Generally speaking, the silica sol used in investment casting is alkaline silica sol with a pH value of 8.5-10. The pH value of silica sol is closely related to the stability of silica sol. From the pH value and gelation curve of silica sol, there are two ranges in which silica sol is relatively stable. One is pH 2-3, and the other is pH 8.5-10. However, the silica sol on the acidic side contains less silanol, and its gelation does not form a three-dimensional structure in the form of siloxane, so it cannot be used as a casting binder. Because the pH value of silica sol is related to the stability of silica sol, one of the maintenance of silica sol coating is the determination of pH value to ensure that the pH value of silica sol is within the specified range. If the pH value of silica sol coating changes slightly, ammonia water or ammonium carbonate solution can be used to make appropriate adjustments. The pH value of silica sol is generally determined using pH test paper. The general frequency is one to two weeks.