Beck, Mihály Tibor

English

Beck Mihály Tibor

University of Szeged

O N T H E C O L O R I M E T R I C D E T E R M I N A T I O N O F B O R O N By M. T. BECK Inst i tu te of Inorganic and Analytical Chemistry, The University, Szeged' (Received September 10, 1957) It has ' been established tha t the method suggested by MONNIER, RUSCONI: and WENGER cannot lead to reliable results: Namely, the boron content of the solution is . only in pa r t responsible for t he changes of extinct ion a n d they are strongly inf luenced by the buffercapaci ty of the sample. The pH of the medium has a very important role in analytical proce-dures based on complex formation. On the basis of our former work [1] we m a d e t h e MONNIER, RUSCONI a n d WENGER'S m e t h o d fo r d e t e r m i n a t i o n of boron [2] the subject of a critical examination. The principle of this method is that the ferric-sulphosalicylic acid com-plex is partially decolourized by fluoride ion and the increasing of absorbancy due to the effect of borax solution is measured: 2 Fe3+ + 2 A — S"" = 2(Fe A—S)3~" 2 (Fe A—S)3~" + 12 F1" = 2 (FeFc)3~ + 2 A— 2 (FeFG):!~ + 3 B3+ = 3 (BF4)1_ + 2 Fe3+ 2 F e 3 + + 2 A — S " ' = 2 (Fe A—S)3"". The authors mentioned in their paper that "the colour is a function of the pH value, and it is necessary therefore, to work to a definite hydrogen ion concentration. With this object , in view we h^ye added phenolphtalein to the borax solutions, followed by hydrochloric acid, 2N, until the indicator is just decolourized." However, it is evident that this procedure cannot eliminate the change of pH during the determination, because the pH of ferric-sulpho-salicylic acid prepared according to the instruction is 1,67. Consequently, it follows that the change of absorbancy can not be at-tributed to the effect of the boron alone, but the boron and the change of pH are jointly responsible for it. The effect of pH and concentration of boron on the absorbancy of ferric-sulphosalicylic acid-NaF system was studied. The colorimetric measure-ments were carried out with Pulfrich photometer, for the pH determinations Radiometer PHM Type 22 instrument was used. First of all we took down — following the authors' instructions — the calibration curve, and the change of pH was determined simultaneously. As from the paper cited it is. "90 M . T . B E C K not clear that the original calibration curve at what wavelength was taken •down, the measurements were carried out with three filters: S42; S47 and Sr<1. Table I contains the result of these experiments. Table I ml 0,2 o/„ borax pH Absorbancy S4 , S47 . S50 0 3,50 0,189 0,166 0,149 4 3,73 0,481 0,413 0,355 6 3,88 0,583 0,607 0,442 8 3,90 0,655 0,681 0,520 Further, we carried out analogous experiments for studying the effect •of pH. In these experiments instead of 0,2 per cent borax solution 0,2 per cent NaHC0 3 solution was applied the pH of which was equal to that of the borax solution. The data are summarized in Table II. Table il ml 0,2 0/0 NaHCO:i pH Absorbancy S42 s47 S«, 0 3,50 0,189 0,166 0,149 3 4,00 0,312 0,335 0,241 5 4,78 0,981 1,180 0,780 8 5,89 2,950 2,950 2,340 Comparing the data of the tables it is evident that the boron determi-n a t i o n a c c o r d i n g to MONNIER, RUSCONI a n d WENGER'S m e t h o d c a n n o t g i v e reliable results because for the change of absorbancy observed the boron content of the solution is partly responsible, and these are significantly influ-enced by the pH of the final solution which is determined by the buffer •capacity of the solution examined. An other inconvenient problem, discernible from the data of tables, undér the above conditions the indicator properties of ferric-sulphosalicylic acid complex has to be taken in consideration as well. Experiments were carried out at constant pH, however, the changes of absorbancy observed were so slight that an advantageous method on the basis, of principle suggested by the authors is not to be expected. References ¡[1] Szabó, Z. G., M. T. Beck: Anal. Chem. 25, 103 (1953). ,[2] Monnier, D., Y. Rusconi, P. Wenger: Anal. Chim. Acta 1, 13 (1947). 

On the colorimetric determination of boron

http://acta.bibl.u-szeged.hu/23479/1/phys_chem_003_089-090.pdf

On the colorimetric determination of boron

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