Method of tracing a well drilling mud

A method of tracing a well drilling mud by determining the concentration of a tracer ion in samples of filtrate from a drilling mud which contains a predetermined concentration of the tracer ion, wherein the tracer ion is the acetate ion, CH.sub.3 COO.sup.-, and is present in the drilling mud in a concentration of from about 1.times.10.sup.-3 to about 5.times.10.sup.-2 ions per liter;

• said method comprising:
• providing said filtrate from a drilling mud which contains said tracer ion;
• determining the acetate ion concentration in the filtrate sample by treating said filtrate with a solution of a strong acid selected from the group consisting of hydrochloric acid and sulphuric acid and distilling the acetic acid thereby liberated at a temperature of from about 105.degree. to about 130.degree. C.;
• and
• determining the strong acidity of the distillate by adding thereto an N solution of a strong base until the pH of the resulting solution is indicated by an indicator to be about 4.2-4.3, and then determining the weak acidity representing the tracer ion concentration by adding a volume V.sub.1 of a base solution until the pH of the resulting solution is indicated by an indicator to be about 8.3.