Additivity of masking

Sodium diethyldithiocarbamate, (C2H5)2N CS S Na+. This reagent is generally used as a 2 per cent aqueous solution it decomposes rapidly in solutions of low pH. It is an effective extraction reagent for over 20 metals into various organic solvents, such as chloroform, carbon tetrachloride, and ethanol. The selectivity is enhanced by the control of pH and the addition of masking agents. 

Considerable inter-element interference effects have been reported on the actual hydride-forming step. Elements easily reduced by sodium borohydride (e.g. silver, gold, copper, nickel) give rise to the greatest suppressions. These interfering ions may be removed by the addition of masking agents that complex with them. 

Strictly specific procedures are unknown and interferences must be minimized by control of conditions (pH, addition of masking agents) according to the nature of the matrix. 

More advanced models try to estimate a time-dependent Signal-to-Mask-Ratio (SMR) for each band used in the coder. Because the knowledge about masking effects is limited at this time and because different theories about additivity of masking or the effects of tonality exist, there is no such thing as the correct psychoacoustic model . 

The determination error is smaller if the absorption of radiation is a consequence of the nature of the analyte itself, as with the coloured ions of transition metals. Conversion of the analyte into a form capable of absorbing radiation in proportion to its concentration requires some additional procedures (such as the use of a chromogenic reagent, pH adjustment, or addition of masking agents), that must be identical in the treatment of standard solutions and of the sample solution. 

Zinc dithionate dissolves in organic solvents, producing a red colour and can be extracted. Although dithizone is not a specific reagent for zinc ions, it is possible, by choosing appropriate conditions for the reaction (pH approximately neutral, addition of masking solution), to apply the method in a largely selective way. 

Silicate Molybdenum blue ICP-AES Spectrophotometry 0.14-1,4mg Si detectors for the determination of orthophosphate and total phosphorus Addition of masking agents... 

In Summary a-Hydroxyketones are available from addition of masked acyl anions to aldehydes and ketones. The conversion of aldehydes into the anions of the corresponding 1,3-dithiacyclohexanes (1,3-dithianes) illustrates the method of reverse polarization. The electrophilic carbon changes into a nucleophilic center, thereby allowing addition to an aldehyde or ketone carbonyl group. Thiazolium ions catalyze the dimerization of aldehydes, again through the transformation of the carbonyl carbon into a nucleophilic atom. 

The determination of several abuse drugs and illicit substances is frequently performed in urine because large volumes of samples can be available and collection is easy and not invasive. However, urine samples might be subjected to adulteration, through substitution, dilution, or addition of masking agents. A method clinically accepted and used to evidence possible adulterations or dilutions in urine samples is based on the evaluation of species typical of the urine composition, as, for example, the creatinine content. Also a urine sample of a tobacco smoker that does not contain cotinine can be suspected of substitution. 

While in most cases the color reactions used are not specific, numerous procedures can be made selective by the proper choice of pH and the addition of masking agents. Recent developments - in double-wavelength spectrophotometry have further increased the specificity of many direct methods. Separation and preconcentration techniques can be applied, but these impose a restriction on the detection limit based on the reproducibility and magnitude of the blank. Theoretical and practical considerations presently set the limit of detection on the most sensitive spectrophotometric methods at 5-20 ng. 

Unique chemistry is associated with the cyclopentenone all five carbon atoms can be functionalized, and the endo-methyl groups of the acetonide assure clean stereoselective addition of the alkenylcopper reagent from the convex side. The use of the acetonide group to control enolate regioselectivity and to mask alcohols should be generally applicable. 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

The chromium can be stabilized in a limited way to prevent surface fixation by addition of formate ions. The formate displaces the sulfate from the complex and masks the hydroxyl ions from forming the larger higher basicity complexes. This stabilization can then be reversed in the neutralization to a pH of about 4.0 and taimage becomes complete. This simple formate addition has decreased the time of chrome tanning by about 50% and has greatly increased the consistent quaHty of the leather produced. 

Isopropyl alcohol is also employed widely as a solvent for cosmetics (qv), eg, lotions, perfumes, shampoos, skin cleansers, nail poHshes, makeup removers, deodorants, body oils, and skin lotions. In cosmetic appHcations, the acetone-like odor of isopropyl alcohol is masked by the addition of fragrance (144). 

Flavor-Masking Deodorant. In addition to its use as a constituent of perfume compositions, vanillin is also useful as a deodorant to mask the unpleasant odor of many manufactured goods. As a masking agent for numerous types of ill-smelling mass-produced industrial products, particularly those of synthetic mbber, plastics, fiber glass, inks, etc, vanillin finds extensive use. It is often the most inexpensive material for the amount of masking effect it provides. Only traces are required for this purpose as the odor of vanillin is perceptible in dilutions of 2 x 10 mg/m of air. Cmde vanillin is acceptable for such purposes. 

Odour. This aspect is important in resins derived from natural sources. Rosins based on wood and gum rosin retain trace quantities of terpenes and have a piney odour. Tall oil rosins retain the typical sour odour of the rosin. Odour can be removed by steam sparging under vacuum before or during esterification of rosins. Addition of odour masks can also be done. 

It is therefore possible to determine cations such as Ca2+, Mg2+, Pb2+, and Mn2+ in the presence of the above-mentioned metals by masking with an excess of potassium or sodium cyanide. A small amount of iron may be masked by cyanide if it is first reduced to the iron(II) state by the addition of ascorbic acid. Titanium(IV), iron(III), and aluminium can be masked with triethanolamine mercury with iodide ions and aluminium, iron(III), titanium(lV), and tin(II) with ammonium fluoride (the cations of the alkaline-earth metals yield slightly soluble fluorides). 

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

Traces of many metals interfere in the determination of calcium and magnesium using solochrome black indicator, e.g. Co, Ni, Cu, Zn, Hg, and Mn. Their interference can be overcome by the addition of a little hydroxylammonium chloride (which reduces some of the metals to their lower oxidation states), or also of sodium cyanide or potassium cyanide which form very stable cyanide complexes ( masking ). Iron may be rendered harmless by the addition of a little sodium sulphide. 

In the back-titration small amounts of copper and zinc and trace amounts of manganese are quantitatively displaced from the EDTA and are complexed by the triethanolamine small quantities of cobalt are converted into a triethanolamine complex during the titration. Relatively high concentrations of copper can be masked in the alkaline medium by the addition of thioglycollic acid until colourless. Manganese, if present in quantities of more than 1 mg, may be oxidised by air and forms a manganese(III)-triethanolamine complex, which is intensely green in colour this does not occur if a little hydroxylammonium chloride solution is added. 

A-Acido imines (R R"C = N —X=0) like /V-acyl (X = CR) /V-sulfonyl [X = S(R)=0]2-7 or /V-diphenylphosphinoylimines [X = P(C6H5)2]3 are masked inline derivatives of ammonia. Compared to the imines themselves these activated derivatives are better electrophiles showing less tendency to undergo undesired deprotonation rather than addition of organometal-lics1812 The apparent advantages of these compounds have been exploited for asymmetric syntheses of amines, amides, amino acids and /J-lactams1-8 I6. 

TV-aluminum imines are another example of masked inline derivatives of ammonia. They are easily synthesized by partial reduction of nitriles with diisobutylaluminum hydride (D1BAL-H)6. Addition of lithium organic reagents to /V-aluminum iniines 7 derived from O-protected cyanohydrins 6 provides a-amino alcohols 8a and 8b in moderate yields and low to good diastereo-selectivities n 12. 

Addition of organometallics to hydrazones provides hydrazines1, which can be converted to the corresponding amines by hydrogenolysis. Hydrazones can therefore be regarded as masked imine derivatives of ammonia. This methodology has been utilized successfully for numerous stereoselective syntheses of amines and amine derivatives2-23. 

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