An apparatus for measuring hydrogen peroxide in a water system on a periodic basis using a colorimetric assay comprising: a measurement cell for receiving a water sample from the water system; a light transmitter configured to emit light at a selected wavelength at the measurement cell; a photodiode receiver configured to receive light passing through the measurement cell; a solvent and a reagent valve; a reagent compound configured to react with hydrogen peroxide to form a reaction product, the reaction product adapted to absorb light at the selected wavelength proportional to the amount of hydrogen peroxide in the water sample; a reagent vial containing the reagent compound and the solvent; a buffer jar for receiving a water sample from the water system, the buffer jar being connected to the measurement cell through a supply valve, the buffer jar comprising an overflow tube for returning overflow water to the water system, for maintaining a constant water volume and constant head pressure in the buffer jar, and for maintaining a turn-over of water in the buffer jar; a first network of pipes for connecting source water of the water system to the buffer jar, the buffer jar to the supply valve, and the supply valve to the measurement cell, and a second network of pipes connecting the reagent vial to the reagent valve, the reagent valve to the measurement cell, and the measurement cell to a drain valve; and a control unit; wherein the control unit is configured to control the supply valve in a time-dependent manner, cause a first colorimetric measurement of a first water sample free of reagent and a second colorimetric measurement of a second water sample mixed with reagent, determine the difference between the first and second measurements, and compare the difference against a pre-determined standard curve of the difference between the first and second measurements versus hydrogen peroxide concentration at different dilutions to determine and report the concentration of hydrogen peroxide in the water sample. a measurement cell for receiving a water sample from the water system; a light transmitter configured to emit light at a selected wavelength at the measurement cell; a photodiode receiver configured to receive light passing through the measurement cell; a solvent and a reagent valve; a reagent compound configured to react with hydrogen peroxide to form a reaction product, the reaction product adapted to absorb light at the selected wavelength proportional to the amount of hydrogen peroxide in the water sample; a reagent vial containing the reagent compound and the solvent; a buffer jar for receiving a water sample from the water system, the buffer jar being connected to the measurement cell through a supply valve, the buffer jar comprising an overflow tube for returning overflow water to the water system, for maintaining a constant water volume and constant head pressure in the buffer jar, and for maintaining a turn-over of water in the buffer jar; a first network of pipes for connecting source water of the water system to the buffer jar, the buffer jar to the supply valve, and the supply valve to the measurement cell, and a second network of pipes connecting the reagent vial to the reagent valve, the reagent valve to the measurement cell, and the measurement cell to a drain valve; and a control unit; wherein the control unit is configured to control the supply valve in a time-dependent manner, cause a first colorimetric measurement of a first water sample free of reagent and a second colorimetric measurement of a second water sample mixed with reagent, determine the difference between the first and second measurements, and compare the difference against a pre-determined standard curve of the difference between the first and second measurements versus hydrogen peroxide concentration at different dilutions to determine and report the concentration of hydrogen peroxide in the water sample.