Visual colorimetry

Kingsley, G. R. Direct biuret method for determination of seriim proteins as applied to photoelectric and visual colorimetry. J. Lab. Clin. Med. (1942), 27,... 

Visual colorimetry, which has been used for over two centuries, is a simplified form of instrumental colorimetry. Because of its low cost, it is used on a daily basis and its precision is often surprising. 

Figure 11.19— Visual colorimetry. To the left, a rotating filter comparison system. By using two cells, one of which contains the sample and the other the blank, it is possible to take into account the normal coloration of the two lubes. Observation is made by comparing the transparency for a specific filter against white light. To the right, a portable reflectomeler that allows comparisons to be made without the human eye. (Reproduced by permission of Merck.)...

Visual colorimetry, probably the oldest analytical method, was last used by the Greeks and Romans. This method found its scientific basis in 1729 when Pierre Bouguer theorised that if a given width of glass absorbs half of the light emitted by a source then double the width will reduce the fight by one quarter its initial value . 

Among all the devices that have been invented for visual colorimetry, one of the most original was described by Jules Duboscq in 1868. Now redun-... 

Visual colorimetry is a simple method and is fairly precise. Essentially it requires the matching of the color of a standard solution with thal of an unknown sample so that when they become identical, they must contain the same amount of colored substance in columns of equal cross-section. At this point... 

Abstract A novel colorimetric method, digital color analysis (DCA), was proposed using a digital color analyzer and was applied to various quantitative analyses using chromatic-ity coordinates and suitable sensors for visual colorimetry based on the characteristics of human visual perception by virtual simulations based on digital color information. On the basis of DCA, we developed a visual colorimetric sensor for Li+, NH4+ and protein determination by the mixing of two kinds of lipophilic dyes, whose optimum mixing ratio... 

Absorption spectrometry is a traditional method used for the measurement of various chemical substances and makes it possible to carry out visual colorimetry allowing easy measurements. Conventional absorption spectrophotometry is the measurement of numerical values such as that of absorbance to carry out qualitative and quantitative analysis. In such cases, if the spectra obtained are complicated, the determination often becomes difficult. However, even if the spectral changes are quite complicated, our eyes recognize them simply as color changes. Determination utilizing the colors themselves is a perceptual method instead of simple absorption spectrophotometry. 

The L a b color space is a visually uniform color space and is quite useful for designing devices suitable for visual colorimetry. The angle originating from the positive direction of the a axis expresses the hues themselves and is defined as the hue angle, hab, expressed by Eq. 29 [13] ... 

To make use of their merits and to erase their drawbacks, we carried out the multicolor visual colorimetry over a large concentration range of protein under the optimum mixing ratio of tetrabromophenol blue and eosin B using L a b color coordinates based on DCA. 

In quality control of colored objects, production requires more consistency than absolute accuracy. After all, there will generally be a product standard having the correct color to which we may reference the color of parts from the current production run. Since the production part and the product standard will be free from the small errors in accuracy, because any small inaccuracies will be the same in measurements of the batch and the standard and thus will subtract out in the different components. What is needed is an instrument with higher day-to-day objectivity than our visual system, and for such an increase in consistency, one must be willing to give up some of the absolute accuracy. This requires an analog simulation of visual colorimetry. 

Amongst the different devices imagined for visual colorimetry measurements, one of the most original was described by Jules Dubose in 1868. This instrument which remained in use until the 1960s permits, due to a system of total reflecting prisms, a juxtaposition in a small drcular field, of the light intensities that have travelled through two identical cells, one of which contains the sample (concentration Q) and the other contains a known standard (concentration Q). The observer sees both fields with one eye, and adjusts the depths of the columns of liquid, of the solution to be measured and of the standard, until the two halves of the field are identical in intensity. When this condition holds, the absorbances are equal. The concentrations of the two solutions are inversely proportional to their depths, which may then be read on the instrument. 

Visual colorimetry, used for more than four centuries, is a simplified form of instrumental absorption spectrometry. Given its modest cost and its frequently surprising precision it is therefore widely practised. 

Figure 9.31 Visual colorimetry. Left, a disc comparator. The use of this apparatus consists of choosing, by rotation of the filter holder, the filter which permits when it is superimposed upon the analytical blank, a match of the colour of the tube. The observation is made by transmission of white light. The central tube contains the sample after treatment. The disc is calibrated for a particular measurement. Right, a portable colorimeter which allows comparisons without evaluation by the human eye (Model 1200, reproduced courtesy of LaMotte Company).

In its simplest form, colorimetry consists of visual matching of the color of the sample with that of a series of standards. A colored compound is first formed by suitably reacting the constituent to be determined, then the colored solutions are racked side-by-side in Nessler tubes for viewing from the top. The approximate concentration of the unknown is estimated by finding which standard most closely matches the unknown in color. Visual colorimetry suffers from poor precision since the eye is not as sensitive to small differences in absorbance as is a photoelectric device. The use of a Duboscq colorimeter constitutes a more refined method of analysis for color comparison. This is equipped with an eyepiece with a split field that permits the ready comparison of beams passing through sample and standard. 

The shape and size of the spot produced on chromatograms are decisively influenced by the amount of solute, the volume of the solution applied, the diameter of the spot at the start, and series of other factors. In an attempt to standardize the quantification in TLC, Mohammad and Tiwari (141) and 0. Shadrin et al. (143) have established a linear relationship between the size of the spot and the amount of inorganic anions. A representative plot is given (Fig. 6). A. Timeibaev et al. (164) have determined Ti in sulfite-cellulose liquor and high speed steel by visual colorimetry or by planimetry using a calibration graph. 

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