Colour solution

Thus, for example, an analysis using coloured solutions can be carried out, where an indicator cannot be used. Moreover, it is not easy to find a redox indicator which will change colour at the right point. Potentiometric methods can fairly readily be made automatic. 

The alkali metals have the interesting property of dissolving in some non-aqueous solvents, notably liquid ammonia, to give clear coloured solutions which are excellent reducing agents and are often used as such in organic chemistry. Sodium (for example) forms an intensely blue solution in liquid ammonia and here the outer (3s) electron of each sodium atom is believed to become associated with the solvent ammonia in some way, i.e. the system is Na (solvent) + e" (sohem). 

Assemble a 250 ml. three-necked flask, fitted with a stirrer, a reflux condenser and a dropping-funnel, as in Fig. 22(A) and (j), p. 43, or Fig. 23(c), p. 46 (or a two-necked flask, with the funnel fitted by a grooved cork (p. 255) to the top of the condenser). Place 40 ml. of ethanol in the flask, and then add 2-3 g. of sodium cut into small pieces. When all the sodium has dissolved, heat the stirred solution on the water-bath, and run in from the funnel 17 g. (17 ml.) of ethyl malonate and then (more slowly) io-2 g. (12 ml.) of mesityl oxide, the reaction-mixture meanwhile forming a thick slurry. Boil the stirred mixture under reflux for i hour, and then add a solution of 10 g. of sodium hydroxide in 50 ml. of water, and continue boiling the pale honey-coloured solution for ij hours more. 

Place about 1 g. of the nitro-hydrocarbon in a boiling-tube and add 5 ml. of cone. HCl and several pieces of granulated tin. Warm the mixture and shake continuously to break up the oily drops of the nitro-compound. When all the oil has disappeared (about 3 minutes heating) pour off the liquid from any undissolved tin into a 100 ml. conical flask. Cool and add cautiously 30% aqueous NaOH solution until the precipitate formed redissolves to give a dark-coloured solution. Cool the latter thoroughly and shake well with about 15 ml. of ether. Separate the ethereal layer in a separating-funnel, wash with water and evaporate the ether in a basin on a previously heated water-bath in a fume-cupboard atoay from all flames. The residue is either... 

Coloured salts loith alkalis. Dissolve a few crystals of o-nitro-phenol in a few ml. of 10% NaOH solution. An orange-coloured solution is produced, and becomes almost colourless on the addition of acids. Alkali restores the orange coloration, and so the solution exhibits the properties of an indicator. 

Nitrosomethylurea. Acetamide method. To a solution of 59 g. of acetamide in 88 g. (28 ml.) of bromine (1) in a 4-litre beaker add dropwise, with hand stining, a solution of 40 g. of sodium hydroxide in 160 ml. of water. Heat the resulting yellow reaction mixture on a steam bath until eflfervescence sets in (2), after which continue the heating for 2-3 minutes. CrystaUisation of the product from the yellow or red coloured solution usually commences immediately. Cool in an ice bath for 1-2 hours, collect the product by suction filtration, wash with a little ice-cold water, and dry in the air. The yield of colourless acetylmethylurea, m.p. 178-180°, is 50 g. 

Methylation with diazomethane may be carried out as follows (FUME CUPBOARD )-. Dissolve 2-3 g. of the compound (say, a phenol or a carboxylic acid) in a little anhydrous ether or absolute methanol, cool in ice, and add the ethereal solution of diazomethane in small portions until gas evolution ceases and the solution acquires a pale yellow colour. Test the coloured solution for the presence of excess of diazomethane by removing a few drops into a test-tube and introducing a glass rod moistened with glacial acetic acid immediate evolution of gas should... 

Solutions of colourless pyrazolidinediones give highly coloured solutions of radicals (113) when treated with lead dioxide. The ESR spectra of these radicals have been recorded (78JOC808). They dimerize to tetrazenes (114) which appear to be indefinitely stable. 

Sulfur dissolves in liquid ammonia to give intensely coloured solutions. The colour is concentration-dependent and the solutions are photosensitive.Extensive studies of this system by several groups using a variety of spectroscopic techniques, primarily Raman,... 

Ability of liquid NHi to dissolve metals giving coloured. solutions reported by W. Weyl. 

As long ago as 1804 C, F. Bucholz observed that sulfur dissolves in oleum to give clear, brighdy coloured solutions which could be yellow, deep blue or red (or intermediate colours) depending on the strength of the oleum and the time of the reaction. These solutions are now known to contain cations, the structure... 

In cases where it proves impossible to find a suitable indicator (and this will occur when dealing with strongly coloured solutions) then titration may be possible by an electrometric method such as conductimetric, potentiometric or amperometric titration see Chapters 13-16. In some instances, spectrophotometric titration (Chapter 17) may be feasible. It should also be noted that ifit is possible to work in a non-aqueous solution rather than in water, then acidic and basic properties may be altered according to the solvent chosen, and titrations which are difficult in aqueous solution may then become easy to perform. This procedure is widely used for the analysis of organic materials but is of very limited application with inorganic substances and is discussed in Sections 10.19-10.21. 

For the titration of colourless or slightly coloured solutions, the use of an indicator is unnecessary, since as little as 0.01 mL of 0.02 M potassium permanganate imparts a pale-pink colour to 100 mL of water. The intensity of the colour in dilute solutions may be enhanced, if desired, by the addition of a redox indicator (such as sodium diphenylamine sulphonate, AT-phenylanthranilic acid, or ferroin) just before the end point of the reaction this is usually not required, but is advantageous if more dilute solutions of permanganate are used. 

As an alternative, a reversible indicator may be employed, either (a) 1-naphthoflavone (0.5% solution in ethanol, which gives an orange-coloured solution at the end-point), or (b) p-ethoxychrysoidine (0.1% aqueous solution, colour change pink to pale yellow). Under these conditions, the measured 25 mL portion of the arsenic solution is treated with 10 mL of 10 per cent potassium bromide solution, 6 mL of concentrated hydrochloric acid, 10 mL of water and either 0.5 mL of indicator (a) or two drops of indicator (b). 

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. 

When plotting the standard curve it is customary to assign a transmission of 100 per cent to the blank solution (reagent solution plus water) this represents zero concentration of the constituent. It may be mentioned that some coloured solutions have an appreciable temperature coefficient of transmission, and the temperature of the determination should not differ appreciably from that at which the calibration curve was prepared. 

Chromium in steel Discussion. The chromium in the steel is oxidised by perchloric acid to the dichromate ion, the colour of which is intensified by iron (III) perchlorate which is itself colourless. The coloured solution is compared with a blank in which the dichromate is reduced with ammonium iron(II) sulphate. The method is not subject to interference by iron or by moderate amounts of alloying elements usually present in steel. 

The intensity and colour of the fluorescence of many substances depend upon the pH of the solution indeed, some substances are so sensitive to pH that they can be used as pH indicators. These are termed fluorescent or luminescent indicators. Those substances which fluoresce in ultraviolet light and change in colour or have their fluorescence quenched with change in pH can be used as fluorescent indicators in acid-base titrations. The merit of such indicators is that they can be employed in the titration of coloured (and sometimes of intensely coloured) solutions in which the colour changes of the usual indicators would... 

Heat Autoclaving or dry heat Temperature-sensitive coloured solution Sealed tubes partly filled with a solution which changes colour at elevated temperatures rate of colour change is proportional to temperature, e.g. Browne s tubes Temperature, time... 

Ag IIl) dithiocarbamate, observed as a red-coloured solution, obtained by the reaction of one mole Bu tds and 1/6 mole [AgCBUjCltc)] in benzene, was reported by Bergendahl and Bergendahl 145). This product is rather unstable as the red solution turns to the blue-coloured Ag(Bu2C fc)2 within ten minutes. 

HC1, was sonicated for similar durations of 10, 20 and 30 min. The blue coloured solution changed again to green, besides, the turbidity and conductance also gradually increased in sonicated solutions. The turbidity increased remarkably after about 15 min, even when sonication was stopped. (Table 9.5c). 

Sulfur dissolves in liquid ammonia to give intensely coloured solutions. The colour is concentration-dependent and the solutions are photosensitive. Several S-N anions are present in such solutions.76,77 The primary reduction products are polysulfides Sx2, which dissociate to polysulfur radical anions, notably the deep blue S3 ion. In a 1M solution, the major S-N anion is cyc/0-[S7N] with smaller amounts of 21 and a trace of 20.76... 

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