[Henderson-Hasselbalch]

My research for my internship involves hydroxylapatite and potential adsorption. Anywho, the other day I was making a diluted solution of KOH from a 0.05 mol L^-1 stock solution. From my calculations, which where checked, the pH should've been around 9.5. However, when I stuck the electrode of the pH meter in the solution it read 7.4! The temperature was around 25^oC and I was using MilliQ water. Why is this? Could CO₂ being dissolved cause such a difference?

PS: I realise I could control the pH with a buffer but in the context of the experiment it would possibly cause interations.

[Borek]

Have you calibrated the electrode?

[Henderson-Hasselbalch]

Have you calibrated the electrode?

I haven't but a PhD student did about a weak ago. I assume that's ok for a pH meter used only a few times each day? It gives pH values for my acidic solutions very close to my calculated pH.

[Borek]

It can be not enough. And if you are measuring 0.05M base pH it should be calibrated using 7 & 10 buffers.

At the same time I did some rough estimates and it seems to me carbon dioxide can be able to lower pH by two units. Try to add a pinch of NaCl to your MilliQ water and measure its pH.

[DevaDevil]

I know from experience that milliQ water has a pH of about 5 due to dissolved CO₂