Abstract: A system for measuring acid concentration in an alkylation process by repeatedly measuring the density, viscosity or velocity of sound through a sample over time as volatile organic compounds (VOCs) are allowed to escape and the sample stratifies. A partial vacuum can be used to accelerate dissipation of light VOCs from the sample. A processor applies a predetermined function to a series of these sound velocity measurements to compute the acid concentration in the sample. The function can be determined by statistical regression against sound velocity measurements taken from samples having known acid concentrations. The system can be used either to generate a read-out to facilitate manual control of the alkylation process, or to automatically regulate the acid feed rate to maintain a desired acid concentration.