Where uncertainties are available, I determine optimal parameters by minimizing the χ 2 -error (neglecting the uncertainty in the independent variable) via the Python package SciPy 24 and the curve fit method of the optimize module (otherwise minimizing the squared sum of residuals via the same method). Uncertainties could not be reproduced for the CdS data (as some error bars were smaller than the data point). To compare the COP model ( ) to the linear fit, I use an F-test for an additional term. I performed the fit in p ∞ -Q m space and also by taking the logarithms of all variables. The difference between the fits is negligible for the sulfur sol data (see S4), and only the results of the logarithmic fit are discussed in section 4.1, but the fits for the CdS data are compared in section 4.2. The form of the COP model in logarithmic space is, p∞,COP = ln