Boron-containing reagents determination

Various chromogenic reagents have been used for the spectrophotometric determination of boron in seawater. These include curcumin [108,109], nile blue [110], and more recently 3,5 di-tert butylcatechol and ethyl violet [111]. Uppstroem [108] added anhydrous acetic acid (1 ml) and propionic anhydride (3 ml) to the aqueous sample (0.5 ml) containing up to 5 mg of boron per litre as H3BO3 in a polyethylene beaker. After mixing and the dropwise addition of oxalyl chloride (0.25 ml) to catalyse the removal of water, the mixture is set aside for 15-30 minutes and cooled to room temperature. Subsequently, concentrated sulfuric-anhydrous acetic acid (1 1) (3 ml) and curcumin reagent (125 mg curcumin in 100 ml anhydrous acetic acid) (3 ml) are added, and the mixed solution is set aside for at least 30 minutes. Finally 20 ml standard buffer solution (90 ml of 96% ethanol, 180 g ammonium acetate - to destroy excess of protonated curcumin - and 135 ml anhydrous acetic acid diluted to 1 litre... 

Place a 5 ml aliquot or less in a 50 ml volumetric flask and make to 5 ml volume with H2S04 (3.6M) if less than a 5 ml aliquot is used. The aliquot should contain 0-100 fig of boron. Add 20 ml of chilled H2S04 (18M) and swirl. Then, by pipet, add 20 ml of carminic acid (0.92 g in H2S04 (18M)). Make to volume with H2S04 (18M) and determine the absorbance in 1 cm cells in a spectrophotometer at 605 nm with a reagent blank in the reference cell. Compare with a standard curve containing 0-100 fig of boron. 

The reagent blank can be used as an indication of the baseline Instrumental detection limit, when its standard deviation is multiplied by the appropriate constant. Data from this solution could be used to answer question 1 above, but in a number of cases cannot be used to answer the second question. The analytical blanks can be used as indications of method detection limits, wherein wet ash digestion or alkali fusion procedures are employed to dissolve the sample. In the case of the fusion blank, this analytical blank also serves as a specific type of matrix blank since the principal constituents in solution are lithium and boron. The concentrated iron and aluminum solutions represent solution matrices that contain the major constituent when the determination of trace elements in these matrices is of concern. These solutions can be used for answering question 2 above. In addition, these blanks also serve as analytical blanks since dissolution procedures were employed in their making. In some cases the differences in calculated detection limits among these solutions is significant."... 

When the ketone or the aldehyde contains a chiral centre, then the use of a chiral boron reagent can result in a matched or a mismatched pair. The two chiral groups will either both favour the same stereoisomer of the product, or will work in opposition to one another. Normally, the reaction is carried out first in the absence of the chiral reagent in order to assess the extent of stereoselectivity afforded by the chiral ketone (or aldehyde) alone. One or both enantiomers of the chiral boron reagent can then be used to promote the reaction and to determine the relative influence of the chiral groups. The matched pair enhances the stereoselectivity, whereas the... 

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