Simply put, drift is background moisture that the Karl Fisher titrator is detecting. What is background moisture? Well, it is moisture that the Karl Fisher titrator (specifically the detector electrode) is detecting inside the vessel -that’s not coming from your sample. Drift or "background moisture" can be the result of having the titration vessel sitting idle for some time where moisture has slowly infiltrated and accumulated inside the vessel, or it may be the result of a leak that is allowing a small amount of moisture to enter the vessel continually. Although we might like to think that the Karl Fisher titrator vessel is air-tight/moisture-tight, it is not. Depending on how well the vessel is sealed there may be a little or there may be a lot of background moisture interference. All Karl Fisher titrators deal with the drift issue. Unfortunately drift cannot be completely eliminated but the good news is that it can be reduced, measured, isolated, and discarded from your test results.
Before a single test is run on a Karl Fisher titrator it must go into a “ready” mode. But before the titrator can go into a “ready” mode it most likely will go through a “pre-titration” mode. During the “pre-titration” mode excess drift (moisture) is detected and removed by the reagent inside the vessel. A “ready” mode ideally will occur when the drift being measured is low and steady/stable – usually below .1 micro grams per second. Once the drift becomes low and stable the Karl Fisher Titrator records the drift level and goes into a “ready” mode and will allow the operator to introduce a sample into the vessel. Upon completion of the test the Karl Fisher titrator adds up all of the moisture detected over the duration of the test and subtracts out the known drift level that was also measured during the test. This process of knowing what the drift was before the test allows the Karl Fisher Titrator to then determine and backout the drift -leaving only the moisture detected from the sample as your result.