A method for preparing an aqueous solution of a defined pH comprising an acid, a base and optionally one or more additives is provided. The method comprises the steps of:
a) calculating the theoretical concentrations of acid and base for the solution to have the defined pH using the Henderson-Hasselbach equation in combination with the Debye Huckel theory for a range of different additive concentrations; b) preparing a sample of the buffer for the range of additive concentrations and measuring the actual pH for each additive concentration; c) calculating a value for delta pH, ΔpH, being the difference between the theoretical pH and the actual pH, for each additive concentration; d) generating a mathematical model describing the relationship of ΔpH with additive concentration; e) selecting the defined pH and additive concentrations; f) using the mathematical model generated in step d) to calculate ΔpH for the defined pH and additive concentration; g) calculating a ΔpH-corrected pH by summing the defined pH and delta pH; h) using the ΔpH-corrected pH to calculate the concentrations of acid and base using the Henderson-Hasselbach equation in combination with the Debye Huckel theory; i) preparing the solution using the concentrations calculated in step h).