As a general rule, the potassium salt of an organic acid is usually quite water soluble, but the common food additive commonly known as Cream of Tartar or Potassium Hydrogen Tartrate is a relatively insoluble amphipathic salt. However both the full double salt called Rochelle Salt, Sodium Potassium Tartrate, and the full acid, Tartaric acid, are both quite water soluble. Titration of Rochelle Salt with a strong acid such as HCl leads first to the formation of a precipitate as the tartrate ion is initially protonated. Further addition of acid causes the precipitate to gradually redissolve. The entire titration involves the the interplay of Ka<sub>1</sub> and Ka<sub>2</sub> with Ksp. These overlapping and competing equilibrium values make for a very interesting analysis involving spreadsheets for an undergraduate Analytical Chemistry or Quantitative Analysis lab course (or an ambitious General Chemistry II lab). Students should be able to calculate values for each of the three equilibrium constants and recover Cream of Tartar as a precipate if they stop at the half-way point of a second titration of Rochelle Salt.

Peculiar equilibrium of Rochelle salt, cream of tartar and tartaric acid in demonstrating acid-base equilibrium and Ksp

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