Masking reagents

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). 

For the addition of an organometailic compound to an imine to give a primary amine, R in RCH=NR would have to be H, and such compounds are seldom stable. However, the conversion has been done, for R = aryl, by the use of the masked reagents (ArCH=N)2S02 [prepared from an aldehyde RCHO and sulfamide (NH2)2S02]. Addition of R"MgX or R"Li to these compounds gives ArCHR"NH2 after hydrolysis. ... 

Buffer masking reagent - Dissolve 250 g of ammonium acetate and 15 g of EDTA, disodium salt, in 400 ml water. Carefully add 125 ml of glacial acetic acid. 

More recently, a somewhat similar assay principle has been reported (Gl) determining cortisol (Fig. 5B) as a model hapten, in which a polydentate ligand [multiply cortisol-substituted poly(L-lysine) with a high MW (418,400)] was used as the masking reagent (Fig. 10). First, the analyte (cortisol) and the polydentate ligand were simultaneously added to the wells on which rabbit anti-cortisol antibody had been immobilized (step i). After incubation (room... 

Generally, the selectivity can be improved by optimizing the reaction conditions, that is, by addition of buffers, masking reagents, etc. Nevertheless, many special reagents have been developed for very special applications in the last four decades, mainly by users of photometric methods, who had no or only limited access to modem sophisticated spectrometric methods. 

Foreman et al. [631] compared the direct method of the chelate formation with the preliminary ashing method for the analysis of beryllium in rat urine. A detailed study showed that both of the methods are satisfactory, whereas testing of column material and packings showed the best results for a PTFE column packed with SE-52. Down to 1 ng/ml of the element could be detected in urine with the use of an ECD and EDTA as a masking reagent and a 0.05 M benzene solution of trifluoroacetylacetone. 

In order to determine S(IV) in rain water, it is necessary to prevent the oxidation of S(IV) between sampling and analysis. The suppressive effect on the oxidation of S(IV) was investigated by the addition of EDTA (Ethylenediaminetetraacetate) or TEA (Triethanolamine) as masking reagents for FeJ+ and Mn. Table III shows the suppressive effect of EDTA or TEA at various pH values. The suppressive effect was not observed between pH 3 and 5, because neither EDTA nor TEA chelate with Fe3+ and Mnz+ at these pH values. EDTA and TEA were found to be very effective for the suppression of the oxidation of S(IV) in solutions having neutral and basic pH values. 

In general, the use of a specific or selective reagent as precipitant for the element is advisable as1 is the use of a masking reagent for interfering elements. In this way a single or double precipitation procedure often becomes sufficient. 

FIGURE 23-12 Schematic illustration of variation in distribution ratio as a function of pH, concentration of extracting reagent HL in the organic phase, concentration of masking reagent A in the aqueous phase, and concentration of an adduct former B in the organic phase. A, pH variable B, [HL] variable C, [A] variable D, [B] variable. (Adty>ted from Schweitzer. )... 

Figure 80 The structure of a sulfonated crown ether usable as an ion size selective masking reagent.

Because of the high selectivity and sensitivity of the postcolumn fluorescence detection of histidine with OPA, the present HPLC method is applicable to a specific and rapid assay of histidine in human semm, blood, and urine after simple pretreatment. A recent paper demonstrated that the postcolumn detection with OPA was applicable to the simultaneous assays of histidine and its major metabolites cis- and frawi-urocanic acids) in human stratum corneum. " The postcolumn detection system was also applicable to the flow injection analysis (FIA) method for the assay of histidine in semm and urine. The FIA method enabled us to determine histidine in blood after pretreatment of the sample with A-ethylmaleimide (masking reagent of glutathione).These methods are useful in the diagnosis of histidinanemia, one of hereditary metabolic disorders characterized by a virtual deficiency of histidine ammonia-lyase. 

Longquan, L., Cheng, W. and Yadong, L. (1997). Separation of cobalt and nickel by emulsion liquid membrane with the use of EDTA as masking reagent. J. Membr. Sci., 135, 173-7. 

Cu, Zn Serum UV-Vis 0.0—2.5 pg mL-1 Cu + Zn (mixed standard solutions) Sequential determinations/pulsed addition of a masking reagent for copper [21]... 

---