Colorimeter functions

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. 

However, these analogues are actually hypothetical. The reason for this is that it is nearly impossible to obtain optical measurement components, such as the source and the detector, whose response to light across the visible spectrum is flat (or nearly so). However, this is not an impossible task and we find that an excellent match can be obtained to the transmission functions of 7.8.21., i.e.-those of the Standard Observer. This is typical for commercially available instruments. Now, we have an instrument, called a Colorimeter, capable of measuring reflective color. 

We find that an excellent match can be obtained to the transmission functions of 7.8.20. This is typical for commercially available instruments. Now, we have ein instrument, called a Colorimeter, capable of measuring reflective color. 

In a sensitive and specific colorimetric method 1,1,1-trichloro-2,2-bis(p-methoxyphenyl)-ethane is extracted from plant or animal tissue, using benzene or petroleum ether as the solvent. The solvent is evaporated at room temperature by a current of air and the residue dehydroha log ena ted with 2% alcoholic potassium hydroxide. By petroleum ether extraction the resulting 1,1-dichloro-2,2-bis(p-methoxyphenyl)-ethylene is removed from the reaction mixture. After the solvent is removed by air evaporation the dehydroha log ena ted methoxychlor is isolated from the nonsaponifiable portion of the fats and waxes by dissolving the residue in hot acetone, chilling, and filtering. After the acetone is removed by air evaporation, the residue is treated with 85% sulfuric acid. This produces a red solution with an absorption maximum at 555 m/z, the intensity of which can be read on a colorimeter and is a function of the methoxychlor concentration. Beer s law is obeyed over the range of 1 to 50 micrograms. 

The colorimeter is designed to convert % transmittance (% T) to absorbance (Abs) (Figure 5). It is also programmed with the slope from the standard curve of absorbance vs. nanomole NO2 and thus derives a nanomole NO2 value for each sampler analyzed. Additionally, the factor of 2.3 nanomoles per ppm hour exposure is incorporated so that the colorimeter can calculate and display a ppm hour value for each sampler by performing the following functions ... 

Tristimulus colorimeters are used to measure color in certain food applications. They combine light source, filters, and photodetectors to reproduce the CIE Standard Observer response functions (Fig. F5.1.8). Colorimeters, having broad band-pass filters, do not measure the spectral data. Without spectral data they cannot offer the choice of either observer or... 

It has proven feasible to take the electrical output from photocells or phototubes and either with or without amplification record the magnitude and duration of the output. The recording may be made either in the form of a line tracing on a moving chart or may be converted to numerical values and printed by a read-out device. Further refinements can be supplied in which the instrument converts optical densities (or transmittance units) to concentration values. More intricate recording colorimeters or spectrophotometers are also available for continuous scanning and recording of complete spectra from ultraviolet to infrared. No further discussion of these will be attempted since they do not serve a normal function in routine clinical chemistry laboratories. 

In Chapter 2 it was demonstrated that the visual system requires three things to function correctly a light source, an object, and a detector. So one designs an instrument with a standard light source, a port for placing a solid sample, and a detector that has the properties of a standard observer. In the visual colorimeter the pri-... 

Figure 3.3. Comparison of analog colorimeter fit (solid lines) to QE 1931 standard observer functions (from Billmeyer and Salzman, 2nd ed.).

This component enables the analogue signal from the colorimeter to be converted into a numerical value which may be directly expressed as analyte concentration. It may often include a control module by which the functioning of the system may be... 

The reaction of penicillin G with NHjOH is carried out in a batch reactor, A colorimeter was used to measure tiie absorbency as a function of time. [1st Ed. P5-10]... 

All the systems discussed here use conventional strip chart recorders for recording the photometer or colorimeter output, and the resulting record is a conventional histogram in which the absorbance of the eluate or eluate-reagent reaction mixture is recorded as a function of time. In addition, some prototype systems of the UV- and carbohydrate analyzers use on-line computers for data storage and processing (Cl, S7). 

The basic components of spectrophotometers are a light source, wavelength selector, absorption cell (cuvette), and photodetector. Colorimeters or absorptiometers commonly use nondispersive wavelength selection (a filter with bandwidth 4 -40 nm) and solid state or simple phototube detectors, while spectrophotometers employ a prism or grating monochromator (with bandwidth down to 0.2 nm) and a photomultiplier. Colorimeters are inexpensive and most appropriate for repetitive measurements of absorption at a fixed wavelength. The more expensive spectrophotometer can also fulfill this function, but its main purpose, by virtue of its accurate and variable wavelength control, is the measurement of absorption spectra. 

Work-simplification devices on the laboratory bench will be detailed under the headings of automatic dispensers, automatic diluters, and flowthrough colorimeters with digital read-out. In each group, examples will be given of equipment of which the authors have had personal experience, but a choice of alternative apparatus will be available in most instances. The instruments mentioned are not necessarily the simplest or cheapest obtainable for performing a particular function. 

However, colorimeters are very rare to see in crude oil chemical laboratories, because as was indicated earlier, almost all the important functions of the colorimeter can be reproduced by modern photometers. 

In a tristimulus colorimeter, three or four filters duplicate the response of the standard observer. The filters, which correspond to the three primary colours in the spectrum (red, green and blue), can be combined to match most colours. The more sophisticated instruments carry a fourth filter to simulate the blue part of the CIE [[X bar]] function shown in Fig. 5.1. The other essential parts of a tristimulus colorimeter are a white light source, an array of photometers and, nowadays, a computer or an interface to one, as shown in Fig. 5.2a. The computer can collect responses as well as carry out data transformations between CIE and other colour scale systems or between different standard white light sources or white diffusers. Hunter and Harold (1987) give a good summary of the transformation formulae employed. Data from a colorimeter is given as a three-point output, commonly CIELAB, HunterLab or 7, x, y. 

As mentioned previously, the now routine integration of a pc means that the data collected from either a colorimeter or spectrophotometer can be transformed to XYZ, CIELAB, HunterLab or other colour system as specified by the user and provided by the software. The initial step from instmment response to XYZ requires the definition of standard observer functions. 

More data are available on decolorization than on other liquid-phase functions of activated carbon, and this is understandable in view of the comparative ease of measuring color. Long before the introduction of modern instrumentation, there were satisfactory visual methods of measuring color which continued in use until relatively recently. Photoelectric colorimeters which are now in general use avoid the personal equation and expedite evaluations. Spectrophotometers offer further advantages, especially when it is desirable to measure the separate adsorption of several different colored bodies. 

As an example of the automation of an HPLC detection module we describe here a filter colorimeter (Hewlett-Packard), the optical scheme of which is shown in Fig. 12.16a. In addition to the UV-visIble light source and the lens system, a disc furnished with peripheral interference filters converts the Incident light beam to monochromatic radiation which Is split into two beams that are sent to a reference and an analytical photodiode, the latter of which is located after the flow cell. The microprocessor allows the automation of a number of functions ... 

Figure 13.15 shows the operational scheme of this automatic tltrator. The heart of the unit Is an INTEL 8080 microprocessor mounted on the central processing unit (CPU) board. The rotary reaction cell assembly can accommodate up to three different sensors for multiple measurements on the same processed sample. Each stepper burette board controls up to two burette dispensing assemblies. Function boards such as the colorimeter board, air burette board, E/I output board and RS-232 printer Interface boards are available optionally. The optional D/A and E/I board is used for closed-loop applications where the tltrator controls the final element such as a control valve. The RS-232 printer Interface board Is useful for troubleshooting the equipment and editing user-defined programs. The Instrument accuracy, repeatability and response time vary widely and depend on the particular type of measurement concerned. The system requires a.c. power, a 75-psl air supply and a dilution water supply for proper operation. The air flow-rate required is of about 50 cm3/mln... 

Colorimetric method. This can be used when either a reactant or a product is coloured (iodine or potassium manganate(VII) are examples). The colorimeter must first be calibrated using solutions of the coloured substance of known concentrations. Then the reactants are mixed and the clock started. The intensity of the colour is measured as a function of time. The concentration of the coloured substance is proportional to the amount of light absorbed. 

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