I am having issues with pH test strips. I have two different pH probes and bromethyl blue indicator that tell me my solution is neutral (7.1) but every time the pH strips read closer to 6.
Does anyone know what could cause the strips to read more acidic than the solution actually is? The strips are not expired and I trust the bromethyl blue indicator along with the two pH probes are trustworthy (they all agree). My solution contains fluoride ions if that matters?
It could be that the solution is too dilute to overwhelm the buffering capacity of the indicator itself.
I suppose it is possible that the reagents in your solution chemically react with litmus and change its nature and ability to ionise or has some bleaching action like hydrogen peroxide. What is the solution that you are measuring?
The solution is from a waste stream that contains mostly KOH/DMSO/Nitric acid. I know there is also some fluoride ions too.
Adam- Your suggestion is interesting. The solution is dilute, but I don't see how that would affect its buffering ability. Could you explain it a little further?
The pH indicator itself can act as a buffer. If the solution being tested is very dilute, the buffering capacity of the indicator could resist a slight pH change due to the solution. However, I think another explanation is the DMSO. pH paper is meant for testing aqueous solutions and may be thrown off by organics if the dielectric constant of the solution is significantly different from water, because that changes the pKa of the indicator.
if your test solutios are very dilute and you are reading the colour change out of the solution in air then absorption of CO2 from the air will cause the wetting solution on the paper to contain H2CO3 which may have more effect in terms of acidity than your test solution
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