Abstract: In this paper,an error equation of single increment method has been derived as follows: ε=-2.3/S(10^ΔE/s)/(10^ΔE/s-1)Δ(ΔE)According to this equation,the optimum conditions of method are : the difference of potentials ΔE should be larger than about 40 mV (for mono-valent ions) or about 30 mV(divalent ions),and the volume of standard solution should not be over 10% of sample solution.