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THEpresent research represents an effort to standardize a colorimetric procedure for the rapid determination of small quantities and the detection of minute changes in the concentration of dissolved silica in natural waters of very low phosphate content.A preliminary study indicated that two general procedures were available. The method of Dienert and Wandenbulcke (4) takes advantage of the yellow silicomolybdate color produced when ammonium molybdate reacts in acid medium with dissolved silica. Isaacs' method (7) attempts to extend the sensitivity of this reaction by reducing the molybdenum in the silicomolybdate complex to the relatively intense molybdenum blue color.

COLORIMETRYis assuming an ever-increasingly important role in analytical chemistry, and recent improvements in colorimeters and colorimetric methods have increased both the accuracy and speed of analysis. Photoelectric instruments (S) designed for use with a series of color filters have largely replaced the visual white light colorimeters in present-day practice. Such instruments are frequently referred to in the German literature as step-photometers or absolute colorimeters. Ashley (1) has chosen to call the practice with such instruments "abridged spectrophotometry" and thereby infers the name "abridged spectrophotometer" for these instruments. It is, perhaps, unfortunate and misleading that many of the instruments are referred to by still other names (2) or simply as photoelectric colorimeters (ß-9,13,14)•The purpose of this paper is to show theoretically and experimentally how photoelectric filter photometers can be used to resolve the intensity of one color in the presence of a second color which may be present in the solution owing to an interfering ion or substance.The treatment presented here for a two-component system can be extended to a three-component system or to even more complex systems. Except in unusual cases, however, the resolution of these multicomponent systems in a filter photometer cannot be performed with a great degree of accuracy. Weigert (12), using the nearly monochromatic radiation available with a spectrophotometer, reports the resolution of a four-component color system of dyes by a method involving the solution of simultaneous equations in which the extinction coefficients of each color occur. Theoretical centration. The slope of this line is numerically equal to the constant, k, in the above equation.Unfortunately, there are many color systems in which the absorption bands of the components are not sufficiently displaced from each other so that spectral separation can be effected. These systems fall into the second group and require a more specialized treatment for resolution. Figure 1 represents the absorption characteristics of a hypothetical system No matter where one selects a filter for component 1, there will be some interference from component 2; however, the maximum in each curve occurs at different wave lengths. By choosing two

A series of nine determinations on NBS sample 130 indicate the precision which may be expected from this procedure. The following results were obtained from determinations made on separate films taken on 3 different days.

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