The Solubility of CaS04: An Ion Exchange-Complexometric Freshman Laboratory
Titration Experiment for the
Several years ago we noticed an article in this Jdurnoll entitled, "A New Twist to K , Evaluation." This article deals with the determination of the ''K.," of various salt d u t i o n ~by passing saturated solutions of the salts through cation exchange resins and titrating the resulting acid produced. We have incorporated a version of this experiment into our general chemistry laboratory using saturated CaSOl solutions. The results obtained by this procedure are always excellent; however, this can not always be said for their interpretation. The problem is that, once the student determines the amount of sulfuric acid produced and hence the original concentration of calcium in solution, the.tendency is to assume that the square of this value is equal to the K,, of CaSOa. (This is even implied to be the case in the article previously referred to above.) However;the value the student obtains, (2.5 i 0.1) X lo-', does not correspond to the K,,, value reported in the students' text which usually is 2.4 X The natural tendency is to assume a misprint, especially when it is pointed out that the solubility of CaS04 given in The International Critical Tables is 0.01537 rn a t 2 5 T , which would also appear to yield a K,, value close to 2.5 X l O W . At this point we suggest that, as a final cheek, the student determine the calcium concentration by an alternative method. For this we have chosen a complexometrie method using EDTA with hydrory napthol blue2 as the indicator. This method, if carried out a t a p H of 12.5 t o 13.0, yields calcium concentrations in excellent agreement with those obtained previously with the ion exchange procedure. Obtaining the same answer by two independent techniques is always gratifying and the student will now he more convinced than ever that the K,, value listed in his text is in error. This is the opportune time to refer the student t o the article by Meites e t al;? entitled "Are Solubilities and Solubility Products Related" or to present one's own lecture on the subject of ion pair formation and how it effects solubilities and K,'s. This lab may he used to introduce the student to two important areas of chemistry with the added advantage that one method serves as a direct cheek on the other. If standardized NaOH and EDTA solutions are provided, our experience has been that this entire laboratory may easily he completed in one 2-3 hr period.
' Hazel, J. F., J. CHEM. EDUC., 43,636 (1966). 2-Disodium salt of 1-(2-Naphthol szo-3,6-disulfonic acid)-2-naphthol-4-sulfonic acid. Available from Mallinckrodt, AR6630. Meitei, L., Pode, J. S. F., and Thomas, H. C., J. CHEM. EDUC., 43,667 (1966). ~~~~~~~
U S . Naval Academy Annapolis, Mal.yland 21402
254 / Journal of ChemicalEducation
Edward Koubek
