The effect of NaCl concentration (non-ideality) was investigated on the solubility of FeCO3 layer. After a layer of FeCO3 was formed on a gold coated crystal, NaCl was incrementally added into the solution and the mass change of the FeCO3 layer was measured with an Electrochemical Quartz Crystal Microbalance (EQCM). It was found that the mass of the precipitated FeCO3 layer did not change with increasing NaCl concentration even though the saturation value of FeCO3 (SFeCO3.) was far below 1 and dissolution of FeCO3 was expected. It was hypothesized that the calculation of SFeCO3 was incorrect due to inaccurate equations for dissociation equilibrium constants or solubility product constant (Ksp). Therefore, the equations for dissociation equilibrium constants taken from Oddo & Tomson 1982 and the Ksp equation borrowed from Sun et al. 2009 were revisited. New equations were proposed for carbonic acid first dissociation equilibrium constant (Kca) and Ksp.

The predicted pH and SFeCO3 values at low pressures over a temperature range of 30°C to 80°C and an ionic strength range of 0 to 4.95 M were in good agreement with the experimental results. The new equations could justify the observations for the effect of NaCl concentration on FeCO3 solubility.

Subject
Precipitation (solids), Hydrogen concentration, Iron carbonate, Saturation, Sodium chloride, Dissolution, Solubility, Quartz, Water chemistry, Constants, Sodium chloride solution, Experimental data, Salt concentration
Keywords:
Iron carbonate layer, FeCO3 solubility, non-ideal solutions, ionic strength, salt concentration, water chemistry model
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