I predominately work with 1% HCl in methanol for extraction of plant phenolics. How can I measure the pH of this? Is pH only for aqueous solutions? What can
The units of pH are moles H+ per liter and the 1%HCl units need to be converted into moles per liter. One mole H+ per liter [in this case it include methanol in units of liter] has a pH of 0.0. In your case pH would most likely be negative. pH buffers for calibrating pH measurements are 4.0 and 7.0, and they need to be 3 pH units from the expected pH. Thus rarely is pH for negative pH values a measured quantity.
Also commercial HCl contains about 37% HCl and 63% water, so 1% HCl (v/v) would equal only 0.37% HCl, not 1% HCl.
pH is merely -log H+ concentration. It doesn't matter what the solution is. The issue is calculating the H+ concentration without knowing the dissociation constant for the proton from its conjugate base. As Kissinger said, you can measure this directly using a pH meter, but there will be a bias error due to the higher diffusivity of H+ in methanol versus water. pH meters depend on the rate of H+ diffusion through a porous glass cover between the solution and the electrode (effectively measuring the current produced). There can be a similar solvent effect on the color of the pH indicator in the strip since the pH indicator depends on its own dissociation constant, which will differ in non-aqueous solutions. Bottom line is that there is no way to know with absolute precision what the effective pH of your methanolic HCl solution is, unless you have an indicator that with a known dissociation constant in that solution. Since MeOH has similar H-bonding properties to water, you might assume that the H+ and Cl- are fully dissociated (probably ture for dilute solutions) and calculate the pH from the effective concentration.
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