Question regarding the drop test for Borates

[flynwill]

The last few times I've check the borate level using the test developed by @JoyfulNoise I've noticed that after adding enough R-0009 to get straw yellow, and then adding R-0010 it usually takes two drops to get the mixture to "baby blue", but the if it is left stirring for a few seconds (about how long it takes to fish the bottle of mannitol powder out of my kit) the solution turns back to a pale green. An additional drop of R-0010 will return it baby blue and this time the change sticks. Same effect happens sometimes after adding the mannitol. For consistency I've been adding that extra drop and counting it in the measurement, but not sure if that is the best practice.

Any thoughts of what's happening? CO2 being absorbed back from the air?

 

[JoyfulNoise]

The last few times I've check the borate level using the test developed by @JoyfulNoise I've noticed that after adding enough R-0009 to get straw yellow, and then adding R-0010 it usually takes two drops to get the mixture to "baby blue", but the if it is left stirring for a few seconds (about how long it takes to fish the bottle of mannitol powder out of my kit) the solution turns back to a pale green. An additional drop of R-0010 will return it baby blue and this time the change sticks. Same effect happens sometimes after adding the mannitol. For consistency I've been adding that extra drop and counting it in the measurement, but not sure if that is the best practice.

Any thoughts of what's happening? CO2 being absorbed back from the air?

pH indicators near their equivalence points can be very sensitive to minute changes in hydrogen ion concentration. Your approach is fine as you want to make sure your solution is in the blue region. Unfortunately the R-0010 is very concentrated and so it doesn’t offer finer control over adjusting pH. It’s fine for titrating but it’s not great for making precise adjustments to pH.

I’ve seen this same drifting effect with phenol red pH indicator when water pH is near the neutral-to-alkaline region (yellow to red). I’ve added drops and mixed and watched as the red color slowly developed a more orange tinge from the yellow. It’s just the normal equilibrium process … some chemical reactions are fast, others are slower and others are affected by secondary reactions (eg, oxidation of DPD indicator by air making the clear solutions drift back to pink if you wait too long to measure CCs).