We use a manual colorimetric analysis method for the determination of dissolved silicic acid concentration in fresh and sea water. Silica reacts with molybdate in an acidic environment to form yellow beta-molybdosilicic acid (silicomolybdate). We add a reducing solution, containing metol and oxalic acid, which reduces the complex to give a blue color and simultaneously decomposes any phosphomolybdate and aresenomolybdate. The resulting heteropoly molybdous-blue complex absorbs light at 810 nm. We determine the concentration by comparing the absorbance signal of the sample with calibration results from prepared standards of varying concentrations.