 Lemons that were not saleable as produce were made into pectin or citric acid, with sulfur dioxide used as a preservative.Sunkist needed to know the acidity of the product at any given time, and the colorimetric methods then in use, such as readings from litmus paper, did not work well because sulfur dioxide interfered with them.Chemist Glen Joseph at Sunkist was attempting to measure the hydrogen-ion concentration in lemon juice electrochemically, but sulfur dioxide damaged hydrogen electrodes, and non-reactive glass electrodes produced weak signals and were fragile.