The ion of potassium, K⁺. There are tests used to monitor high (>5000 mg/L) or low (<5000 mg/L) potassium ion (K⁺) concentration in water-base muds. The test for high
concentration, as specified by the API, is based on the insolubility of potassium perchlorate. A volume of mud filtrate is mixed with an excess of NaClO₄ in a centrifuge tube. White KClO₄ is precipitated by the reaction. After the
tube has spun in a centrifuge to settle the white sediment in the tube, the amount of precipitate is read and compared to a calibration chart that relates sediment to concentration of K⁺ in the filtrate sample. The test to monitor low
potassium ion (K⁺) concentration in water-base muds, as prescribed by API, is a titration procedure using quaternary ammonium salt solution (QAS) as reagent. Potassium ion is first precipitated as the tetraphenylborate (TPB) salt by adding an
excess of sodium tetraphenylborate. After filtering out the solid, the amount of TPB not reacted with K⁺ ion is titrated with a standard QAS solution. The endpoint is purple-to-light blue color change.

The test for high potassium ion
concentration was first applied to drilling fluids by Ron Steiger at Exxon Production Research Co. And has proven to be a reliable way to measure K⁺ ion concentration in a mud at the wellsite, which allows the mud engineer to maintain the
proper level when drilling through hydratable shales. Before a direct test for K⁺ ion was available, KCl in a mud was monitored by chloride analysis. Although simple to perform, this analysis was misleading and counterproductive because after
drilling shale with a new KCl mud for awhile, the beneficial K⁺ ion was consumed by the clays but the Cl^- ion remained in the mud. Eventually, the wellbore shales hydrated, fell into the hole and created severe mud and drilling
problems. As a result of improper analytical methods, early potassium muds earned an undeservedly negative reputation.

For optimal shale stability, K⁺ ion must be continually maintained by adding KCl (or some other K salt) as fast as it
is consumed. The advent of a direct field-worthy method for K⁺ analysis, the result of Dr. Steiger's efforts, was a breakthrough in the use of water mud for drilling troublesome shale.
Reference:
Steiger RP: "Fundamentals and Use
of Potassium/Polymer Drilling Fluids to Minimize Drilling and Completion Problems Associated with Hydratable Clays," Journal of Petroleum Technology 34, no. 8 (August 1982): 1661-1670.