Colorimeters

The tendency of the color to become darker with time is often indicative of chemical degradation. The test is conducted with the aid of a colorimeter (NF T 60-104 and ASTM D 1500) and by comparison with colored glass standards. The scale varies from 0.5 to 8. The French specifications stipulate that diesel fuel color should be less than 5, which corresponds to an orange-brown tint. Generally, commercial products are light yellow with indices from 1 to 2. 

Ammonia present in very small quantities in solution may be estimated by comparing the intensity of colour produced with Nessler s reagent (p. 439) with standard colours, using a simple form of colorimeter called a Nessleriser . 

The change in colour when one ligand is replaced by another can be used to determine the coordination number thus if the colour change is measured in a colorimeter as the new ligand is added, the intensity of new colour reaches a maximum When the metal/ligand ratio is that in the new complex. 

Color-forming reactions Color generation Colorimeters Colorimetric methods... 

Measurement of Whiteness. The Ciba-Geigy Plastic White Scale is effective in the visual assessment of white effects (79), but the availabihty of this scale is limited. Most evaluations are carried out (ca 1993) by instmmental measurements, utilising the GIF chromaticity coordinates or the Hunter Uniform Color System (see Color). Spectrophotometers and colorimeters designed to measure fluorescent samples must have reversed optics, ie, the sample is illuminated by a polychromatic source and the reflected light passes through the analy2er to the detector. 

A measure of the color developed by impurities when trimeUitate esters are produced can be correlated with the anhydride color measurement. The method measures the color difference in light transmittance between a trimellitic solution and a 3.0 N sodium hydroxide solution as a reference. The difference in light transmittance or AH (total color difference) is obtained using a colorimeter. 

Instmments like colorimeters and spectrophotometers are used less often. These ate used primarily for manufacturing control of printing inks. Ftequentiy, however, inks other than yellow, magenta, and cyan ate used for spot-color appHcations, and in those instances a spectrophotometer ensures the correct match of an ink blend to standard. 

The science of color measurement has been explored by various authors (127,128). AATCC evaluation procedure no. 6 describes a method for instmmental measurement of color of a textile fabric. AATCC evaluation procedure no. 7 may be used to determine the color difference between two fabrics of a similar shade. Instmmentation may be either a spectrophotometer for measuring reflectance versus wavelength, or a colorimeter for measuring tristimulus values under specified illumination. If a spectrophotometer is used, however, the instmment must be equipped with tristimulus integrators capable of producing data in terms of CIE X, Y, and Z tristimulus values. 

Colorimeters. Also known as tristimulus colorimeters, these are instniments that do not measure spectral data but typically use four broad-band filters to approximate the jy, and the two peaks of the x color-matching functions of the standard observer curves of Figure 7. They may have lower accuracy and be less expensive, but they can serve adequately for most industrial color control functions. Examples of colorimeters are the BYK-Gardner Co. XL-835 the Hunter Lab D25 series the Minolta CA, CL, CS, CT, and CR series (the last of these is portable with an interface) and the portable X-Rite 918. 

In addition to fiber length, strength, and fineness, two other properties that have significant bearing on fiber and yam properties are color and trash measurements, which have been accompHshed by instmmentation such as the Colorimeter and the Shirley Non-Tint Analyzer. 

Farb-messer, m. colorimeter, -messung, /. colorimetry, -mine, /. colored lead (for pencils). -mischer, m. color mixer. mittel, n. coloring agent, -miihle, /. color mill, -muster, n. color pattern color sample, -nuance, /. color shade, tint, -pflanze, /. dye plant, -rakel, rn.. f. color doctor, -raster, m. color screen, -reaktion, /. color reaction. 

The sensitivity can, however, be improved if the technique of derivative spectrophotometry (Section 17.12) is employed. The development of inexpensive photoelectric colorimeters has placed this branch of instrumental chemical analysis within the means of even the smallest teaching institution. 

Application of Beer s Law. Consider the case of two solutions of a coloured substance with concentrations c, and c2. These are placed in an instrument in which the thickness of the layers can be altered and measured easily, and which also allows a comparison of the transmitted light (e.g. a Duboscq colorimeter, Section 17.5). When the two layers have the same colour intensity ... 

A colorimeter can, therefore, be employed in a dual capacity (a) to investigate the validity of Beer s Law by varying c, and c2 and noting whether equation (11) applies, and (b) for the determination of an unknown concentration c2 of a coloured solution by comparison with a solution of known concentration c,. It must be emphasised that equation (11) is valid only if Beer s Law is obeyed over the concentration range employed and the instrument has no optical defects. 

D. Balancing method (Section 17.5). This method forms the basis of all colorimeters of the plunger type, e.g. in the Duboscq colorimeter. The comparison is made in two tubes, and the height of the liquid in one tube is adjusted so that when both tubes are observed vertically the colour intensities in the tubes are equal. The concentration in one of the tubes being known, that in the other may be calculated from the respective lengths of the two columns of liquid and the relation [equation (11)] ... 

E. Photoelectric photometer method (Section 17.6). In this method the human eye is replaced by a suitable photoelectric cell the latter is employed to afford a direct measure of the light intensity, and hence of the absorption. Instruments incorporating photoelectric cells measure the light absorption and not the colour of the substance for this reason the term photoelectric colorimeters is a misnomer better names are photoelectric comparators, photometers, or, best, absorptiometers. 

Plunger-type colorimeters. The plunger-type of colorimeter with two halves of the field of view illuminated by the light passing through the unknown and standard solutions respectively was invented by J. Duboscq of Paris in 1854. Various improved modifications of the instrument were subsequently developed by manufacturers of optical apparatus. 

Use of Duboscq-type colorimeter. The colorimeter must be kept scrupulously clean. The cups and plungers are rinsed with distilled water and either dried with soft lens-polishing material or rinsed with the solution to be measured. 

Owing to optical and mechanical imperfections of some makes of colorimeters, it is sometimes found that the same reading cannot be obtained in the adjustment for illumination when the cups are filled with the same solution and balanced. In such a case one of the cups (say, the left one) is filled with a reference solution (which may be a solution containing the component to be determined) of the same colour and approximately the same intensity as the unknown and the... 

Immediately the determination has been completed, empty the cups and rinse both the cups and plungers with distilled water. Leave the colorimeter in a scrupulously clean condition. 

Photoelectric colorimeters (absorptiometers). One of the greatest advances in the design of colorimeters has been the use of photoelectric cells to measure the intensity of the light, thus eliminating the errors due to the personal characteristics of each observer. 

Light filters. Optical filters are used in colorimeters (absorptiometers) for isolating any desired spectral region. They consist of either thin films of gelatin containing different dyes or of coloured glass. 

Visual methods have been virtually displaced for most determinations by methods depending upon the use of photoelectric cells (filter photometers or absorptiometers, and spectrophotometers), thus leading to reduction of the experimental errors of colorimetric determinations. The so-called photoelectric colorimeter is a comparatively inexpensive instrument, and should be available in every laboratory. The use of spectrophotometers has enabled determinations to be extended into the ultraviolet region of the spectrum, whilst the use of chart recorders means that the analyst is not limited to working at a single fixed wavelength. 

Proportionality between colour and concentration. For visual colorimeters it is important that the colour intensity should increase linearly with the concentration of the substance to be determined. This is not essential for photoelectric instruments, since a calibration curve may be constructed relating the instrumental reading of the colour with the concentration of the solution. Otherwise expressed, it is desirable that the system follows Beer s law even when photoelectric colorimeters are used. 

Procedure. Take an aliquot portion of the unknown slightly acid solution containing 0.1-0.5 mg iron and transfer it to a 50 mL graduated flask. Determine, by the use of a similar aliquot portion containing a few drops of bromophenol blue, the volume of sodium acetate solution required to bring the pH to 3.5 1.0. Add the same volume of acetate solution to the original aliquot part and then 4 mL each of the quinol and 1,10-phenanthroline solutions. Make up to the mark with distilled water, mix well, and allow to stand for 1 hour to complete the reduction of the iron. Compare the intensity of the colour produced with standards, similarly prepared, in any convenient way. If a colorimeter is... 

Fora filter colorimeter use a blue filter (maximum transmission 400-420 nm) a wavelength of 410 nm is employed for a spectrophotometer. In the latter case, the effect of iron, nickel, chromium(III), and other coloured ions not reacting with hydrogen peroxide may be compensated by using a solution of the sample, not treated with hydrogen peroxide, in the reference cell. 

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