1. Clark, W. M. (1928). The determination of hydrogen ions. 3rd ed. Lond.
2. Findlay, A. (1933). Introduction to physical chemistry. Lond.
3. Kolthoff, I. M. (1926). Indicators, p. 13. Lond.
4. Schmidt, J. (1932). A textbook of organic chemistry. (English ed.). Lond.
5. Sidgwick, N. V. (1927). The electronic theory of valency. Lond.