Varying Speciation

The experiments with varying speciation were performed with KCl, NaCl, KOH, K₂CO₃, CaCl₂, H₂SO₄, and MgSO₄ as the counter-diffusing electrolyte. In all cases, the initial concentration of KI was 0.10 mol/L. The concentration of each counter-diffusing electrolyte was varied to eliminate the osmotic pressure gradient across the ceramic specimen; it is not asserted that the values used here were ideal.

Table 2:Magnitude of the slope for data shown in Fig. 3(a), regressed to a linear model for t(v1−1+v2−1) < 8 h/cm3. For each system, the KI concentration was 0.10 mol/L. Also shown are the values for the formation factor F used to produce the solid curves Fig. 3(b). The uncertainties shown are the estimated from standard deviations reported by the regression software, with a corresponding coverage factor of k = 1.
Counter Solution	concentration (mol/L)	|Slope| (cm3/h)	F
K2CO3	0.0769	0.1581±0.0014	11.7
H2SO4	0.0859	0.1688±0.0075	9.6
MgSO4	0.1660	0.1867±0.0022	13.5
KCl	0.1000	0.1978±0.0029	11.4
NaCl	0.1000	0.2120±0.0024	11.8
KOH	0.0879	0.2333±0.0034	11.2
CaCl2	0.0731	0.2801±0.0089	12.8

The relative concentration differences Δ/Δ₀ are shown in Figs. 3, and the analyses of the data are divided between the "short" time and the "long" time results. In Fig. 3(a), a linear model is regressed to the data for which t(v₁⁻¹+v₂⁻¹) < 8 h/cm³. The results from the linear regression analyses are shown in Table 2, and the corresponding lines are shown in Fig. 3(a). The uncertainties reported are the estimated standard deviations reported by the regression software, and have the corresponding coverage factor [42] k = 1. Based on these results, the CaCl₂/KI system and the K₂CO₃/KI system represent the extrema, and are shown in Fig. 3(b) over a longer time, along with the KCl/KI system as an example of nearly ideal behavior.

The results from the linear regression given in Table 2 give an indication of the effect speciation can have on the observed diffusion coefficient. The magnitude of the slopes shown in the table varies by nearly a factor of two. This contrasts the previous 20 % change in the slopes due to changes in concentration.

Figure 3: Concentration difference Δ of iodide (filled symbols) as a function of scaled time. In (a) results from linear regression (solid lines) for t(v₁ ^-1+v₂⁻¹) < 8 h/cm³; the calculated slopes are reported in Table 2. In (b) the electro-diffusion equation (solid lines) is used to estimate the formation factor F at long times; the calculated values of F are shown in Table 2. The error bars represent an estimated standard deviation.