I'm attempting to determine biochar surface functionality via Boehm titration following Fidel et al. 2013 (10.2134/jeq2013.07.0285), where they call for an acid wash (0.05 M HCl) to remove soluble alkalys and a subsequent rinsing with DIW to EC < 10 uS/cm, at which point rinsate pH should be ~5. This, they mention, is important as it sets the lower pKa value of functional groups measured by the technique and, they continue, those with pKa values lower than 5 are thus NOT accurately measured.

My problem lies in the fact that, upon rinsing with DIW to EC < 10 uS/cm, I consistently get pH values of ~9 in the rinsate. Thus, from my understanding, samples reacted with NaHCO3 and Na2CO3 would give little information, if any, on functional groups from the lower pKa range (5-10).

A solution I contemplated was rinsing with increasingly diluted HCl but it invariably raises EC above the target value.

Any orientation regarding the indispensability of the rinsate's pH being ~5 or other strategies to raise it while maintaining the target EC would be very appreciated.