Colorimeter, Chemical

an optical device for measuring the concentration of substances in solutions. The operation of colorimeters is based on the property of colored solutions of absorbing light passing through them. The absorption increases with increasing concentration c of the coloring substance. All colorimeter measurements are taken in monochromatic light in

Figure 1. Optical diagram of a KOL-1M visual chemical colorimeter. Color balancing of the two fields, which correspond to the standard and test solutions and are observed through the eyepiece (6), is achievedby varying the thickness I of the layer of the of the test solution by moving the plunger, a glass rod (3), to which the instrument scale is related. (1) light source, (2) and (2’) cells with the test and standard solutions, (3) and (3’) plungers, (4) prism, (5) interchangeable colored light filters.

the region of the spectrum that is most strongly absorbed by the particular substance in the solution and most weakly absorbed by the solution’s other components. Therefore, colorimeters are equipped with light filters; the use of various light filters with narrow spectral ranges for transmitted light makes possible separate determination of the concentration of the various components of the same solution.

Colorimeters are divided into visual and objective (photoelectric) types. In visual colorimeters (see Figure 1), the light passing through the solution being measured illuminates one part of the field of vision, and light passing through a solution with a known concentration of the same substance is incident on the other part. By changing the thickness I of the layer of one of the solutions being compared or the intensity I of the light beam, the viewer attempts to make the color tones of the two halves of the field of vision indistinguishable to the eye. The concentration of the solution under study may be determined from the known relationships for I, l, and c (the Bouguer-Lambert-Beer law).

Figure 2. Schematic diagram of an FEK-M photoelectric compensating colorimeter. The light from the source (1) passes through the test solution in the left side of the instrument (numbers without primes) and through the standard solution in the right side (numbers with primes); the difference between the signals of the selenium photocells (9) and (9’) is recorded by the galvanometer (14). Non-calibrated photometer wedges (10) and (11) are used to set the galvanometer at zero in the absence of solutions. The optical compensation—that is, reduction of the difference in the signals of detectors (9) and (9’) to zero after mounting the cells with the solutions (6) and (6’) — is achieved using a slit stop (12) with a reference drum, dial, or scale (13). (2) and (2’) converging lenses, (3) and (3’) mirrors, (4) and (4’) light filters, (5), (5’) and (7), (7’) lenses, (8) and (8’) prisms.

Photoelectric colorimeters give higher accuracy of measurement than the visual type. Selenium and vacuum photocells, photomultipliers, photoresistors, and photodiodes are used as light detectors in photoelectric colorimeters. The strength of the photocurrent of the detectors is determined by the intensity of the incident light and thus by the extent of absorption of the light beam in the solution (absorption increases with increasing concentration). In addition to photoelectric colorimeters with reading of the photocurrent strength, compensating colorimeters (see Figure 2) are also common; in this type the difference in the signals corresponding to the standard and test solutions is set to zero (compensated) by an electric or optical compensator (for example, a photometer wedge). The reading in this case is taken from the compensator scale. Compensation makes possible minimization of the effect on accuracy of measurement conditions, such as temperature and instability of properties of the colorimeter’s parts. The readings of colorimeters do not immediately give concentration values for the substances studied. Conversion to concentration values requires the use of calibration curves obtained by measuring solutions of known concentrations.

Colorimeter measurements are relatively simple and rapid. The accuracy of such measurements is often not less than that of more complicated methods of chemical analysis. The lower limits for determinable concentration are 10^–3 to 10^–18 moles per liter, depending on the type of substance.