Color development browning

Dissolve a few drops of nitromethane in 10% sodium hydroxide solution. Add a few crystals of sodium nitrite and shake. Now add dilute sulphuric acid drop by drop. A brownish-red coloration develops, but fades again when an excess of acid is added. The sulphuric acid has thus liberated nitrous acid, which has in turn reacted with the nitromethane to give a nitrolic acid, the sodium salt of which is CH3NO2 + ONOH = CH(N02) N0H + HgO reddish-brown in colour, probably owing to mesomeric ions of the type ... 

The MauIg Color Reaction. The procedure for this test consists basically of three sequential treatments of lignified material with 1% potassium permanganate, 3% hydrochloric acid, and concentrated ammonium hydroxide. A red-purple color develops for hardwoods and a brown color... 

After drying in a stream of cold air the chromatogram is immersed in the reagent solution for 1 s and heated to 70 —75°C for 3 — 5 min (until the color develops optimal intensity). Red-colored zones are usually formed on a white background occasionally, however, yellow to brown chromatogram zones, which gradually fade, are formed on a white to pale yellow background [1],... 

Detection and result The developed chromatogram was dried in a stream of cold air, immersed in the reagent solution for 1 s and heated to 80 °C for 20 — 30 min (until optimal color development occurred). Yellow to brown-colored zones were produced on a pale yellow-colored background these were suitable for quantitative analysis. The detection limits for cholesterol hRf 20 — 25) and coprostanol 25 — 30) were a few nanograms per chromatogram zone. 

Consider what happens when a sample of N204, a colorless gas, is placed in a closed, evacuated container at 100°C. Instantly, a reddish-brown color develops. This color is due to nitrogen dioxide, NO2, harmed by decomposition of part of the N2O4 (Figure 12.1, p. 324) ... 

Note When combined with thin-layer chromatographic separation the reagent provides a specific detection method for nitrate and nitrite. The color development is often completed within a few minutes on silica gel plates. In the absence of ammonia vapor traces of oxides of nitrogen in the laboratory atmosphere can slowly cause the background to become reddish-brown. The simultaneous presence of the following ions in the chromatogram zones interferes with the detection of nitrate/nitrite I , 10J, IO4, MoO and H2PO2. 

A bright emerald green color (usually) develops as the addition proceeds. If the iodine is impure, a brown color develops with no decrease in yield. 

To perform the ninhydrin test a solution of ninhydrin in ethanol or acetone (0.2% w/v) is sprayed lightly over the plate. Brown to purple colors develop slowly (within 1-2 min) upon heating to 60-90 °C. The spray is stable for several weeks. 

Konovaloff Reaction. A color test for primary or secondary nitro-compds. Samples are treated with Na hydroxide soln, the salt formed is extrd with w, and ether is added to this extr. Upon dropwise addition of ferric chloride, a red to reddish-brown color develops in the ether... 

When a sample of N2O4, a colorless and poisonous gas, is placed in a closed container, a reddish-brown color develops due to the presence of nitrogen dioxide, NO2. As soon as some NO2 is formed, the reverse reaction can occur simultaneously with the forward reaction. 

Bromine.—Of the three halogens, chlorine, bromine, and iodine, bromine has the least eventful history. Its elemental nature and its relation to chlorine and iodine were recognized from the very first. While studying the mother-liquid which remains after the crystallization of salt from the water of the salt-marshes of Montpellier, A. J. Balard was attracted by the intense yellow coloration developed when chlorine water is added to the liquid. A. J. Balard digested the yellow liquid with ether decanted off the supernatant ethereal soln. and treated this with potassium hydroxide. The colour was destroyed. The residue resembled potassium chloride but unlike the chloride, when heated with manganese dioxide and sulphuric acid it furnished red fumes which condensed to a dark brown liquid with an unpleasant smell. 

Heavy Metals. — On dissolving 1 gm. of potassium sulphocyanate in 20 cc. of water, and adding to the solution a few drops of ammonium sulphide solution, no precipitate should form, nor should a brown or green color develop. 

When alcoholic solutions of hydroquinone and quinone are mixed, a brown-red color develops and a green-black 1 1 complex crystallizes that is known as quinhydrone. This substance is a charge-transfer complex (Section 24-6C), with the diol acting as the electron donor and the dione as the electron acceptor. Quinhydrone is not very soluble and dissociates considerably to its components in solution. 

As with the Basic Protocol, this method can be used to assess latent DPO activity by addition of SDS. This simple assay system may also be used to investigate the effect of DPO inhibitors by adding suitable quantities of test compound dissolved in buffer (Ferrar and Walker, 1996, 1999). However, it must be remembered that, in this assay system, the prevention of color development (enzymic browning) may be due to chemical reactions between the inhibitor and the reaction products preventing the formation of the colored end-product, rather than the actual inhibition of the enzyme. This problem may be avoided by use of the 02-electrode assay (see Basic Protocol). 

Pyrogallol-sodium phosphate solution Dissolve 10 g sodium dihydrogen phosphate (NaH2P04 H2O) and 10 g of pyrogallol in 100 ml of double-distilled water, bubble with nitrogen to dissolve. Prepare daily and discard when brown coloration develops. Add 10 ml of this solution to the gas washing column. 

Frying temperature was found to be the crlterial parameter that determined the flavor quality in Chinese pork bundle. Cooked meat aroma increased as the heating temperature varied from 134°C to 172°C., as shown in Figure 3. Below 130°C neither cooked meat aroma nor brown color developed. Slightly higher temperatures have been reported for the optimum flavor formation in fried potato chips at 180°C (2), and roasted beans at 200°C (3). 

At room temperature, methylene trimethylarsorane (73) is a colorless crystalline compound, mp 33°C, which sublimes in a vacuum at 20°C/0.1 mm Hg. It is rapidly decomposed above 33°C trimethylarsine and polymethylene are the main products formed. Blue and brown colors develop and some gas is evolved, predominantly ethylene. With trimethylphos-phine, a methylene transfer reaction takes place yielding trimethylarsine and methylene trimethylphosphorane ... 

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