How often does Karl Fischer reagent change? How to choose between pyridine free and pyridine containing?

Pyridine-containing reagents are indeed more decisive in endpoint determination and traditionally have better stability, but pyridine waste liquid is troublesome to dispose of and has a strong odor, so we basically stopped using them later. Pyridine-free formulations are now mature in many brands and are generally sufficient for routine moisture testing of lyophilized powders. Accuracy mainly depends on calibration and sample handling, not simply on whether pyridine is present or not. It is recommended to continue using pyridine-free reagents. If the endpoint is unstable, first check the titration vessel, electrode, and desiccant, and test with a new batch of pyridine-free reagent. If the sample has special interference, then consider dedicated aldehyde-ketone reagents or pyridine-containing reagents; pyridine-containing waste liquid must be collected separately as nitrogen-containing organic waste and must not be mixed into general organic waste liquid, and the disposal cost will be somewhat higher.

In terms of stability, Merck and Honeywell's pyridine free reagents have a good reputation, but their prices are relatively high; Domestic products such as Siyou can also be used, and the price of 500ml needs to be quoted on the same day, with a significant difference between imported and domestic products. We freeze dry powder in batches of more than ten batches, with small quantities. When buying 500ml single component, it is easy to absorb moisture after opening the bottle. Later, we changed to small bottle packaging or directly bought smaller specifications, and used sealed bottle caps with drying tubes. If the drift is too large, first confirm the sealing of the titration cup and waste liquid bottle, and replace the molecular sieve. Brand is not the only variable, storage methods have a greater impact. Merck Honeywell、 Domestic four friends can first buy a bottle to test, see the calibration stability and endpoint repeatability, and then decide which one to use for a long time.

We also use the volumetric method, and it is generally not recommended to seal the pyridine free single component separately for more than a month, especially in the south where humidity is high and frequent bottle opening can cause moisture absorption. Instead of memorizing the time, it's better to look at drift and calibration: check the drift value before starting up every day. If it significantly increases compared to opening a new bottle, or if the recovery rate drifts to the edge required by the method using sodium tartrate dihydrate calibration, it should be replaced. The quantity of more than ten batches per week is not large. It is recommended to write the date on the bottle when opening, prepare a drying tube, and immediately seal it tightly after use; If it's almost a month, even if it hasn't been used up, it's recommended to downgrade or replace it. If the drift is large and the endpoint is unstable, first check the sealing of the titration cup and the molecular sieve, it may not necessarily be due to the reagent itself.