Schiff's Reagent Glycol Test - Controlling False Positives

The Fluid Life Corporation is a commercial oil analysis laboratory situated in Edmonton, Alberta Canada. As with most laboratories, they have run across samples where the common Schiff’s Reagent Test for glycol (ASTM D2982), to detect antifreeze contamination, produces false positive readings. The standard explanation for the inaccurate readings is that aldehydes present in the new oil, perhaps from solvent refining, react with the Schiff’s reagent. When makeup fluid is added to the used crankcase oil in normal service the aldehydes it may carry are added to the blend producing the false reading. Here is a quick and simple way to rule out false positives when conducting the Schiffs Reagent Test:
ASTM D2982 requires that the glycol first be oxidized in the oil using a solution of HCl and HIO3. The process produces an aldehyde which reacts with the Schiff’s reagent, yielding a positive color change (see Figure 1). However, by conducting the test twice the risk of a false positive can be eliminated. Before following the standard procedure, first use the Schiff’s reagent without going through the preliminary oxidation step. Any oil that contains an aldehyde from another source (such as new oil) will show a color change. Oil that really does contain glycol from antifreeze will not produce a color change before oxidizing the glycol. This is obviously quick and simple and is practical in many instances.

There is one caveat worth mentioning. If the oil is contaminated with any other chemical which, when oxidized using this procedure, forms an aldehyde, it will still give a false glycol positive. A customer of The Fluid Life Corporation coming from the railroad industry has reported such a case. They have, on occasion, observed a false positive reading from a crankcase sample where no makeup oil was added.

Figure 1