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 (Pcrit-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. Pcrit-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 (Ccrit-Cl–) and OH− concentration (COH–), namely Ccrit-Cl– = 1.6COH–. If Ag+ could react with only partial OH− and Cl−, there existed an exponential relationship among Ccrit-Cl– and COH–, Ag+ concentration (CAg+), volume of AgNO3 solution (VAg+) as well as the volume of NaCl + NaOH solution (VOH–+ Cl–), namely Ccrit-Cl– = 0.00695COH– + 0.608CAg+VAg+/VOH–+ 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 (Cd). Calculated Cd 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.