The Pierce Borate buffer sold by Thermo Fisher:
20X Borate Buffer is ideal for preparing sodium borate buffer solutions for use in protein modification procedures requiring amine-free buffer at an alkaline pH. 20X Borate Buffer makes 50 mM borate at pH 8.5 when diluted to 1X with water.
Did so using fresh 18.2 Megohm water and get a pH of 8.9 +/-0.5 using several independently calibrated pH meters with glass electrodes, fresh calibrators.... The pH paper suggests 8.5, so does my Isfet pocket pH meter using the same calibrators
Any suggestions on what I could do wrong?
Does the product specifications sheet clearly state that x20=1M? Perhaps the concentration is different, meaning that the buffer is less susceptible to pH changes from water (which also contains hydronium ions aka protons).
You can always complain to Thermo, but perhaps it would be quicker to titrate your own buffer using boric acid and NaOH.
looks like its an artifact of borate buffers that they respond different to dilution when measured in different measurement systems, particularly glass electrodes. It could be a consequence of low conductivity. Boric acid / sodium tetraborate has an osmolarity of 61 or 31 for a 50 of 20mM buffer. Sodium-tetraborate / HCL comes in at 151mM and seems to have less of a discrepancy. Boric Acid NaOH however seems to show a similar problem overestimating pH. I made one up according to www.aatbio.com/resources/buffer-preparations-and-recipes/sodium-borate-buffer-ph-8-5 but end up with pH 8 on the glass electrode and 8.3 on the ISFET and the pH paper, but 8.8 when diluted on the glass electrode and 8.5 on the ISFET when diluted 1:50 to 20mM. Any ideas welcome
found a brilliant website to makeup buffers at
https://www.cusabio.com/m-296.html
which gives you the amount of salts to weigh in to achieve a given molarity and pH. Tried it for 1M and 0.1M with sodium tetra-borate-decahydrate /boric acid and got the same problem that the pH shifted down by 0.5pH units when using the higher molarity. Interrestingly, the pH paper stays constant, so who should I trust?
When adjusting the buffer diluted from the pH 8.5 stocksolution (pH 9 to 8.5 using HCl) the pH paper gives a pH of 8.0, thus 0.5 pH units lower than the electrodes. Hope someone has a chance to reproduce this effect or disprove it.
as of today, 3month down the line I still await an answer from Fisher. Just chased the email to see if they are still alive :-)
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