Abstract: This paper describes the method for the preparation and purification of materials used in construction of high precision saturated cadmium sulfate standard cells from general technical grade chemicals. Forty acid cells (0.049 N H₂SO₄) were studied, readings of emf were taken at intervals during the period of 1973 to 1979, the average change in emf is 0.33μVper year.Various factors are briefly discussed in the article. The following processes will take place during the aging period of the cells: the dissolution of alkali constituents from the inner surface of the glass container into the cells and the slight reaction of cadmium in the negative electrode with the acid in the solution, both these two processes will cause the increase of the emf. While another process, the gradual growth of the crystal sizes of the mercurous sulfate, will decrease the emf of the cells. Experimental data are given for these two factors.