Colorimeter Duboscq

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 ... 

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)] ... 

Drop time in polarography, 597, 608 Dropping mercury electrode 608, 628 Dry ashing 114 Dry box lOl Drying reagents 99 comparative efficiencies of, (T) 99 Drying of precipitates 119 Duboscq colorimeter 656 Duplication method 701... 

In actual practice, the so called visual nephelometer (comparator type) have been more or less superseded by the photoelectric instruments Nevertheless, a Duboscq Colorimeter with a slight modification may be used conveniently for nephelometric analysis, for instance ... 

The Duboscq Colorimeter should always be maintained meticulously neat and clean. The clear-glass-tubes and the plungers are either rinsed with distilled water or with the solution to be measured. 

The various components of a Duboscq Colorimeter are as follows A = Clear glass tube with opaque bottom,... 

Introduction of photoelectric cells led to the replacement of the Duboscq colorimeter and so to quantitative spectrophotometric methods of analysis which met biochemical requirements. This introduction of spectrophotometry as a routine procedure was one of the earliest technological advances underpinning the elucidation of biochemical pathways between 1930-1960. Micromanometric methods also became available about the same time, and offered a means to measure cell respiration. Manometry was developed in Warburg s laboratory in Berlin and was one of the main techniques used by H.A. Krebs in his studies on the citric acid and urea cycles (Chapters 5 and 6). 

A more refined method uses the Duboscq colorimeter. This instrument features a dual-matched optical system. Uniformly intense light is incident upon both colorimeter tubes and the difference in absorption of the standard and unknown solutions is compensated for by adjusting the thickness of solution through which light passes. When the two colors match Cs =C,bJbx. 

The solution thus prepared is compared in the Duboscq colorimeter with the 8 mm. layer of the dichromate solution. 

With the Duboscq colorimeter, comparison is made with a standard liquid, but the nature of this has not been definitely fixed some use solutions of caramelised sugar of given colouring intensity, others a o-i%-iodine solution. 

Procedure. 10 c.c. of the spirits, brought to 50% strength, are placed in one test-tube and 10 c.c. of the standard solution in another, 4 c.c. of Schiff s reagent being added in each case and the tubes then dosed, shaken and left for 20 minutes. If the two liquids then have about equally intense colorations, they are compared in the colorimeter. Otherwise the test is repeated, a less quantity being taken of the alcohol giving the deeper coloration and this made up to 10 c.c. with pure 50% alcohol this is necessary because the intensities of the coloration are not proportional to the aldehyde-content, except when the differences are small. When sensibly similar colorations are obtained, the liquids are compared in the Duboscq colorimeter. 

For this purpose a standard solution containing 0-005 gram of furfural per litre of pure 50% alcohol is prepared, the spint to be tested being also brought to 50% concentration. 10 c c of each of the two liquids are treated at the same time with 10 drops of aniline and 1 c c of concentrated acetic acid. If, after 20 minutes, the colorations are approximately similar, they are compared in the Duboscq colorimeter, if, however, the two colorations differ greatly, the test is repeated m the manner described for the determination of the aldehydes (tee above). The proportion of furfuraldehyde present is readily calculated from the colorimeter readmgs... 

For substances absorbing in the visible region of the spectrum, simple instruments called colorimeters are often used in place of spectrophotometers for measuring concentrations. In the earliest form of this instrument, the Duboscq colorimeter, the observer visually matched the unknown and standard solutions. These visual instruments have now been superseded largely by photoelectric devices, but the Duboscq colorimeter illustrates well the principle involved, and its use will be described briefly. 

A solution containing 2.5 mg of ammonia per dm was treated with Kessler s reagent, which produces a yellow color. An unknown solution was treated similarly. When the two solutions were matched in the Duboscq colorimeter, the depths were 12.5 mm for the standard solution and 17.0 mm for the unknown. Calculate the concentration of the unknown solution. 

A colored solution of concentration 0.50 mol dm" was placed in a Duboscq colorimeter and compared with an unknown solution of the same substance. The observed depths were 14.5 mm for tlfe standard and 9.6 mm for the unknown. Calculate the concentration of the unknown. 

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 Duboscq Colorimeter is an instrument used to analyze samples by differences in color intensity. Explain how an accuracy of 2% is achieved. 

Based upon this law, color comparison instruments soon developed. One of the first instruments to use the absorption of light to determine concentration was the visual color comparator. The user visually compared the transmitted light from the sample and a standard solution and adjusted the path length until the transmitted light from both solutions appeared to have the same intensity [50]. Figure 6.7 shows an early color comparator called a Duboscq Colorimeter. 

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