Dissolve approximately 0,1 % by mass of analytical reagent grade anhydrous ammonium
chloride (NH4Cl), of a purity of not less than 99,8 %, in de-ionized water, having a conductivity
of not greater than 1 mS/m to give a resistivity of (3,95 ± 0,05) Ωm at (23 ± 1) °C.