A method for the determination of a film forming amine concentration in a liquid by adding a reagent to the liquid to form a colored complex of the amine and the reagent, wherein the colored complex is measured photometrically, the method comprising the steps of: a) Providing a buffer solution of a weak acid having a pKa ≥ 4.5 and a strong acid having a pKa ≤ 1; b) Diluting an aliquot of the buffer solution with a given volume of water, and determining a pH value of said diluted buffer solution; c) Adding a given amount of said reagent to said diluted buffer solution and measuring an initial absorbance of said diluted buffer solution containing the reagent; d) Preparing a sample solution by adding a given volume of the liquid containing the film forming amine to an aliquot of the buffer solution and measuring a pH value of the sample solution; e) Adjusting the pH value of the sample solution to match with the pH value of the diluted buffer solution by adding a calculated amount of the strong acid, wherein the amount of the strong acid to be added to the sample solution is calculated using a first calibration curve or an algorithm based on the formula: log Ac − = log HAc total / K HAc + pH sample, wherein [Ac -] is the anion concentration of dissociated weak acid in the solution, [HAc] total is the total concentration of the weak acid in the solution, K HAc is the association constant of the weak acid, and pH sample is a measured pH of the solution; and f) Adding a given amount of said reagent to the pH adjusted sample solution to form the colored complex and measuring the absorbance of the pH adjusted sample solution containing the reagent and the colored complex, wherein the concentration of the film forming amine in the liquid is calculated from the measured absorbance of the pH adjusted sample solution containing the reagent and the colored complex using a second calibration curve or an algorithm based on the formula Amine = A sample − A o / f; wherein [Amine] is the concentration of the film forming amine in a sample solution, A sample is the measured absorbance of the sample solution containing the reagent and the colored complex, A o is the measured initial absorbance of the diluted buffer solution containing the reagent, and f is a factor corresponding to the slope of the second calibration curve a) Providing a buffer solution of a weak acid having a pKa ≥ 4.5 and a strong acid having a pKa ≤ 1; b) Diluting an aliquot of the buffer solution with a given volume of water, and determining a pH value of said diluted buffer solution; c) Adding a given amount of said reagent to said diluted buffer solution and measuring an initial absorbance of said diluted buffer solution containing the reagent; d) Preparing a sample solution by adding a given volume of the liquid containing the film forming amine to an aliquot of the buffer solution and measuring a pH value of the sample solution; e) Adjusting the pH value of the sample solution to match with the pH value of the diluted buffer solution by adding a calculated amount of the strong acid, wherein the amount of the strong acid to be added to the sample solution is calculated using a first calibration curve or an algorithm based on the formula: log Ac − = log HAc total / K HAc + pH sample, wherein [Ac -] is the anion concentration of dissociated weak acid in the solution, [HAc] total is the total concentration of the weak acid in the solution, K HAc is the association constant of the weak acid, and pH sample is a measured pH of the solution; and [Ac -] is the anion concentration of dissociated weak acid in the solution, [HAc] total is the total concentration of the weak acid in the solution, K HAc is the association constant of the weak acid, and pH sample is a measured pH of the solution; and f) Adding a given amount of said reagent to the pH adjusted sample solution to form the colored complex and measuring the absorbance of the pH adjusted sample solution containing the reagent and the colored complex, wherein the concentration of the film forming amine in the liquid is calculated from the measured absorbance of the pH adjusted sample solution containing the reagent and the colored complex using a second calibration curve or an algorithm based on the formula Amine = A sample − A o / f; wherein [Amine] is the concentration of the film forming amine in a sample solution, A sample is the measured absorbance of the sample solution containing the reagent and the colored complex, A o is the measured initial absorbance of the diluted buffer solution containing the reagent, and f is a factor corresponding to the slope of the second calibration curve wherein the concentration of the film forming amine in the liquid is calculated from the measured absorbance of the pH adjusted sample solution containing the reagent and the colored complex using a second calibration curve or an algorithm based on the formula Amine = A sample − A o / f; wherein [Amine] is the concentration of the film forming amine in a sample solution, A sample is the measured absorbance of the sample solution containing the reagent and the colored complex, A o is the measured initial absorbance of the diluted buffer solution containing the reagent, and f is a factor corresponding to the slope of the second calibration curve