pH changes in Ca(OH) 2 solutions have been studied using a low sodium selective glass electrode.
Successive additions of MgCI 2 and MgSO 4 both lower pH whereas Na2SO 4 contamination leads to an increase in pH and NaCI causes no pH change at all. The amount of salt addition required to change pH alters in a manner determined by the amount of excess solid Ca(OH) 2 present in solution.
Vol. 17, No. 5 C.J. Newton and J.M. Sykes measured drop in pH were suggested by Gjorv and Vennesland (I0):
a) The glass electrodes used to monitor pH showed significant sodium ion sensitivity.
b) Exhaustion of excess Ca(OH) 2 by carbonation allowed pH to decrease from 12.6 when NaCI additions were made in the presence of CO 2.
However, subsequent workers have restated that the pH of sat. Ca(OH) 2 solution is significantly depressed by NaCI contamination (3, II, 12). Hausmann and Azlz and Mansur have speculated loss in pl{ may be influenced by the common ion effect between [Ca 2+] and [Na +] in aqueous solutions and that this pH change is important in locallsed corrosion of steel in concrete (7,11,12).
Glass electrodes give a rapid estimation of [H +] ion concentration by a simple measurement of potential in the test solution against a standard reference electrode. This method relies upon the linear (Nernstian) potential response of the glass electrode over a calibrated range of pH. However, this technique has fallen into disfavour when testing solutions of high alkalinity because the calibration of many glass electrodes becomes non-llnear at high pH values ( 14). Problems may arise from three sources: a) Difficulty in preparing and preserving standard solutions for calibration.
b) Enhanced sodium ion selectivity of glass electrodes (15).