Varying Concentration

The experiments with varying concentration were performed with either potassium chloride or sodium chloride as the counter-diffusing electrolyte. Plots of the experimental results are shown in Figs. 2. For both the KCl/KI and the NaCl/KI systems, the data exhibit virtually linear behavior on the semi-logarithmic axes. The apparent linear behavior suggests that these systems behave in a nearly "ideal" manner, and can be characterized accurately by Fick's law in Eqn. 1. This ideal behavior is caused by the nearly equal self-diffusion coefficients for potassium, chloride, and iodide. The corresponding electrical diffusion potential across the sample is relatively small.

Figure 2: Concentration difference $\Delta$ of iodide (filled symbols) for the (a) NaCl/KI and the (b) KCl/KI systems; the time t is scaled by the vessel volumes v₁ and v₂. The lines shown are the result of the linear regression analyses reported in Table 1. The measurement uncertainties were approximately the size of the symbols used.

The magnitude of the slope calculated from the data at each concentration is reported in Table 1. The uncertainties reported are the estimated standard deviations reported by the regression software, and have the corresponding coverage factor [42] k = 1. The slopes for the NaCl/KI systems decreased by 18 % as the iodide concentration increased. The slopes of the KCl/KI systems increased by 9 % as the iodide concentration increased. Therefore, it is possible to observe both decreases and increases in the apparent diffusion coefficient as the concentration increases.

While the differences among slopes, being greater than the estimated uncertainties, suggest that the concentration dependence is statistically significant, for practical use, these variations are minor. Any chemical reactions that might occur within the cementitious matrix can alter the diffusion coefficient, easily overwhelming a 20 % variation in the apparent diffusion coefficient due to variations in the concentration.

Table 1: Absolute value of the slopes calculated from the data for the NaCl/KI and the KCl/KI systems in Figs. 2, regressed to a linear model. The uncertainties shown are the estimated from standard deviations reported by the regression software, with a corresponding coverage factor of k = 1.
Species	concentration (mol/L)	|Slope| (cm3/h)
NaCl/KI	0.010/0.010	0.2365±0.0051
	0.100/0.100	0.2120±0.0024
	1.024/1.000	0.1931±0.0059
KCl/KI	0.010/0.010	0.1923±0.0039
	0.100/0.100	0.1978±0.0029
	1.000/1.000	0.2101±0.0024