Depth of colour

It is advisable, wherever possible, to make a preliminary determination of the concentration of the unknown solution by adding from a burette a solution of the component in known concentration to a Nessler tube containing the reagents diluted with a suitable amount of water until the depth of colour obtained is practically the same as that of an equal volume of the unknown solution also contained in a Nessler cylinder and standing at its side. A series of standards on either side of this concentration is then prepared. 

By matching the intensity and depth of colour with standard stains, the proportion of arsenic in the substance may be estimated. Thus, a stain equivalent to the 1 ml standard stain obtained by performing on 10 g of a substance implies that the proportion of As is 1 part per million. 

Extraneous Colouring Matters.—Most of the products in question are coloured a more or less deep yellow by means of organic colouring matters or, sometimes, saffron, the depth of colour depending on whether enhancement of the natural colour of the flour or imitation f the colour imparted by eggs is desired. In Italy the use of picric acid, Victoria yellow, Martius yellow and metanil yellow for this purpose is prohibited. 

With the former instrument, comparison is made with a double disc of yellow glass of definite depth of colour (standard colour), and when a 100 mm. layer of a liquid corresponds in colour with the standard, the colour of the liquid is taken as 1 thus, the colour of any liquid is obtained by dividing 100 by the depth in mm. of the layer necessary to equal the standard colour. The value obtained is then referred to 100 of pure sugar, allowance being made for the sugar content of the juice and for the extent to which it is diluted for the determination of the colour. 

The importance Of the determination of the depth of colour is diminished in consequence of the fact that raw sugars may be coloured with coal-tar colours. The latter may be tested for by extracting with alcohol or Other solvent, fixing on wool and characterising by the methods given in the cases of macaroni, etc., or textiles. 

Salleron takes as unit a wine which, in a layer 3 mm. deep, exhibits an intensity of colour equal to that of one of the tints of his wine-colorimeter scale. Thus, a wine which in this apparatus requires a thickness of 1-5 mm. would have a depth of colour double that of the unit. 

The two liquids to be compared are placed in the two vessels and the glass cylinders adjusted until the two halves of the field appear equally illuminated the respective depths of colour of the two liquids are then inversely proportional to the thicknesses of the layers. Three observations should be made with varying depths of the two liquids and the mean of the three results taken. In the case of wine, a o-i% solution of Bordeaux red is used as comparison liquid. 

Old masters were almost always given a final coat of varnish, partly to enhance the depth of colour (known as colour saturation) and partly as a necessary protective layer in the days when buildings were heated with open fires and stoves, which gave off damaging fumes... 

Traces of turbidity and of colour are also readily distinguished in micro test-tubes (50 x 8 mm) or in micro centrifuge tubes. As a general rule, these vessels are employed in testing dilute solutions so as to obtain a sufficient depth of colour. The liquid in a micro centrifuge tube or in a test-tube may be warmed in a special stand immersed in a water bath (compare Figs. 11.22-23) or in the apparatus depicted in Fig. 11.47. The latter is constructed of thin aluminium or... 

Some superstitions were based on the belief that coral could change colour. It was thought that, when worn by men, coral was red worn by women it was paler and on the sick it was very pale. A variation of this belief was that the depth of colour was an indication of the health of the wearer, with deep, red coral signifying excellent health, and the palest shades signifying very poor health. 

Basic Lead Chromates.—The basic salt, PbaCrOj, PbCrO. PbO, or 2PbO.CrOg, is formed as a red powder by the action of cold caustic soda solution on the normal chromate. It is also produced by mixing together lead chromate and oxide in presence of water, and is known commercially by various names—chrome red, orange chrome, Derby red, Chinese red—according to its depth of colour. It is converted by... 

A The general rule that the depth of colour of a compound increases with the number of atoms constituting the molecule, does not apply in all cases for example, the above dyestuff is redder, if the connecting benzene ring is replaced by a higher hydrocarbon. 

Colorimetric analysis depends upon the production of a coloured solution in which the depth of colour changes quantitatively as the... 

Up to 10% of pigment by weight of cement may be added depending on the depth of colour required. Used with white Portland cement it is possible to produce pastel shades. 

E. E. Free showed that the difference in the colour of the yellow colour of the powder and the orange colour of the crystals is due to differences in the grain-size. The largest crystals are obtained with hot dil. soln. The size of the precipitated crystals increases continuously with decrease in cone. The colours of the precipitates obtained from the most cone, and the most dil. soln. are lighter than is the case with precipitates from soln. of intermediate cone. The curve shows a maximum depth of colour at moderate cone. So that size of grain alone does not suffer to explain the results. 

Increases in refining will increase the depth of colour produced. 

A shaped body, not densely sintered, is immersed in a solution of permanganate of potash over a certain period of time. Should there be laminations, the suction speed and the capillary degree of absorption of the structure are inconsistent. When such an impregnated sintered body is cut open, differences in the depth of colour and in the flow path of the coloured liquid are apparent. 

Original breathalysers used a bag and crystals similar to this set-up. Today, breathalysers use the same reaction, but in a more sophisticated form, linked to electronic determination of the depth of colour produced. They can be bought quite cheaply over the internet. 

Urine is a clear yellowish fluid the depth of colour of which depends largely on its concentration. The colour is due principally to the pigment urochrome the origin of which is unknown. 

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