Citrate-stannous reagents were prepared in lyophilized form and each vial contains: 3.7 mg of dihydrated trisodic citrate, 5.5 mg of anhydrous dextrose, 2.0 mg of sodium chloride and 95mg of dihydrated stannous chloride (SnCl2.2H2O).
The stannous salt was previously standardized by titulation procedure using Ce(SO4)2 as previously described (1).
The assay calibration curves were prepared to the range of 0 to 20 ppm of SnCl2.2H2O. The standard solution of SnCl2.2H2O was prepared disolving 0.04g of the salt in 6.25 mL of concentrated HCL and diluted to 25 mL in a checked volumetric bottle with previously nitrogenated distilled water.
The spectrophotometric methodology employed is based in the colour development when Sn+2 reacts with sodium molibdate in the presence of potasium thiocianate in chloridric medium (2, 3, 4). The colour reaction was developed as follow: 4 mL of 3M HCl was introduced in a 10 mL checked volumetric bottle followed by 1 mL of 0.01M Na2MoO4.2H2O freshly prepared solution, 2 mL of 1.5 M KSCN solution and the ideal aliquot of the Sn+2 solution. The final volume was obtained with 3M HCl. The lyophilized reagent was reconstituted with 10 mL of previously nitrogenated 3M HCl and 4 mL of this solution were assayed as described to the standard solution. After 20 minutes of reaction, the absorbance of the mixtures were determined (Spectrophomether, U2010, Hitachi) at 460nm.
The stability of the coloured reaction was studied for 60 minutes. The sequence of reagents introduction in the reaction mixture and the influence of the kit constituints in the reaction were also studied. In the last case, 4 mL of the reconstituted lyophilized reagent were introduced in the standards preparations.
The Sn(II) was also determined by atomic absortion spectrophotometry (Spectrophotometer Z-5300, Hitachi). In this case, the standard curves were prepared in a range of 0 to 40 ppm of SnCl2.2H2O.