Spent another day at DNR in Olympia running water samples. Hollie suggested I try drawing acid directly from the stock container to remove the chance of changing acid concentration by mixing old acid and new acid in the titrator reservoir. I did this, as well as mixing the acid bottle before each sample run and still found a frustrating amount of variation in CRM samples. Micah was available to watch my technique and we decided it did not appear to be caused by operator error.
We re-checked the probe calibration and found that it had drifted 5mV at 3.5pH in the week since it was last checked, so we adjusted the titrator script and ran a set of CRMs in the middle of the day and found better consistency with results, but they were still reading higher than intended. We decided that in the future checking the probe calibration at the beginning of each processing day would be a good move.
After thinking of other potential reasons for consistently high CRM readings, we checked the calibration of the balance used to weigh out samples, thinking a higher than expected mass reading could lead to higher than expected TA readings, but after finding a reference weight in the nearby geology lab, we found that the scale was accurate within 0.01g, so that wasn’t the culprit. The only other thing we could think of, was potentially the acid burette was dispensing an incorrect volume of acid. Micah said that he would contact Mettler-Toledo regarding potential ways to test this/ways to calibrate it, and will go from there!