meter; 0.1 M HCl is pH 1 and 0.01 M HCl at 25 C corresponds to pH 2. Work in diluted solutions (i.e., aspirin concentration
Andrei Blasko Thank you for your response. Unfortunately I am unable to repeat my experiment, however this is not a major issue as it's only a grade 12 science report and the results are not expected to be perfect.
I have indeed seen this image and my results (although there are many errors including the pH meter) do kind of show a similar trend to this image.
My problem is that I am unsure of why this trend occurs and the mechanism behind it - I am not very familiar with the complex specific acid catalysis mechanism - however I thought that assuming there is enough water then the lower the pH the faster the rate of hydrolysis. I do not understand why that's not the case.
If you know anything else about this, I'd be so appreciative of your reply.
If the pH meter is calibrated with old buffer standards, the whole pH scale is shifted and the rates will correspond to the wrong pH: pH 1 will be 1.5-2 and pH 2 will be 2.5-3 and this the difference in rates.
Andrei Blasko I was wondering if you have any knowledge on the acid catalysed ester hydrolysis mechanism? and how pKa affects the rate?
The bigger the difference in pKa's of the acid and base comprising the ester, the more stable the ester is to hydrolysis. On the other hand, the strength of the acid [H+], used in the hydrolysis has a big influence of the catalysis.
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