Calculation of chloride concentration at color change boundary of AgNO3 colorimetric measurement

AgNO₃ colorimetric measurement of chloride in cement-based materials can be implemented owing to chemical reactions between Ag⁺ and Cl⁻ or OH⁻ ions, which result in the precipitation of AgCl and Ag₂O. When the mass ratio of AgCl to precipitated AgCl + Ag₂O reaches a critical value (P_crit-AgCl), a color change boundary appears. In this paper, chloride concentrations required for the formation of the critical mass ratio of AgCl to precipitated AgCl + Ag₂O derived from the AgNO₃–NaCl–NaOH system were calculated based on the solubility products of silver hydroxide and silver chloride, and mass conservation of the chemical reactions. P_crit-AgCl was determined as 0.65 through a series of AgNO₃ colorimetric measurements on cement-based materials. Calculated results indicated that if Ag⁺ could react with all OH⁻ and Cl⁻, there existed a linear relationship between the critical chloride concentration for color change (C_crit-Cl^–) and OH⁻ concentration (C_OH^–), namely C_crit-Cl^– = 1.6C_OH^–. If Ag⁺ could react with only partial OH⁻ and Cl⁻, there existed an exponential relationship among C_crit-Cl^– and C_OH^–, Ag⁺ concentration (C_Ag⁺), volume of AgNO₃ solution (V_Ag⁺) as well as the volume of NaCl + NaOH solution (V_{OH^–+ Cl^–}), namely C_crit-Cl^– = 0.00695C_OH^– + 0.608C_Ag⁺V_Ag⁺/V_{OH^–+ Cl^–}. It can be concluded that the alkalinity of cement-based materials, concentration and volume of sprayed AgNO₃ solution and volume of pore solution have effects on chloride concentration at the color change boundary (C_d). Calculated C_d values ranged between 0.03 and 0.96 mol/L (in pore solution) or 0.011% to 2.27% (by the mass of binder), respectively. This explains why reported range of chloride concentration at the color change boundary in publications varied in a broad range.