Methods and systems to determine a concentration of bromide ions in an aqueous sample are disclosed. The method involves the oxidation of bromide ions to bromine, followed by bromination of a colored or fluorescent reporter compound which may be detected by spectrophotometric means. The relative change in color or fluorescence upon bromine binding to the reporter compound may then be used to determine a quantitative concentration of bromide ions in the sample. The system utilizes a photocatalytic coating in a sample chamber, a source of reporter compound in fluid communication with the sample chamber, light sources that may activate the photocatalyst and excite the reporter compound, an optical detection unit capable of receiving a light signal from the second light source after it has passed through the sample chamber, and various pumps, valves or injection syringes that regulate the flow of sample and reporter compound into and out of the sample chamber.