There’s an established protocol for measuring the pH of solid soap. You make a 1% soap solution in hot water and then measure the pH. Just let the piece of soap sit in hot water until it completely dissolves.
Maybe the people who are prone to develop eczema should just apply a light lotion after showering/bathing, since those are closer to our skin’s pH than the soap.
I wonder if in the DT article the hot water was exacerbating the peoples’ sensitivity to soap’s pH?