Two new standards for the pH scale

Abstract

Two new primary standards for the NBS pH scale, supple me nting the fiv e already available, are proposed. The new refere nce solution s are the 0.05 molal soluti on of potassium dihydrogen citrate and a mixture of sod ium bi carbonate and sodium carbonate, each 0.025 molal. The ci trate so lution has a pH of 3.776 at 25°C and is more stable und er ce rtain conditions th an either the tartrate or phthalate standards. The carbona te solution has a pH of 10.012 at 25 0c. It therefore extend s the NBS pH scale above its present upper limit of 9.180 at thi s temperature. Reference values pH(S), based on emf meas-uremen ts of cell s with hydroge n electrodes and silver-silver chloride electrodes, have been assigned to these two sta ndard solutions at 11 temperatures from 0 to 50°C.