I was using a bucket to temporarily store alkalinity solution (baked sodium bicarb though the weight at the end was 430 instead of 375 So I assume it didn't completely react). The pH of this solution was 9.9. I then poured this into another container and briefly washed out the bucket and then put new RODI water ( like 3 gallons) in it. There was some residue/precipitation on the sides which I figured would not be potent enough to cause issues. The pH according to a recently calibrated pinpoint marine pH checker is 10.3. How is this possible? Is it possible that the precipitate was purer carbonate and this is influencing the pH even if it is much more dilute?