The effect of ammonia on the redox reaction of cobalt(II) with potassium ferricyanide was studied potentiometrically as a function of pH, the concentration of ammonia and concentration of cobalt using as platinum as an indicator electrode and saturated calomel electrode as a reference electrode. In the presence of ammonia the formal potential of the Co(III)/Co(II) system is lower than the [Fe(CN)6]3-/ [Fe(CN)6]4- couple at a pH of 9.0 – 11.0, and hence it is possible to oxidize it quantitatively with potassium ferricyanide which is not possible otherwise. Thus, the addition of complexing agent increases the sharpness of the endpoint of titration with ferricyanide. The precision and accuracy of the method have been assessed by the application of lack – of- fit test and other statistical methods. Overall mean recovery and the mean standard analytical errors obtained were 99.81% and 0.0084 respectively. The interference study was carried out to check the possible interference of other metal ions.