The response time to the same type of pH standard solution is usually 10-15 min (A), e.g. from the secondary phthalate pH standard solution (pH 4.008) to the primary phthalate pH standard solution (pH 4.008). The response time in the case of the second-point calibration is usually 20-30 min (B), e.g. the primary oxalate pH standard solution (pH 1.679) to the primary phthalate pH standard solution (pH 4.008).