Contaminant yellowing

Allow the reddish-yellow solution to cool to about 40° and then filter off at the pump the polyene which has separated this filtration should be performed as rapidly as possible to avoid contamination with lead acetate, and a Buchner funnel of not less than 6 cm. diameter should therefore be used to avoid clogging the filter. Wash the crude production the filter with... 

The cft)ject of the intermediate flask is to trap any water which may distil with the amine this water is generally coloured yellow and if allowed to pass into the hydrochloric acid in the receiver will contaminate the product. 

To obtain crystalline perbenzoic acid, dry the moist chloroform solution with a little anhydrous sodium or magnesium sulphate for an hour, filter, and wash the desiccant with a little dry chloroform. Remove the chloroform under reduced pressure at the ordinary temperature whilst carbon dioxide is introduced through a capillary tube. Dry the white or pale yellow residue for several hours at 30-35° under 10 mm. pressure. The yield of crystalline perbenzoic acid, m.p. about 42°, which is contaminated with a little benzoic acid, is 22 g. It is moderately stable when kept in the dark in a cold place it is very soluble in chloroform, ethyl acetate and ether, but only shghtly soluble in cold water and in cold hght petroleum. 

Most of the final product producing recipes in this book will provide for the chemist to take up the final free base product in DCM. Usually the freebase oil in the DCM is dark. Used to be that Someone-Who-ls-Not-Strike (SWINS) would have to distill the freebase to get clear yellow oil before crystallizing because when SWINS used ether or ethanol as a crystallization solvent, the colored crap would contaminate the final product. But not with DCM. Even with the grungiest (well...not too grungy) freebase, the crystals that come out are pure snow. The DCM so strongly solvates the contaminants that none remain in the mass of crystalled final product. The filter cake is sooooo clean even in the darkest solvent ... 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

Chelerythrine crystallises from alcohol in colourless, prismatic leaflets, m.p. 207°, containing one molecule of alcohol. The alkaloid absorbs carbon dioxide from the air, becoming yellow. The solutions fluoresce blue when the alkaloid is contaminated with its oxidation product, which is formed by mere exposure of solutions to air. The salts, which are quaternary, are intensely yellow. The hydrochloride, B. HCl. HjO, forms citron-yellow needles, and the sulphate, B. H2SO4.2HjO, golden-yellow needles, sparingly soluble in water the platinichloride, B2. HaPtCl. ... 

Disulfur dichtoridc was added to an aqueous solution of ammonia to give a yellow precipitate of sulfur contaminated with S4N4 y. Pharm. Chim. 11, 315 (1835). 

In the crystallization of these hydrated salts from aqueous solutions it is essential that a low pH (high level of acidity) is maintained, otherwise hydrolysis occurs and yellow impurities contaminate the products. Similarly, if the salts are redissolved in water, the solutions turn yellow/brown. The hydrolytic processes are complicated, and, in the presence of anions with appreciable coordinating tendencies, are further confused by displacement of water from the coordination sphere of the iron. However, in aqueous solutions of salts such as the perchlorate the following equilibria are important ... 

Tannin-containing woods can also suffer from dark-brown stains produced by alkali, usually from a concrete or a detergent source. Conversely, fair-faced concrete can be similarly stained by the run-off water from unprotected hardwood surfaces. Some woods, e.g. afzelia, ayan and idigbo, give yellow contaminating dyes with alkaline detergents and should not be used in washrooms and kitchens. 

Hontela A, Dumont P, Duclos D, Fortin R. 1995. Endocrine and metabolic dysfunction in yellow perch, Perea flavescens, exposed to organic contaminants and heavy metals in the St. Lawrence River. Environ Toxicol Chem 14 725-731. 

Taste, odour, discoloration problems (yellowing, mystery contaminants)... 

HPLC-UV is a popular technique to analyse textile dyes extracted from polyester fibres [697], acidic dyes from wool fibres [698] and basic dyes from acrylic fibres [699]. HPLC provides better sensitivity and resolution than TLC [697-699]. GE-RPLC has been used for the determination of 18 disperse dyes (e.g. Navy D-2G-133, Orange CB, Yellow D-3R and Red D-2G) extracted from polyester [700]. Compared with the traditional TLC method, HPLC offers lower detection limits, better observation of contaminant peaks, and reproducible quantitative results. HPLC has also been used to determine azo dyes [701,702]. 

An automotive component supplier manufacturing a fuel delivery system module noticed contamination, appearing as whitish-yellow particles, in one unit. Some initial testing included both FTIR spectroscopy and SEM. However, additional testing was requested in order to independently and more accurately determine the source of the contamination. Preliminary testing indicated that the... 

Samples of three different polypropylene (PP) cups were submitted for analysis of contaminants found on their surfaces. Several cups that showed a yellow contaminant and both large and small cups with a green contaminant were... 

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