Colour appearance

This test is extremely sensitive and usually sufficient ferric ions are present in an ironlll) salt to give some coloration. The blood red colour appears to be due to a complex... 

To determine which halogen is present, take 1-2 ml. of the filtrate from the sodium fusion, and add dilute sulphuric acid until just acid to litmus. Add about 1 ml. of benzene and then about 1 ml. of chlorine water and shake. A yellowish-brown colour in the benzene indicates bromine, and a violet colour iodine. If neither colour appears, the halogen is chlorine. The result may be confirmed by testing the solubility of the silver halide (free from cyanide) in dilute ammonia solution silver chloride is readily soluble, whereas the bromide dissolves with difficulty, and the iodide not at all. 

To the same solution add a few drops of phenol phthalein, and titrate it against N/10 NaOH until a pink colour appears, which will indicate the total acidity of the bath. This is approximately 35 to 37 for a concentration of 5% for a hot process and 60 to 64 for a concentration of 10% for a cold process. Obtain the standard total acidity of the hot or cold process chemicals from the manufacturer. 

Notes. (1) For elementary students, it is sufficient to weigh out accurately about 1.7 g of sodium oxalate, transfer it to a 250 mL graduated flask, and make up to the mark. Shake well, Use 25 mL of this solution per titration and add 150mL of ca 1M sulphuric acid. Carry out the titration rapidly at the ordinary temperature until the first pink colour appears throughout the solution, and allow to stand until the solution is colourless. Warm the solution to 50-60 °C and continue the titration to a permanent faint pink colour. It must be remembered that oxalate solutions attack glass, so that the solution should not be stored more than a few days. 

Hutchings, J., Colour, Appearance, Expectations and the Food Industry, Kluwer/Ple-num. New York, 2002. 

The longer it persists the more difficult it is to remove. So it is important to act immediately there are signs of algal growth. That first sign is most probably the greenish colour appearing in the pool water. 

PVC (200 mg) was dissolved in 20 mL THF and precipitated with 50 mL EtOH. Several drops of 0.1 % pyrocatechol violet solution were added to the heated filtrate until a blue colour appeared. This solution was titrated with 0.001 M EDTA until the change via green to yellow. In the presence of Mg, Ca, and Zn, Eriochrome Black was added before titration. 

Figure 9.10 Illustration of Lambert s law the absorbance A of a glass of juice is proportional to the optical path length /, so holding the glass against a white card makes its colour appear twice as intense because the path length has been doubled. The width of the beam here is proportional to its intensity...

A number of colours appear when the visible or white light falls on a compound. The colour produced depends or how the white light reacts with the compound. A number of cases may arise ... 

Figure 5 Polymerisation curve for a reaction carried out in the calorimeter (see Ref. [1], Experiment SGP 14). T0 = - 19 °C, [St] = 0.264, [HClO4]0=7.3 x 103M. When the colour appears and the rapid reaction sets in, [St] = 2.7 x 10"2M...

Note Soon after the addition of ammonium thiocyanate a white precipitate of silver thiocyanate is formed first and then a reddish-brown colour appears that fades out completely upon shaking thereby leaving a white precipitate of silver thiocyanate. The end-point is indicated by the appearance of a permanent faint reddish brown colour that does not vanish upon shaking. 

C. If a blue or blue-black colour appears in receiver 6 (explain its origin), lower the evaporation temperature of the carbon tetrachloride to 50 °C. Perform chlorination during 15 minutes. 

Compounds of Vanadium in Lower Oxidation States. Melt paraffin in a porcelain bowl or test tube. Put seven test tubes with stoppers onto a test tube stand. Pour 10 ml of a sodium vanadate solution into a small 50-ml flask, add 10 ml of a 20% sulphuric acid solution, and put in 8-10 small pieces of granulated zinc. Watch how the solution s colour changes. As a new colour appears, pour off 1-2 ml of the solution into each of three of the tubes. In one of them, pour melted paraffin over each layer of liquid with a new colour. Close the other two tubes with their stoppers and pour the following portion of the solution with a new colour into two empty tubes with stoppers. 

On treatment with phenylhydrazine, mercury fulminate undergoes reduction to free mercury. The phenylhydrazine changes colour from olive, grey (at the moment when mercury is set free) to reddish-brown. Several hours after the addition of alcohol and dilute sulphuric acid, a red-violet colour appears (Langhans [35]). This reaction may be used for the qualitative detection of mercury fulminate. 

A drop reaction for tervalent arsenic consists in treating a drop of the sample on filter paper with hydrochloric acid and a 0-5 per cent, aqueous solution of kairin (N-ethyl-8-hydroxytetrahydroquinoline hydrochloride) on adding a drop of aqueous ferric chloride and warming the test paper, a reddish-brown colour appears.3 The test is sensitive to 6 x 10 1° g. Mercury, lead and copper interfere. 

A solution of 4,4-dimethoxy-2,6-dimethylheptane (10 g, 53.5 mmol) and zinc chloride (0.72 g, 5.35 mmol) in ethanol (40 ml) was cooled to 0°C. H2S was bubbled through the solution during 30 min. After a few minutes a red colour appeared. The extraction was executed by partition between pentane and a 0.1 n HC1 solution. The organic solution was then washed with water, dried onmgS04 and concentrated. The crude material was purified by flash chromatography with pentane as the eluent, giving 2,6-dimethylheptanethione (2d) (5.09 g, 32 mmol, 60%). 

To a solution of LDA (40mmol) in THF (200ml) cooled at -45°C, methyl dithioacetate (3.92 ml, 40 mmol) was added dropwise. The yellow colour rapidly disappeared. The mixture was stirred for 5 min. 2-Cyclohexenone (3.92 ml, 40 mmol) was then added dropwise. A yellow colour appeared. The resulting mixture was stirred for 15 min. An aqueous solution of ammonium chloride was added and the mixture partitioned between ether and brine. The organic layer was washed with brine, dried with magnesium sulfate and concentrated. Methyl (cyclohexanone-3-yl)dithioacetate (3) (6.75 g, 33.4 mmol, 83%) was isolated by flash chromatography on silica gel using cyclohexane/ethyl acetate (9 1) as the eluent. 

To 10 c.c. of the milk arc added 15 c.c. of raw milk known to be genuine and 3 drops of aqueous 2% paraphenylenediamine solution (freshly prepared) a blue colour appears in presence of hydrogen peroxide. This reaction is highly sensitive. 

Procedure. From 3 to 5 drops of this reagent are placed on a quantity of the flour spread out and pressed flat, as in determining the colour (see 2, above) and any coloration noted. With bleached flour a pink or red colour appears after a few seconds or, in any case, before the lapse of a minute, whereas untreated flour is not coloured.1... 

Schiff s reaction. 10 c.c. of the alcohol (50%) and 4 c.c. of Schiff s reagent (see below r Determination) are shaken together in a test-tube and allowed to stand. The presence of aldehydes is shown by the appearance, either immediately or after some time, of a more or less intense red coloration, which should be observed after about 20 minutes. If no colour appears after this time, the liquid is free from aldehydes. 

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