This project was designed to improve measurements of the acidity of natural waters. The acidity of solutions in the form of a variable designated as pH is one of the most frequently measured chemical variables in the world. Solution pH provides important, and often essential, information on the rates of important natural chemical reactions and on the relative concentrations of dissolved chemicals in rivers, lakes, estuaries, and the ocean. The pH of the world's natural water bodies is currently decreasing each year due to excess carbon dioxide added to the atmosphere relative to carbon dioxide concentrations in the preindustrial era. Although most measurements of pH are made by using electrodes and recording the voltages produced by these electrodes when they are immersed in natural freshwater or seawater, this project involved pH measurements that are obtained by examining the color of solutions to which very small amounts of pH-responsive indicators have been added. The colors that are developed are measured with instruments (spectrophotometers) that specify color on scales that are very precise and accurate. Using colorimetric measurements, differences in the acidities of seawater samples can be specified within approximately one tenth of one percent. In this project four types of pH-responsive indicators were purified, and the properties of the purified pH indicators were characterized over a broad range of both temperature and solution salt-content. Subsequent to this project's characterizations, the solution pH of natural solutions can be characterized without a need for any additional calibrations. The four different indicators characterized in this project allow for pH measurements of solutions whose acidities range over a factor of one million. Measurements can be obtained over temperatures that range from freezing to the physiological temperature of the human body, and over a range of salt content ranging from lake and river water to the saltiest waters encountered in the ocean. Last Modified: 05/10/2022 Submitted by: Robert H Byrne