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ANALYST, FEBRUARY 1991, VOL. 116 199 Determination of Humic Acid and Iron(iii) by Solid-state Spectrophotometry to Study Their Interactions Kunio Ohzeki, Miyoko Tatehana, lshoshi Nukatsuka and Ryoei lshida Department of Chemistry, Faculty of Science, Hirosaki University, 036 Hirosaki, Japan A simple and sensitive method for the determination of humic acid has been developed. The method is based on the adsorptive enrichment of humic acid using a finely divided anion-exchange resin, collection of the resin on a membrane filter by filtration as a circular thin-layer, and direct measurement of the absorbance of the resultant thin-layer of resin by densitometry at 470 nm. Up to 80 pg of humic acid in 100 ml of sample solution can be determined, the limit of detection being 1.3 pg.The effect of iron(ll1) is masked with ethylenediaminetetraacetic acid (EDTA). Iron(ll1) is also determined by densitometry at 600 nm after enrichment on the thin-layer of resin as a complex with ammonium pyrrolidine dithiocarbamate (ammonium pyrrolidin-I-yldithioformate) (APDC), the limit of detection being 0.06 pg of iron(1ii). In the presence of humic acid, the blank value is obtained by masking the iron(iii) as the EDTA complex, and calculating the net absorbance due to the APDC complex. The methods have been used to investigate the effect of humic acid on the formation of filterable iron(il1) species, which can pass through a 0.45 pm membrane filter. The possibility of characterizing humic acid based on the formation of a complex with iron(i1i) has been shown. Keywords: Humic acid determination; iron(iii) determination; densitometry; interaction between iron(///) and humic acid; filterable iron species The importance of speciation in a water sample for under- standing the toxicity, bioavailability, bioaccumulation and transport of a particular element has been previously evalu- ated.' Filtration with a 0.45 pm membrane filter is a universally applied first step to separate the particular element into the fractions termed dissolved and particulate.1.2 Humic acid has been recognized to play an important role in the mobilization, transportation and immobilization of metals in water.3 As iron is the most abundant heavy metal in water, the interactions between iron and humic acid have been widely investigated.3-9 While the association of colloidal particles of iron hydroxide with humic substances and the existence of iron hydroxide coated with humic substances have been widely accepted, 1-7 the formation of dissolved complexes or chelates between the functional groups of the humic substances and iron has also been accepted as significant in order to keep the iron in solution.3-5 The solubility of humic acid-iron com- plexes has been found to be dependent on the humic acid : iron ratio.6 To understand the speciation of iron in water, it is important to study the effect of humic acid on the formation of filterable iron species which pass through a 0.45 pm membrane filter .The aims of this work were to develop simple methods for the determination of humic acid and iron(ii1) at the hundred ppb level and to carry out fundamental studies of the interactions between humic acid and iron(rr1).Experimental Apparatus A Shimadzu CS-920 chromatoscanner was used for the densitometric measurement of the resin-phase absorbance. The apparatus has the capacity to transform convex calibra- tion graphs into linear graphs. A Hitachi-Horiba Type M-711 pH meter was also used. Toyo KG-25 and KG-47 filter holders with Toyo 0.45 pm membrane filters of cellulose nitrate and 0.40 pm Nucleopore filters were used for the filtration of the sample solution. A Toyo KG-25 filter holder with Toyo 0.65 pm membrane filters was used to collect the anion-exchange resin particles for the preparation of the thin-layer of resin. Reagents Unless stated otherwise, all reagents used were of analytical- reagent grade.Water, purified using an osmotic membrane, was distilled twice and used. 10 Iron(1rr) standard solution, 1000 ppm, pH 1 . Prepared from ammonium iron(ii1) sulphate dodecahydrate. A working solution, containing 10 pg ml-1 of iron, was prepared by dilution of the standard solution with dilute sulphuric acid, maintaining a pH of about 1. Ammonium pyrrolidine dithiocarbamate (ammonium pyrrolidin-1 -yldithioformate) (APDC) solution, 0.4% mlv. Sodium acetate solution, 30% mlv. Sodium perchlorate solution, 4 mol dm-3. Humic acid solutions. Humic acids obtained from Wako Pure Chemical and Nacalai-tesque were used as received. The sodium salt of humic acid obtained from Aldrich was converted to the acid form by precipitation from 0.1 mol dm-3 hydrochloric acid. About 0.1 g of each humic acid was accurately weighed and dissolved in 50 ml of a 0.1 rnol dm-3 sodium hydroxide solution using sonication for 30 s.The pH was adjusted to between 6 and 7 by the addition of hydrochloric acid and the solution was then filtered using a 0.45 pm membrane filter of 47 mm diameter. As the filters clogged rapidly, four or five were required to filter 50 ml of the solution. The resultant filtrate was diluted to 500 ml with water. The used membrane filters were air-dried and the amount of residue on each weighed. The concentration of humic acid in the filtrate solution was determined by subtracting the residue on the filters." The concentrations of humic acid solutions from Wako, Nacalai-tesque and Aldrich were 142, 120 and 158 ppm, respectively. Anion-exchange resin suspension (ARS), 7.8 pequiv ml- I.Prepared from a macro-reticular type Amberlyst A-27 resin in the chloride form according to the method reported.lO General Procedure for the Determination of Humic Acid in the Presence of Iron( 111) A 45 ml aliquot of an acidified sample solution (pH about 1.2) containing less than 80 pg of humic acid and less than 5.0 pg of iron(ri1) is placed in a 100 ml beaker. A 0.5 ml portion of 10 mmol dm-3 ethylenediaminetetraacetic acid (EDTA), 1.25 ml of 4 mol dm-3 sodium perchlorate and 2 ml of 30% sodium200 ANALYST, FEBRUARY 1991, VOL. 116 acetate solutions are added successively. The final volume is adjusted to 50 ml with water. The pH of the resultant solution is 4.7.Then 1.3 ml of ARS are added to the solution and the mixture is stirred for 5 min. The resin particles are collected on a 0.65 ym membrane filter by filtration under suction and a circular thin-layer of approximately 17 mm diameter and 0.03 mm thick is prepared. The wet membrane filter holding the thin-layer of the anion-exchange resin is placed on a white plastic plate in the densitometer. The integrated absorbance of the humic acid concentrated in the resin phase is measured, with the use of the linearizer, at 470 and 600 nm by scanning the thin-layer over an area of 24 x 30 mm2. The absorbance measured at 470 nm is used for the determination of humic acid; the absorbance measured at 600 nm is used as the blank value for the determination of iron(m), as described below.General Procedure for the Determination of Iron(m) With APDC in the Presence of Humic Acid The procedure for the determination of iron(Ir1) as the APDC complex is the same as for the determination of humic acid, except that a 0.5 ml portion of 0.4% APDC is added to the sample solution in place of the EDTA solution, and the absorbance of the resultant coloured thin layer of the anion-exchange resin is measured at 600 nm. The net absorbance due to the iron(n1)-APDC complex is determined by subtracting the blank value obtained in the presence of EDTA, as described above. General Procedure to Study the Effect of Humic Acid on the Formation of Filterable Iron Species A 45 ml aliquot of the sample solution containing 500 yl of the 10 ppm iron(Ir1) solution and 1.25 ml of a 4 mol dm-3 sodium perchlorate solution is placed in a 100 ml beaker and various amounts of humic acid are added up to about 80 yg.The solution is then stirred for 1 min using a magnetic stirrer. A 0.3 ml aliquot of a 30% sodium acetate solution is added and the final volume adjusted to 50 ml with water, the pH value of the resultant solution being 5.6. After stirring for 1 min the solution is filtered through a 0.45 pm membrane filter of cellulose nitrate under suktion from a water pump. The filter is then removed from the filter support and the support washed with 0.5 ml of 6 mol dm-3 hydrochloric acid and 5 ml of water. The washings are combined with the filtrate and the whole is allowed to stand for 30 min. Duplicate experiments are carried out to obtain a set of filtrates and then each filtrate is subjected to the determination of humic acid and iron(II1).Results and Discussion Determination of Humic Acid Humic acid has been determined by various methods, including ultraviolet/visible spectrophotometry,3,6.8,9,12-14 fluorimetry,l5J6 chemiluminescence, 17 and thermal-lens spec- trometry.18 Of these, spectrophotometry has been the most widely used. As there are similarities in the absorption spectra of humic acid and iron(rI1) hydroxide, masking the effect of the iron12 or removal of the iron altogether14 is usually required before the determination of humic acid is possible. The simultaneous determination of iron and humic acid is carried out by measuring the absorbances at different wavelengths.19 In this paper, a method of determination based on the enrichment of humic acid with the use of a finely divided anion-exchange resin followed by the densitometric measure- ment of the resin-phase absorbance is proposed.The details of the method are studied according to the general procedure described under Experimental, both with and without the addition of iron(1n). Absorption spectra Absorption spectra of humic acid collected in the resin phase showed an absorption maximum at 470 nm, as shown in Fig. 1, whereas, the absorbance of a humic acid solution obtained by the usual spectrophotometric method simply decreased with increasing wavelength in the visible range, no maximum being found in the spectrum.3.19 Effect of the amount of ARS The effect of the amount of ARS was investigated according to the general procedure, with the addition of different amounts of ARS of up to 2.0 ml.With increasing amounts of added ARS, the absorbance of the resultant thin-layer of resin increased sharply and reached a maximum at 0.5 ml of ARS. The absorbance then gradually decreased with increasing amounts of added ARS; this was probably because the resultant thin-layer of resin became thicker than the effective optical path length. Effect of p H The absorbance of humic acid collected in the resin phase from solutions with various pH levels was found to increase slightly with increasing pH in the range studied (1.5-7.5) as reported.13 The absorbance, however, was almost constant in the pH range 3.4-5.8. The pH of the solution for the determination of humic acid was, therefore, fixed at pH 4.7 and the determination of iron(n1) with APDC was carried out at pH 4.7, as described later.0.4 1 400 500 600 Wavelengthhm Fig. 1 Absorption spectra of humic acid and iron(ii1)-APDC complex in the thin layer of the anion-exchange resin: A, humic acid from Wako, 71.0 pg; B, iron(m)-APDC complex, iron(m) 5.00 pg; C, resin blank 100 s Y a, C ([I e 8 50 a (D 0 0.1 0.2 0.3 0.4 0.5 Salt concentration/mol dm-3 Fig. 2 Effect of salt concentration on the fixation of humic acid to the thin-layer of the anion-exchange resin. Humic acid (Wako), 71.0 pg; anion-exchange resin, Amberlyst A-27,lO. 1 pequiv (1.3 ml of ARS of 7.8 pequiv ml-1); sample volume, 50 ml. A , Sodium chloride: B, sodium perchlorate. The absorbance of the thin-layer of resin prepared without the addition of the salt was taken as 100%ANALYST, FEBRUARY 1991, VOL.116 201 Effect of sodium perchlorate The effect of salt concentration on the resin-phase absorbance was investigated using sodium chloride and sodium perchlor- ate. While a constant absorbance was observed in the presence of sodium chloride in the range from 0.05 to 0.5 rnol dm-3, as shown in Fig. 2, it decreased markedly with increasing amounts of added sodium perchlorate. Consequently, it was clear that the fixing of humic acid was suppressed in the presence of the perchlorate ion. Although the adsorption of humic acid on the anion-exchange resin of styrene-divinyl- benzene copolymer was reported to be irreversible ,12,21 the above results indicated that there was competition between the perchlorate ion and the anion of humic acid for the ion-exchange sites.The recovery could be improved by increasing the amount of added ARS to more than 1.3 ml, however, the use of large amounts of resin could result in the following disadvantages: firstly, a longer time period for the collection of the resin particles by filtration; and secondly, the decrease of the absorbance of the resultant thin-layer of resin with increasing thickness, as already described. As the interactions between iron(iii) and humic acid were investi- gated in the presence of 0.1 rnol dm-3 sodium perchlorate, the calibration graph for the determination of humic acid was prepared in the presence of 0.1 rnol dm-3 sodium perchlorate in order to compensate for the recovery loss of humic acid due to the perchlorate ion.Effect of sample volume The recovery of humic acid was examined for various volumes of sample solution ranging from 30 to 100 ml, all containing 71.0 pg of humic acid from Wako in the presence of 0.1 rnol dm-3 sodium perchlorate. A constant recovery of the humic acid was obtained from each of the different sample volumes. Effect of iron(ii1) The absorbance of the thin-layer of rcsin prepared from sample solutions containing a known amount of humic acid and various amounts of iron(1ii) was found to increase with increasing amounts of added iron(ii1); this is probably owing to the formation of iron(ii1) hydroxide and the iron(ri1) complex with humic acid. 19 The effect of iron(iii) was successfully masked by the addition of EDTA.The resultant iron(m)- EDTA complex was not retained by the anion-exchange resin in the presence of 0.1 mol dm-3 perchlorate ion. Calibration graph A calibration graph was prepared for the determination of humic acid using the general procedure without the addition of iron(iii). A graph showing good linearity was obtained for levels of up to about 80 pg of each humic acid. The regression lines of the calibration graph for the determination of humic acid from Nacalai-tesque, Wako and Aldrich were: y = 1293x + 7558; y = 1125x + 7646; and y = 1056x + 7394, respectively, where y is the integrated absorbance in arbitrary units including the blank value due to the anion-exchange resin, and x is the amount of humic acid in pg.The relative standard deviation (RSD) for 42.6 pg of humic acid from Wako was 3.2% (n = 5) and the limit of detection (LOD) was 1.3 pg, based on three times the standard deviation of the blank value. As stated above, the recovery of humic acid was independent of the sample volume for values of up to 100 ml, accordingly, the regression equation was valid for sample volumes of up to 100 ml. Determination of Iron(u1) With APDC Ammonium pyrrolidine dithiocarbamate has been widely used as a solvent extraction reagent and a spectrophotometric reagent for copper and other metals.22,23 Iron(1Ir) was reported to form a black precipitate of Fe(PDC)3 on reaction with APDC.22 The determination of iron with APDC by thin-layer spectrophotometry has already been reported,24 but the details of the method were not revealed.The details of the method are according to the general procedure described above, both with and without addition of humic acid. Absorption spectrum The absorption spectrum of the iron(ii1)-APDC complex in a thin-layer of the anion-exchange resin showed a small maximum at about 600 nm, while the absorbance spectrum of the blank thin-layer decreased with increasing wavelength, as shown already in Fig. 1. Accordingly, the subsequent measurements were carried out at 600 nm. Effect of amount of ARS The effect of the amount of ARS on the absorbance of the iron(ii1)-APDC complex in the resultant thin-layer of the anion-exchange resin was examined with the addition of various amounts of ARS up to 2.0 ml. Although the iron(Ir1)-APDC complex could be collected on the membrane filter directly, without the addition of the anion-exchange resin, reproducible absorbance was only obtained by fixing the complex in the thin-layer of the anion-exchange resin.The resin-phase absorbance increased with increasing amounts of added ARS, up to 0.5 ml and then gradually decreased. The increase in the absorbance with increasing amounts of added ARS, up to 0.5 ml, was probably because the optical path-length would be increased with the increasing thickness of the resultant thin-layer. The decrease in the absorbance with increasing amounts of added ARS, above 0.5 ml, was probably because the resultant thin-layer became thicker than the optical path-length. These tendencies are characteristic of absorption measurements by densitometry.When the absorb- ance is measured with the use of a conventional spectro- photometer it increases with increasing thickness of the thin-layer. Effect of p H The effect of pH on the absorbance of the iron(ir1)-APDC complex was studied according to the general procedure described above, in the pH range 1.3-6.9. The absorbance of the complex in the thin-layer of the anion-exchange resin was found to be constant and maximum in the pH range 3.0-5.5, as was observed in the extraction of the iron(ii1)-APDC complex with 2,6-dimethylheptan-4-one.25 Effect of sodium perchlorate The absorbance of the iron(ii1)-APDC complex in the thin- layer of the anion-exchange resin showed a constant and maximum value in the presence of sodium chloride or sodium perchlorate in the range 0.05-1 .O rnol dm-3.Effect of sample volume The effect of sample volume on the fixing of iron(iii), as the APDC complex, to the anion-exchange resin was examined with various sample volumes ranging from 20 to 100 ml. The recovery of iron was independent of the sample volume studied. Hence, the calibration graph was valid for sample volumes of less than 100 ml. The sensitivity of the densito- metric determination of iron with bathophenanthrolinedi- sulphonate (BPS)26 is higher than with the proposed method, but the reduction of iron(iii) to iron(1i) is essential before the colour producing reaction with BPS can occur. Effect of foreign ions The effect of iron(ii), copper(ii), nickel(ii), cobalt(ii) and zinc(i1) on the determination of 3.0 pg of iron(ii1) was examined.The results are summarized in Table 1. Iron(rr) was202 ANALYST, FEBRUARY 1991, VOL. 116 Table 1 Effect of foreign ions on the determination of iron(i1i). Amount of iron(iir), 3.00 pg; sample volume, 50 ml Amount Ion added/ yg Fe"t 3.00 Cu" 3.00 1.00 Ni" 3.00 1.00 CO" 3.00 1.00 Zn" 6.00 3.00 Iron( 111) found*& 2.99 2.86 2.98 2.81 3.08 3.03 2.98 3.08 2.95 Error (% ) -0.3 -4.6 -0.7 -6.3 2.7 1 .o -0.7 2.7 -1.7 * Results of duplicate determinations. t In the presence of hydroxylamine hydrochloride. Table 2 Effect of humic acid on the determination of iron(1ii) Amount of Humic acid Number of iron(rrr)/yg added*/yg Iron found/yg experiments 5.00 2.00 14.2 2.06 42.6 2.00 71.0 2.08 Mean 2.05 SDt 0.21 14.2 5.10 42.6 5.11 71 .O 5.02 Mean 5.08 SD 0.09 * Humic acid from Wako.t SD = standard deviation. found to react with APDC in a similar manner to iron(iii). Consequently, in the presence of iron(i1) the total amount of iron can be determined. A small negative error was observed in the presence of the same amount of copper(i1) and nickel(ii), however, the concentrations of these ions in common water samples are much lower than that of iron(111).27 Effect of humic acid In the presence of humic acid, the sum of the absorbances from the iron(ii1)-APDC complex and humic acid was measured at 600 nm. The absorbance of humic acid was measured with the use of another aliquot of the sample solution after masking the iron(ii1) with the EDTA complex and the difference calculated. The effect of humic acid was thus effectively eliminated, as shown in Table 2.Calibration graph The calibration graph for the determination of iron(rri) was prepared according to the general procedure without the addition of humic acid. The calibration graph showed good linearity, the correlation coefficient ( r ) being 0.9998 in the concentration range 1.0-5.0 pg of iron(il1). The regression equation of the calibration graph was y = 21878~ - 4774, where y is the net value of the integrated absorbance in arbitrary units and x the amount of iron(Ir1) in pg. For the determination of less than 1.0 pg of iron(1ri) the regression equation of y = 17969~ was used. The RSD for 4.0 pg of iron(iii) was 2.4% ( n = 5 ) and the LOD was 0.06 pg, based on three times the standard deviation of the blank value. Absorption of Iron(m) on the Membrane Filter Effect of p H on the formation of iron(iii) hydroxide The effect of pH on the formation of the filterable iron(ii1) species was investigated by the use of a 0.45 pm membrane fitter of cellulose nitrate.The solution pH was adjusted with 100 C .- C 2 - 0 2 4 6 8 10 12 14 PH Fig. 3 Effect of pH on the adsorption of iron on to a 0.45 pm membrane filter of cellulose nitrate from 50 ml of solution containing 5.00 yg of iron(m) and 0.1 mol dm-3 sodium perchlorate. Solid line indicates the theoretical curve for the formation of iron(m) hydroxide. The following equilibria and the stability constants were used for the calculation, the ionic strength being 0.1 mol dm-320 Fe(OH)3(s) = Fe(OH)2+ + 20H- log K S I = - 26.16 Fe(OH)3(s) = Fc(OH)~+ + OH- log Ks2 = - 16.74 Fe(OH)3(s) + OH- = Fe(OH)?- 2Fe(OH)3(s) = Fe2(OH)24+ + 40H- log K = - 50.8 log Ks4 = -5 100 0 2 4 6 8 10 12 PH Fig.4 Effect of pH on the fixing of the humic acid and iron on to the 0.45 ym membrane filter of cellulose nitrate. Amount of iron(iii), 5.00 pg; amount of humic acid (Wako), 14.2 yg; sample volume, 50 ml. A, Adsorption of iron(m) in the presence of humic acid; B, adsorption of humic acid in the absence of iron(iI1); C, adsorption of humic acid in the presence of iron(m) the use of sodium hydroxide and dilute hydrochloric acid solutions. The amount of iron(1n) in the filtrate was deter- mined by the proposed method and plotted against the pH of the filtrate, as shown in Fig. 3. Iron(iri) was almost quantita- tively collected on the membrane filter as a pale yellow precipitate from solutions in the pH range of about 6-10.The results of the calculation to estimate the effect of pH on the formation of iron(r1r) hydroxide are also illustrated in Fig. 3. There were good similarities between the pH dependence of the formation of iron(m) hydroxide and the adsorption of iron(iii) on to the membrane filter. Effect of pH on the adsorption of humic acid on the membrane filter The effect of pH on the adsorption of humic acid on to the membrane filter of cellulose nitrate was examined both in the absence and presence of iron(ii1). The solution pH was adjusted with the use of hydrochloric acid, sodium acetate, ammonium acetate and sodium hydroxide solutions. Humic acid was not retained by the membrane filter from.solutions with a pH of above about 6, as shown in Fig.4. In the presence of iron(iiI), however, some of the humic acid became fixed on to the membrane filter together with iron(1ii) above about pHANALYST, FEBRUARY 1991, VOL. 116 203 4.5. As the solution pH increased above 8, most of the humic acid added appeared in the filtrate together with iron(iii). When each sample solution of different pH was allowed to stand for 20 min before the filtration, no significant difference was found in the pH dependence of the formation of the filterable iron species in the presence of humic acid. It was safely concluded that there are strong interactions between iron(ii1) and humic acid above about pH 4.5. Effect of humic acid concentration The effect of the concentration of humic acid on the formation of the filterable iron(ri~) species was examined at pH 5.6 and 8.4, respectively.The results obtained with the use of humic acid from Nacalai-tesque are shown in Fig. 5. Until the added amount of humic acid reached a certain level, both iron and humic acid were not found in the filtrate, however, after that, the amount of filterable iron(iii) species was found to increase with increasing added amount of humic acid and then reached a constant and maximum value. Almost the same results were obtained at pH 8.4 except that the plot was shifted to the left of that obtained at pH 5.6. As it was postulated that the iron found in the filtrate was accompanied by a fixed amount of humic acid (as shown in Table 3) it was expected that the complexation between iron(111) and humic acid occurred in a fixed molar ratio.Assuming the formation of a 1 : 1 complex, the relative molecular mass of humic acid could be calculated, as shown in Table 3, in which the results obtained with humic acids from Wako and Aldrich are also included. Among the 5.0 4.0 0-l 5 . w 2" 3.0 - .- Lc C -u .- : 2.0 Lc S 2 - 1 .o 0 60 0-l 3. . 4- 40 Y- C TI S 3 0 .- Lc z 20 .- s I 0 20 40 60 Humic acid added/yg Fig. 5 Effect of humic acid concentration on the formation of the filterable iron(iI1) species: A, the maximum amount of iron(m) in the filtrate; B, humic acid in the filtrate which is expected to be complexed with iron(1Ii) of A. 1 and 3, pH 8.4; and 2 and 4, pH5.6. Initial amount of iron(In), 5.00 pg; sample volume, 50 ml; ionic strength, 0.1 mol dm-3; and membrane filter.0.45 pm cellulose nitrate humic acids examined, that of Aldrich was found to be most effective for the formation of the filterable iron(1ri) species; in the presence of about twice the amount of humic acid over 5 pg of iron(iii), more than 90% of the iron was converted to the filterable species. As the average relative molecular mass of humic acid was estimated to be in the range 600-1000,28 the formation of iron-humic acid complexes with molar ratios of 2 : 1 and 3 : 1 could also be expected.3 Effect of other complexing agents The effect of complexing agents, other than humic acid, on the formation of the filterable iron(rI1) species was also examined at pH 5.6. As expected, nitrilotriacetic acid (NTA) was found to react with iron(ni) to form a filterable 1 : 1 complex, as shown in Fig.6. On the other hand, citrate, tartrate and pyrophosphate ions were much more effective than NTA, even when they were present at less than the stoichiometric amounts required for the formation of a 1 : 1 complex with iron(m), as considerable amounts of iron(m) were found in the filtrate. The effect of standing time on the formation of a filterable iron(i1i) species was examined with the use of 50 ml portions of a sample solution containing 5.0 pg (89.5 nmol) of iron(ii1) and 20 nmol of pyrophosphate. The amount of iron in the resultant filtrate was independent of standing time for up to 30 min. Accordingly, it was assumed that citrate, tartrate and pyrophosphate ions react with iron(ii1) to form filterable complexes in which the molar ratio of iron to each ligand 0 20 40 60 80 100 Ligand addedhmol Fig.6 Effect of complexing agents on the formation of the filterable iron(1ir) species: A, sodium pyrophosphate; B, sodium citrate; C, sodium tartrate; D, NTA; and E, sodium oxalate. pH 5.6, other conditions as in Fig. 5 Table 3 Results for the determination of iron and humic acid in 50 ml portions of sample solution containing 5.00 vg of iron(m) and various amounts of humic acid after filtration (see Fig. 5) Relative Amount of Amount of molecular Source of iron in humic acid Ratio of mass of humic acid PH A*/% in B*/pg B to A humic acid? Aldrich 5.6 4.53 12.0 2.6 142 8.4 4.54 11.4 2.5 140 Nacalai-tesque 5.6 4.00 19.0 4.8 268 8.4 4.29 20.3 4.7 262 Wako 5.6 3.25 18.4 5.7 31 8 8.4 3.96 22.1 5.6 313 * A and B areas found in Fig.5 . j- The formation of a 1 : 1 iron-humic acid complex was assumed.204 ANALYST, FEBRUARY 1991, VOL. 116 exceeded 1. However, oxalate, acetate and phosphate ions were not effective for the formation of filterable iron(ir1) species. More than 20 pmol of phosphate ion and 4.5 mmol of acetate ion were required for the conversion of 5.0 pg of iron(m) to the filterable species. Effect of Sample Volume on the Determination of Filterable Iron(m) The effect of sample volume on the determination of the filterable iron(1rr) species was examined in the presence of humic acid. The 50 ml portions of sample solution of pH 5.6, containing 5.0 pg of iron(1ii) and 35 pg of humic acid from Nacalai-tesque, were prepared and filtered successively through a 0.45 pm membrane filter of cellulose nitrate, with diameters of 25 and 47 mm, and a 0.4 pm Nucleopore filter with a 25 mm diameter under suction. The amount of iron(m) in each filtrate decreased throughout the number of filtration runs as shown in Fig.7. As already shown in Fig. 4, about 20% of the iron(Ii1) was fixed on to the membrane filter together with the humic acid from the 50 ml of sample solution, and consequently, the filter may be clogged with the particulate iron(m) species during successive filtrations. A marked decrease in iron(rrr) concentration in the filtrate was observed when a sample of water was filtered with the use of a Nucleopore filter.2 A membrane filter of 47 mm diameter was much better than that of 25 mm diameter for preparation for the determination of filterable iron(Ir1) species.The experi- ments were also carried out in the presence of citrate, tartrate, nitrilotriacetate and pyrophosphate ions. Each complexing agent was added to the sample solution such that a part of iron(rr1) was converted to the filterable species and the remainder was present as the particulate hydroxide. With the 0 I I 1 I I I I 1 0 1 2 3 4 5 6 7 8 Number of fraction Fig. 7 Effect of filtration on the determination of the filterable iron(ii1) species and humic acid. Membrane filters used were; A , 0.45 ym cellulose nitrate of 47 mm diameter; B, 0.45 ym cellulose nitrate of 25 mm diameter; and C, 0.40 ym Nucleopore of 25 mm diameter.Initial amount of iron(m), 5.00 yg; and humic acid (Nacalai-tesque), 36.0 yg. Amount of each fraction, 50 ml addition of citrate, tartrate and nitrilotriacetate ions the concentration of iron(n1) in the resultant filtrate was constant regardless of filtered sample volumes for volumes of up to 400 ml (8 filtration runs). In the presence of pyrophosphate, however, the concentration of iron in the filtrate decreased with an increasing number of filtration runs, as was observed in the presence of humic acid. Determination of Iron and Humic Substances in River-water An aliquot of a river-water sample was divided into 50 ml portions, and each portion was filtered successively through a 0.45 pm membrane filter of cellulose nitrate, of 47 mm diameter, within 3 h of sampling.Another series of filtrates was prepared with the use of a fresh membrane filter. A series of sets of filtrates was thus prepared. Each set of filtrates was then subjected to the procedure for the determination of iron and humic acid. Coloured species in each filtrate collected in the resin phase were determined as for humic acid with the use of a calibration graph prepared from the Aldrich humic acid. The results are shown in Table 4 (the results obtained for a tap-water sample are included for comparison). The amount of iron in each 50 ml filtrate of the river-water sample was almost constant for 6 filtration runs. As the total concentration of iron was found to be 81 ppb (Table 5), it was concluded that 85% of the iron in the river-water was from particulate species and 15% from filterable species.However, the classification of iron in the tap-water sample was meaningless because the concentrations of the filterable iron and therefore the particu- Table 4 Effect of filtration on the determination of filterable iron and humic substances in river- and tap-water samples. The 50 ml portions of the water samples were successively filtered through a 0.45 ym membrane filter of 47 mm diameter River water* Tap-water$ Humic Number of Iron substances Iron filtration found found? found runs (PPb) (PPb) (PPb) 1 11.2 75 24.1 2 12.2 87 8.5 3 11.3 82 0.0 4 12.8 81 0.0 5 13.3 82 6 12.3 88 7 9.3 70 8 10.0 77 9 7.6 70 10 5.4 54 - - - - - - * Collected from a tributary of the Iwaki river. t Calibration graph prepared with the humic acid obtained from 3: Collected after running for more than 30 min.The total Aldrich was used. concentration of iron was 102.4 k 4.0 ppb (n = 6). Table 5 Results of the determination of iron and humic substances in river-water Iron Humic substances* Total Filterable Total Filterable River-water (ppb) (PPb) (PPb) (PPb) A t 81.0 f 1.6 12.4 f 0.7 126 f 7 84 f 3 n = 6 n = 5 n = 6 n = 5 B 185.4 f 3.1 17.3 f 0.7 126 f 3 13 f 2 n = 6 n = 5 n = 3 n = 4 * Humic acid from Aldrich was taken as the standard. t River-water A is a tributary of the Iwaki river B.ANALYST, FEBRUARY 1991, VOL. 116 205 late iron were dependent on how much of the tap-water was filtered. The results also indicated that the hydroxide might be the predominant iron species in tap-water.The classification of humic substances in the river-water sample into filterable and particulate species was significant, as a constant amount of humic substances was found in the filtrate, see Table 4. For the determination of the total concentration of iron, the water sample was acidified to pH 1.2 and filtered through a 0.45 pm membrane filter of cellulose nitrate of 47 mm diameter. On the other hand, the determination of total humic substances was carried out as follows. A 1 1 portion of the water sample was acidified to pH 1.2 and 2.0 ml of 0.1 mol dm-3 EDTA solution were added. The solution was allowed to stand for 2 h, after which the pH was adjusted to approximately 11 by the addition of sodium hydroxide and the resultant solution was filtered through a 0.45 pm membrane filter.The pH of the filtrate was adjusted to 4.7 and the coloured species in the filtrate determined as described previously. The results are summarized in Table 5. The total concentration of humic acid in the river-waters was found to be almost the same, whereas a significant difference was found between the concentrations of filterable iron(rr1) species in each water. Conclusion Simple and sensitive methods for the determination of iron and humic acid have been developed and the effects of humic acid and other complexing agents, such as citrate, tartrate, pyrophosphate and nitrilotriacetate ions, on the formation of filterable iron(m) species were investigated. These ligands were found to be effective for the formation of filterable iron species.Except for the nitrilotriacetate ion which formed a 1 : 1 complex with iron(m), the formation of iron-rich com- plexes in which the molar ratio of iron to each ligand exceeded 1 was demonstrated. The total amount of iron and humic substances found in the river-waters examined could be classified into their respective filterable and particulate species. References 1 Florence, T. M., Talanta, 1982, 29, 345. 2 Lexen, D. P. H . , and Chandler, I. M., Anal. Chern., 1982, 54, 1350. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Schnitzer. M., Hurnic Substances in the Environment, Marcel Dekker, New York, 1972, pp. 54 and 230. Rashid, M. A., and Leonard, J. D., Chern. Geol., 1973, 11,89. Perdue, E. M., Beck, K. C., and Reuter, J. H.. Nature (London), 1976,260,418. Picard, G. L., and Felbeck, G. T.. Jr., Geochirn. Cosrnochirn. Acta, 1976, 40, 1347. Moore, R. M., Burton, J . D., Williams, P. J., and Yound, M. L., Geochirn. Cosrnochirn. Acta, 1979, 43, 919. Tipping, E., Geochirn. Cosrnochirn. Acta, 1981, 45, 191. Hiraide, M., Ishii, M., and Mizuike, A., Anal. Sci., 1988, 4, 605. Ohzeki, K., Minorikawa, M., Yokota. F., Nukatsuka, I . , and Ishida, R., Analyst, 1990, 115, 23. Hiraide, M., Tillekeratne, S. P., Otsuka, K., and Mizuike. A . , Anal. Chirn. Acta, 1985, 172, 215. Nagayama, M., Goto, K., and Yotsuyanagi, T., Kogyo-Yosui, 1963, (61), 24. Hiraide, M., Arima, Y., and Mizuike. M.. Anal. Chirn. Acta, 1987, 200, 171. Nomizu, T., Sanji, M., and Mizuike, A., Anal. Chirn. Acta, 1988,211,293. Almgren, T., Josefsson, B., and Nyquist, G., Anal. Chirn. Acta, 1975, 78, 411. McCrum, W. A., Anal. Proc., 1986, 23, 307. Marino, D. F., and Ingle, J . D., Jr., Anal. Chirn. Acta, 1981, 124. 23. Power, J. F., and Langford, C. H., Anal. Chern., 1988,60,842. Carpenter, P. D., and Smith, J . D . , Anal. Chim. Acta, 1984, 159, 299. Snoeyink, V. L., and Jankins, D., Water Chemistry, Wiley. New York, 1980, pp. 264-267. Miles, C. J., Tsuchall, J. R., Jr., and Brezonik. P. L., Anal. Chern., 1983, 55, 410. Shindo, E., and Morito, M., Kagaku no Ryoiki, 1966, 21,206. Cheng, K. L., Ueno, K., and Imamura, T., Handbook of Organic Analytical Reagents, CRC Press, Boca Raton, FL, 1982, p. 397. Ohzeki, K.. Toki, C., Ishida. R., and Saitoh, T., Analyst, 1987, 112, 1689. Bone, K. M., and Hibbert, W. D., Anal. Chirn. Acta, 1979,107, 219. Shriadah, M. M. A., and Ohzeki, K., Analyst, 1986, 111, 555. Martin, J.-M., and Mwybeck, M., Mar. Chern.. 1979, 7, 173. Wilson. M. A., Vasallo, A. M., Perdue, E. M., and Reuter, J. H., Anal. Chem., 1987, 59, 551. Paper 01023461 Received May 25th, 1990 Accepted October 19th, 1990

ISSN:0003-2654    

DOI:10.1039/AN9911600199

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1991, VOL. 116 207 Extraction-Spectrophotometric Determination of Sulphur Dioxide N. Balasubramanian and B. S. M. Kumar Department of Chemistry, Indian Institute of Technology, Madras-600 036, India A sensitive spectrophotometric method for the determination of trace amounts of sulphur dioxide after fixing in a modified buffered formaldehyde solution is described. The reaction of iodate with sulphur dioxide in the presence of acid and excess of chloride leads to the formation of ICI which is stabilized as the I&- ion. The species formed reacts with 2’,7’-dichlorofluorescein to form 2‘,7’-dichloro-4’,5’-diiodofluorescein and is extracted into a solvent mixture of 15% isopentyl acetate in isopentyl alcohol. The colour system obeys Beer’s law in the range 0-40 pg of sulphur dioxide.The relative standard deviation is 3.5% for ten determinations of 15 pg of sulphur dioxide. The effect of interfering gases on the determination is discussed. The method has been applied to the determination of sulphur dioxide at low concentrations and the results obtained were compared with the widely used pararosaniline method. The method can be used to determine as low as 2 pg of sulphur dioxide. Keywords: Sulphur dioxide; 2’,7’-dichlorofluorescein; spectrophotometry; gas analysis; gas permeation device Air pollution due to sulphur dioxide has arisen mainly as a consequence of the widespread use of sulphur and its compounds in manufacturing and industrial processes and the increased use of fossil fuels as a source of energy. It has been estimated that the combustion of coal and petroleum products contributes 70 and l6%, respectively, to man-made sulphur dioxide pollution.1 As the presence of sulphur dioxide in ambient air is known to be a health hazard, the development of analytical methods for its determination has attracted considerable attention.The pararosaniline method,’ after absorbing sulphur diox- ide in 0.1 mol dm-3 tetrachloromercurate (TCM) solution, has been widely used for the colorimetric determination of sulphur dioxide in the atmosphere, because of its simplicity, sensitivity and specificity. However, the analysis must be carried out carefully with close attention to temperature, pH and dye purity.3 Further, the TCM absorbent has some disadvantages, e.g., the use of highly toxic and expensive mercury(i1) chloride and the instability of the complex.4 In order to overcome these disadvantages, Dasgupta et al.5 advocated the use of a buffered formaldehyde solution for trapping sulphur dioxide.The trapped sulphur dioxide is stable for about 30 d without appreciable loss of fixed sulphur dioxide. A highly sensitive method for the determination of sulphur dioxide, after trapping it in a buffered formaldehyde absorber solution, has been reported by Selvapathy et al.6 The reaction of sulphur dioxide with iodate in an acidic medium containing chloride ions to produce ICL- ions, formed the basis of their method. The IC14- formed an ion pair with pyronine G and was extracted into benzene for spectrophotometric measure- ment at 535 nm. This approach, however, has the disadvan- tage of a high blank value and poor colour stability (10 min after extraction into benzene).This paper describes the study and evaluation of the variables which govern the interaction of the ICI, generated by the reaction between iodate and sulphur dioxide, with 2’,7’-dichlorofluorescein in order to provide the basis for a selective and sensitive spectrophotometric method for the determination of sulphur dioxide. Experimental Apparatus Absorbance measurements were made by using a Carl Zeiss PMQ I1 spectrophotometer with 10 mm quartz cells. Fritted glass bubblers with suitable suction devices were used for trapping sulphur dioxide from air. The air flow-rate was measured using a rotameter. Reagents All chemicals used were of analytical-reagent grade and distilled water was used for preparing the reagent solutions.Standard sulphur dioxide solution, 350 pg ml- 1. Prepared by dissolving 0.4 g of anhydrous sodium sulphite in 500 ml of water and standardizing iodimetrically.7 A suitable volume of this solution was diluted using a 7 mmol dm-3 solution of formaldehyde to give a solution containing 5 pg ml-1 of sulphur dioxide. This solution remained stable for at least 1 month. Potassium iodate, 0.4% mlv. Sodium chloride, 6% mlv. Sulphamic acid, 2.5% mlv. Mercury(I1) chloride, 0.03% mlv. Sodium hydroxide, 4.5 and 4 mol dm-3. The above solutions were prepared by dissolving appro- priate amounts of the reagent in water. 2’ ,7’-Dichlorofluorescein solution, 0.01 YO mlv. Prepared by dissolving 0.1 g of the dye in 4 ml of 1 mol dm-3 sodium hydroxide and diluting to 1 1 with water.Acetate-acetic acid buffer (PH 6.20). Prepared by dissolv- ing 29.6 ml of glacial acetic acid in about 200 ml of water. The pH was adjusted to 6.2 by the addition of ammonia solution using a pH meter. The solution was diluted to 250 ml with water. Buffered formaldehyde trapping solution. Prepared by diluting 500 pl of formaldehyde solution, 1.36 g of sodium acetate trihydrate and 600 p1 of glacial acetic acid to 1 1 with water. The solution was 7 mmol dm-3 in formaldehyde and 10 mmol dm-3 in sodium acetate and acetic acid and had a pH of 4.76 at 25 “C. Sulphuric acid, 4.25 mol dm-3. Prepared by diluting 236.1 ml of sulphuric acid (sp.gr. 1.84) to 1 1 with water. Isopentyl acetate-isopentyl alcohol mixture, 15% vlv. Pre- pared by diluting 15 ml of isopentyl acetate to 100 ml with isopentyl alcohol. Procedure Sampling Air samples were collected by drawing 10-100 1 of air through a fritted glass bubbler containing 15 ml of buffered formal- dehyde trapping solution for a period of 20-200 min at a rate of208 ANALYST, FEBRUARY 1991, VOL.116 0.5 1 min-1. The volume of the solution was made up to 50 ml with the trapping solution prior to determination. Determination Into a 50 ml calibrated flask were placed 5 ml of potassium iodate, 2 ml of 0.01% 2’,7’-dichlorofluorescein, 1 ml of 6% sodium chloride and 1 ml of 4.25 rnol dm-3 sulphuric acid. A 15 ml aliquot of the buffered formaldehyde trapping solution containing not more than 40 yg of sulphur dioxide (fixed as the hydrogen sulphite addition compound) was treated with 1 ml of 4.5 mol dm-3 sodium hydroxide, used to decompose the complex.The solution was then introduced into the 50 ml calibrated flask through a long-stemmed funnel with the tip kept well immersed in the reagent solution to avoid the loss of sulphur dioxide. The solution was made up to about 40 ml with water, mixed thoroughly and allowed to stand for 5 min. Then, 1 ml of 4 mol dm-3 sodium hydroxide and 5 ml of acetate buffer (pH 6.20) were added and the solution was diluted to 50 ml with water and mixed thoroughly. It was then transferred into a 125 ml separating funnel and extracted with 5 ml of a solvent mixture containing 15% isopentyl acetate in isopentyl alcohol for 1 min.The organic layer was separated and transferred into a test-tube and treated with about 1 g of anhydrous sodium sulphate to remove trace amounts of water. The absorbance of the organic extract was measured at 535 nm in 10 mm cells against a reagent blank which was taken through the entire procedure. The concentration of sulphur dioxide was established by reference to a calibration graph which was prepared by treating 0-8 ml of standard sulphur dioxide solution (containing 0-40 pg of sulphur dioxide) with 15 ml of the buffered formaldehyde trapping solution and following the procedure described above. Results and Discussion Preliminary studies were carried out using 5 ml of a solution of 0.4% potassium iodate, 1 ml of 6% sodium chloride and 2 ml of 0.01% 2’,7’-dichlorofluorescein.The resulting solu- tion was treated with 15 yg of sulphur dioxide (fixed as the hydrogen sulphite addition compound) in 15 ml of buffered formaldehyde trapping solution, after treating it with 1 ml of 4.5 rnol dm-3 sodium hydroxide solution, used to decompose the formaldehyde-hydrogen sulphite complex.5 After allow- ing it to stand for 5 min, the solution was treated with 1 ml of 4 mol dm-3 sodium hydroxide and 5 ml of acetate buffer and diluted to 50 ml with water. The solution was transferred into a 125 ml separating funnel and extracted for 1 min using a solvent mixture containing 15% isopentyl acetate in isopentyl alcohol. The result indicated that the extraction of the iodinated product into the solvent mixture was fairly selective a 0.4 C m f! 5: a 0.2 a 0 D 420 460 500 540 580 Vn m Fig.1 Absorption spectra measured against solvent blank. A, Reagent; B , SO2 (7.5 pg); C, SO2 (15 pg); and D, SO2 (30 pg) as the absorbance of the blank at 535 nm was low and that of the sample was high (Fig. 1). The optimum acidity for the formation of the ICL- species for subsequent reaction with 2’,7’-dichlorofluorescein to form 2’ ,7’-dichloro-4‘ ,5’-diiodofluorescein was first established. A constant and maximum absorbance was obtained when the acidity of the reaction medium was greater than 0.04 rnol dm-3. Subsequent studies were carried out using solutions maintained at 0.05 rnol dm-3 for the formation of 2’,7’- dichloro-4’ ,5 ’-diiodofluorescein. Although the reaction was carried out in acidic medium for the formation of the dichlorodiiodo compound, the control of pH was essential for the selective extraction of 2’,7’-dichloro- 4’ ,5 ’-diiodofluorescein.Different buffer solutions were inves- tigated for the selective extraction and the optimum pH for the extraction was found to be in the range 6.0-6.4. This could be achieved by the use of 5 ml of acetate buffer (pH 6.20). Similar studies revealed that 2.5 ml of 0.4% potassium iodate, 1 ml of 0.01% 2’,7’-dichlorofluorescein and 0.5 ml of 6% sodium chloride solution were sufficient to provide a constant and maximum absorbance. The formation of 2’,7’-dichloro-4’,5‘-diiodofluorescein was found to be almost instantaneous and mixing the phases for about 30 s was found to be sufficient for its quantitative extraction into the isopentyl acetate-isopentyl alcohol solvent mixture.The colour system, after extraction, was found to be stable for 1 h after which a gradual decrease in absorbance was observed. A variety of solvents were investigated for the selective extraction of 2’ ,7’-dichloro-4’ ,5’-diiodofluorescein. In sol- vents such as benzene, hexane, isobutyl methyl ketone (IBMK), diisopropyl ether and isopentyl acetate, the absor- bance values for the blank and sample were found to be almost zero. In solvents such as butan-1-01, propan-2-01 and isopentyl alcohol, the absorbance values of the blank and sample were both very high. Hence solvent mixtures such as benzene- propan-2-01, IBMK-propan-2-01, acetone-butan-1-01, IBMK -butan-1-01, benzene-isopentyl alcohol, IBMK-isopentyl alcohol, benzene-acetone, IBMK-acetone and isopentyl acet- ate-isopentyl alcohol were investigated as extracting solvents.Binary mixtures of benzene with acetone, propan-2-01, butan-1-01 and isopentyl alcohol produced high blank values. Similar results were obtained with the binary mixtures of IBMK with acetone, propan-2-01, butan-1-01 and isopentyl alcohol. Only a solvent mixture consisting of isopentyl acetate and isopentyl alcohol proved to be satisfactory, as the extraction of the iodinated compound was found to be selective and maximum only in this instance, with low blank values. Constant absorbance was observed when the concen- tration of isopentyl acetate was in the range 10-19% in isopentyl alcohol. Above 19% of isopentyl acetate, the absorbance of the sample showed a gradual decrease while below 10% of isopentyl acetate an increase in the blank absorbance was observed.Hence, studies were carried out by extracting 2’,7‘-dichloro-4’,5‘-diiodofluorescein into 5 ml of a solvent mixture containing 15% of isopentyl acetate in isopentyl alcohol. A linear calibration graph was obtained over the range 0-40 pg of sulphur dioxide. The precision of the proposed method was confirmed by establishing the concentration of ten samples containing 15 yg of sulphur dioxide. The mean recovery was found to be 14.5 pg with a relative standard deviation of 3.5%. Nature of the Extracted Species The formation of 2’,7’-dichloro-4’,5’-diiodofluorescein from 2’,7‘-dichlorofluorescein under the experimental conditions described and the extraction of the former into the solvent mixture is responsible for the observed colour.The formation of the diiodo compound can be explained on the basis of the iodination of the dichloro compound as a result of the reactionANALYST, FEBRUARY 1991, VOL. 116 209 21c12- I I I I CI O W H CI + 21CI I\ + 2 CI- - CI O m : : + ‘ ‘ 2HCI 2‘,7’-Dichlorofluorescein 2’,7’-Dichloro-4’,5’-diiodofluorescein Fig. 2 Species responsible for the colour system of potassium iodate with sulphur dioxide in acidic medium, to generate iodine in accordance with equation (1). 2103- + 5so2 + 2H+ + 12 + 5so3 + H2O 5SO2 I2 (1) (320.31 pg of SO2 253.81 pg of 12) When experiments were carried out to iodinate 2’,7’-di- chlorofluorescein using stoichiometric amounts of iodine solution (prepared by equilibrating solid iodine with water in the absence of iodide) in acidic medium, a very low absorbance (0.004 A) was obtained indicating that the iodination reaction was not taking place as expected.However, when the same reaction was carried out in the presence of potassium iodate and a stoichiometric amount of iodine, a high absorbance (0.195 A, for 11.88 pg of I2 = 15 pg of SO2) was observed. This observation can be explained by the fact that the reaction of iodine with iodate in an acidic medium leads to the formation of ICI which is stabilized as ICI2- in the presence of chloride ions8 in accordance with equations (2) and (3). 212 + 103- + 6H+ + 5C1- + 5IC1 + 3H2O (2) 5IC1 + 5C1- 5ICI2- ( 3 ) Iodine monochloride is widely used as an iodinating agent in aromatic substitution reactions9 and the formation of ICl under the experimental condition used here is highly fav- oured.When experiments were carried out using sulphur dioxide or equivalent amounts of iodine solution in the presence of potassium iodate and chloride ions, identical absorbance values were obtained, indicating that the species responsible for the iodination of 2’ ,7’-dichlorofluorescein is IC1, and the observed colour is due to the extracted 2’ ,7’-dichloro-4‘,5’-diiodofluorescein in the solvent mixture, as shown in Fig. 2. Trapping Solution Tetrachloromercurate is one of the most widely used absorb- ing reagents for fixing atmospheric sulphur dioxide.2 The TCM absorber has several disadvantages.4 Several other reagents have been proposed for the absorption of atmos- pheric sulphur dioxide, including monoethanolamine,’o hydroxamic acids,” ethylenediaminetetraacetic acid12 and morpholine.13 All these reagents have various advantages and disadvantages. Dasgupta et al.5 have advocated the use of a formaldehyde (7 mmol dm-3) solution buffered at pH 4.2 using 1 mmol dm-3 potassium hydrogen phthalate (KHP) as trapping solution; the fixed sulphur dioxide is stable for about 30 d without appreciable loss. The trapping solution permits a sampling rate of 0.25-0.4 1 min-1, preferably at 0.3 1 min-1,14 for the quantitative collection of atmospheric sulphur dioxide. For the sampling of sulphur dioxide in ambient air a sampling rate higher than 0.4 I min-1 is preferred. Attempts were made to modify the trapping solution so that it could be used at a higher sampling rate and for a longer duration of sampling.In this study known amounts of sulphur dioxide were generated using an ‘H’-type permeation device developed by Balasubraman- ian et al. l5 This simple permeation device, made with PTFE as the permeation medium, generates sulphur dioxide at 145.6 Table 1 Collection efficiency of the trapping solutions. Sampling rate, 0.5 1 min-l Sulphur Sulphur dioxide dioxide Trapping Time/ exDected/ found/ Recovery solution 7 mmol dm-3 HCHO- 1 mmol dm-3 KHP 7 mmol dm-3 HCHO- 10 mmol dm-3 KHP 7 mmol dm-3 HCHO- 50 mmol dm-3 KHP 7 mmol dm-3 HCHO- 10 mmol dm-3 KHP (PH 4.2) (PH 4 4 (PH 4 4 (PH 4.9) 7 mmol dm-3 HCHO- 10 mmol dm-3 sodium acetate- acetic acid (pH4.76) h 1 2 2 3 3 4 3 4 5 4 5 6 * Average of three values.Pg 8.74 17.47 17.47 26.21 26.21 34.94 26.21 34.94 43.68 34.94 43.68 52.42 Pg 8.61 10.0 17.15 25.0 25.8 25.83 25.83 33.98 26.15 33.99 43.07 51.67 ( Y o ) 98.51 57.24 98.16 95.37 98.44 73.93 98.55 98.5 74.84 98.5 98.60 98.57 Table 2 Variation of sampling rate. (Trapping solution, 7 mmol dm-3 HCHO-10 mmol dm-3 NaOAc-HOAc buffer pH 4.76.) Sampling period, 6 h. The amount of sulphur dioxide expected was 52.42 yg in all instances Sampling rate/ Sulphur dioxide Recovery Sample 1 min-1 found*/ yg (%) 1 0.4 52.38 99.92 2 0.5 51.67 98.57 3 0.6 51.78 98.78 4 0.7 51.72 98.66 5 0.8 47.50 90.61 6 1 .o 40.83 77.89 * Average of three values. Table 3 Variation of absorption efficiency in the sampling period. (Trapping solution, 7 mmol dm-3 HCHO-10 mmol dm-3 NaOAc- HOAc buffer pH 4.76.) Sampling rate, 0.5 1 min-l Sulphur Sulphur Time/ dioxide dioxide Recovery Sample h expected/yg found*/yg (YO) 1 4 34.94 34.90 99.89 2 6 52.42 51.67 98.57 3 7 61.15 60.85 99.51 4 8 69.89 68,42 97.90 * Average of three values.ng min-1 (214 k 5 ppb) when dry air is passed through the system at a flow-rate of 0.26 k 0.01 1 min-1. The air, containing sulphur dioxide being emitted from the permeation device, was mixed with clean dry air and this gas mixture was sampled through 15 ml of the trapping solution at various sampling rates, as desired for the experiments. The fixed sulphur dioxide , after sampling, was determined by following the proposed method. A sulphur dioxide recovery of >98% is taken as a sign of quantitative absorption in the trapping solution.Three approaches were investigated in an effort to improve the absorption efficiency of the trapping solution. First, the buffer (KHP) concentration of the absorbing solution was changed from 1 to 10 mmol dm-3 and further increased to 50 mmol dm-3. This approach permitted a210 ANALYST, FEBRUARY 1991, VOL. 116 sampling rate of 0.5 1 min-1 for 3 h. Second, keeping the buffer concentration at 10 mmol dm-3, the pH of the trapping solution was adjusted to 4.9 (instead of 4.2). This approach permitted a sampling rate of 0.5 1 min-1 for 4 h. Third, an attempt was made to replace the phthalate buffer system with 10 mmol dm-3 sodium acetate-acetic acid buffer (pH 4.76). This change permitted a sampling rate of 0.5 1 min-1 for 6 h. The results of this study are given in Table 1.After selecting the 10 mmol dm-3 acetate buffer system (pH 4.76), the sampling rate was varied from 0.4 to 1 1 min-1 for a sampling period of 6 h. This study indicated that a maximum sampling rate of 0.7 1 min-1 can be used without loss of sulphur dioxide (Table 2). Experiments were also performed to establish the optimum sampling period by sampling air at 0.5 1 min-1 for different sampling periods ranging from 4 to 8 h (Table 3). This study indicated that the buffer system permits a sampling period of 7 h. Based on the above study it is clear that the modified trapping solution can be employed successfully for sampling sulphur dioxide at a sampling rate of 0.5-0.7 1 min-1 for 6 h. The stability of the collected sulphur dioxide samples was studied by preparing a standard sodium sulphite solution in the buffered formaldehyde solution containing 7 mmol dm-3 formaldehyde-10 mmol dm-3 sodium acetate buffer (pH 4.76), and establishing the concentration of sulphur dioxide by taking samples after different periods of time.This sulphite solution was found to be stable for 25 d in the buffered formaldehyde solution without any deterioration (Fig. 3). Effect of Interfering Species The effect of common air pollutants on the determination of 15 pg of sulphur dioxide was studied by introducing the gas under examination into the trapping solution in the form of anions, together with the sulphur dioxide. Nitrogen dioxide, when present in excess of 5 pg, caused a low recovery of sulphur dioxide.The negative interference of up to 500 pg of I I I I I 10 20 Ti m e/d 30 Fig. 3 HCHO-10 mmol dm-3 NaOAc-10 mmol dm-3 HOAc (pH 4.76) Stability curve for fixed SOz. Trapping solution, 7 mmol dm-3 nitrogen dioxide was overcome by the incorporation of 1 ml of a 2.5% solution of sulphamic acid together with potassium iodate and other reagent solutions. Hydrogen sulphide interfered seriously at all levels causing positive errors. The interference of up to 5 pg of hydrogen sulphide was overcome by the addition of 1 ml of a 0.03% solution of mercury(r1) chloride to the trapping solution after sampling and develop- ing the colour by following the recommended procedure. No interference was observed in the presence of bromate (1000 pg), chromate (1000 pg), nitrate (2000 pg) and Hg" (250 pg) in the determination of 15 pg of sulphur dioxide.Application of the Method The proposed method was used for the determination of low concentrations of sulphur dioxide generated from two per- meation devices (type H and type M) developed by Balasubra- manian et al.15 The simple permeation devices (H and M), made with PTFE as the permeation mediums for sulphur dioxide, generate 214 k 5 and 125 k 4 ppb of sulphur dioxide, respectively, when dry air is passed through the system at a flow-rate of 0.26 k 0.01 1 min-1. The sulphur dioxide concentrations were established using a Seres Model SF 30 pulsed UV fluorescence monitor capable of measuring 1-10000 ppb of sulphur dioxide with an accuracy of k 1%. Air samples from the permeation devices (H and M), after mixing with air, were collected in 15 ml of the buffered formaldehyde trapping solution at a flow-rate of 0.5 1 min-1.The final volume of the trapping solution after sampling was made up to 50 ml with the buffered formaldehyde solution. A 15 ml aliquot of this solution was analysed using the proposed procedure and the standard pararosaniline procedure.2 Although the sampling rate was maintained at 0.5 1 min-1, the concentration of sulphur dioxide in ppb was calculated by taking the volume of air passing through the permeation devices (0.26 1 min-1). The calculated molar absorption coefficients for the pararosaniline method and the proposed method are 3.2 X 104 and 4.3 X 103 dm3 mol-1 cm-1, respectively. The value for the proposed method is less by one order of magnitude because it is calculated by taking into account the use of 5 ml of solvent mixture for the extraction of the compound from 50 ml of aqueous solution.The results obtained are shown in Table 4 from which it is clear that the concentrations of sulphur dioxide obtained by both methods are comparable. Conclusion Sulphur dioxide can be precisely determined down to a level of 2 pg using the proposed procedure. The calibration graph is rectilinear in the range 0-40 pg of sulphur dioxide. The Table 4 Determination of sulphur dioxide generated from the permeation device. Sampling rate, 0.5 1 min-1; and sampling time, 6 h. The volume of solution taken for analysis was 15 ml in all instances* Concentration of sulphur dioxide Pararosaniline 2' ,7'-Dichlorofluorescein method method Estimated Permeation dioxide/yg in dioxide/yg in dioxide dioxide& in dioxide& in dioxide of sulphur H 15.11 50.36 205.52 15.25 50.83 206.7 - 16.0 53.33 216.98 16.00 5;.33 216.98 214 k 5 15.85 52.83 215.45 16.0 53.33 216.98 - M 9.20 30.67 125.3 9.0 30.0 122.24 - 9.5 31.67 128.7 9.0 30.0 122.24 125 ? 4 9.3 31.0 126.44 9.4 31.33 127.97 - Sulphur Sulphur Sulphur Sulphur Sulphur Sulphur concentration device 15 ml 50 ml (PPb) 15 ml 50 ml (PPb) dioxide (ppb)? * Sampling solution was made up to 50 ml with buffered formaldehyde before analysis. t Determined by using a Seres SF 30 pulsed ultraviolet fluorescence monitor.ANALYST, FEBRUARY 1991, VOL. 116 21 1 relative standard deviation is 3.5% for ten determinations of 15 pg of sulphur dioxide.By using the modified trapping solution, air can be sampled for periods of up to 7 h at a sampling rate of 0.5 1 min-1 without a decrease in the recovery of sulphur dioxide. In addition, the fixed sulphur dioxide is stable for 25 d. The application of the proposed method to the determination of trace amounts of sulphur dioxide generated from two permeation devices has been used to demonstrate the usefulness of the method. References 1 Robinson, E.. and Robins, R. A., Sources, Abundanceand Fute of Gaseous Atmospheric Pollutants, Final Report, SRI Project Sec. 8501, Stanford Research Institute, Menlo Park, CA, 1968. West, P. W., and Gaeke, G. C., Anal. Chem., 1956, 28, 1816. Scarringelli. F. P., Saltzmann. B. E., and Frey, S . A., Anal. Chem., 1967, 39, 1709. Pate, J. B., Lodge, J . P., and Wartburg, A. F., Anal. Chem., 1962.34, 1660. Dasgupta. P. K . , DeCesare, K., and Ulrey, J . C., Anal. Chem., 1980,52, 1912. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Selvapathy, P., Ramakrishna, T. V., Balasubramanian, N., and Pitchai, R., Analyst, 1987, 112, 1139. Vogel’s Text-book of Quantitative Inorganic Analysis, Including Elementary Instrumental Analysis, English Language Book Society, Longman, London, 1978, p. 384. Vogel’s Text-book of Quantitative Inorganic Analysis, Including Elementary Instrumental Analysis, English Language Book Society, Longman, London, 1978, p. 386. March, J., Organic Chemistry Reaction Mechanism and Struc- ture, Wiley, New Delhi, 3rd edn., 1986, p. 478. Bhatt, A., and Gupta, V. K., Analyst, 1983, 108, 374. Chaube, A., Baveja, A. K . . and Gupta, V. K . , Analyst, 1984, 109, 391. Humphrey, R. E., Ward, M. N., and Hinze, W., Anal. Chem., 1970, 42, 698. Raman, V., Rai, J . , Singh, M., and Parashar, D. C., Analyst, 1986, 111, 189. Dasgupta, P. K., Anal. Chem., 1981, 53, 2084. Balasubramanian, N., Selvapathy, P., and Pitchai, R., Indian J . Environ. Health, 1988, 30. 360. Paper OlO2121 K Received May 14th, 1990 Accepted September 28th, 1990

ISSN:0003-2654    

DOI:10.1039/AN9911600207

出版商:RSC    

年代:1991

数据来源: RSC

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ANALYST, FEBRUARY 1991, VOL. 116 213 Studies on the Co-colour Reaction of Platinum(iv) and Palladium(ii) With 4,4'-Bis(dimethylamino)thiobenzophenone Zhao-Lan Liu, Wen-Bao Chang, Jian Hong and Yun-Xiang Ci* Department of Chemistry, Peking University, Beijing 100871, People's Republic of China The sensitive co-colour reaction of Ptlv and Pd" with 4,4'-bis(dimethylamino)thiobenzophenone (TMK) in the presence of ascorbic acid and Triton X-I00 was investigated. The effect of pH on the absorbance of the complex was studied using an acetic acid-sodium acetate buffer, the optimum pH range being between 2.8 and 4.5, in the presence of ascorbic acid and Triton X-100. The orange-red complex exhibits an absorption maximum at 530 nm with a molar absorptivity of 1.96 x lo5 dm3 mol-1 cm-1 for Pd" (in the presence of 10 pg of Ptlv) and 2.98 x lo5 dm3 mol-1 cm-1 for Ptlv (in the presence of 10 pg of Pd").The Pt : Pd : TMK ratio in the complex was found to be 1 : 2 : 12. Beer's law is obeyed over the concentration range 0-0.8 ppm of Pd" and 0-0.48 ppm of PtlV in the presence of 10 pg of P t l V and 10 pg of Pdll, respectively. The optimum conditions for the co-colour reaction of Ptlv and Pd" with TMK and the interferences of foreign ions are reported. The proposed method has been used successfully for the spectrophotometric determination of platinum and palladium in synthetic samples. Based on the experiments, the structure of the complex and the mechanism of the co-colour reaction are proposed and discussed. Keywords : Pla tin um ; palladium; 4,4' - bis (dim e th ylam ino) thio benzop hen one; co-co lo ur reaction; spectro- photometry The reagent 4,4'-bis(dimethylamino)thiobenzophenone (Thio-Michler's ketone) (TMK) has been used for the spectrophotometric determination of mercury,' gold,2-4 silver,5.6 platinum ,7 palladium8 and indium .9 The colour reaction of platinum with TMK in the presence of ascorbic acid (vitamin C) (VC) occurs only in a boiling water-bath for 15-20 min.Neither platinum nor palladium alone gives a colour reaction with TMK in the presence of VC and Triton X-100 (TX-100) at room temperature. However, an orange- red complex formed immediately when TMK was added to a solution containing PtIV and Pd" in the presence of both VC and TX-100 at room temperature. This co-colour reactionlo has been exploited for the development of a spectrophoto- metric method for the determination of platinum and pallad- ium with high sensitivity and reasonably high selectivity. Experimental Reagents All reagents were of analytical-reagent grade. Palladium( 11) standard solution.A stock standard solution containing 1 mg ml-1 of palladium was prepared by dissolving 100.0 mg of palladium metal (99.9%) in 3-5 ml of hot aqua regia and evaporating the solution almost to dryness. The residue was treated with 5 ml of concentrated hydrochloric acid and evaporated to a small volume (1 ml or less). This treatment was repeated three times to destroy any nitroso complexes formed. The resulting residue, after the final evaporation, was dissolved in and diluted to 100 ml with 1 rnol dm-3 hydrochloric acid.Appropriate dilutions of the stock solution were made as required. Platinum(1v) standard solution. A stock standard solution containing 1 mg ml-1 of platinum was prepared in the same way as for the palladium(ii) solution. 4,4'- Bis(dimethylamino)thiobenzophenone solution. A 0.02% d v solution of TMK was prepared in ethanol and stored in an amber-coloured bottle under refrigeration. Ascorbic acid solution. A 2% d v solution of ascorbic acid was prepared in de-ionized water, stored in an amber- coloured bottle under refrigeration, and replaced every 3 d. * To whom correspondence should be addressed. Triton X-100 solution. A 5% m/v solution of TX-100 was prepared in de-ionized water. Buffer solutions. Buffer solutions in the pH range 2.5-6.5 were prepared using 5 rnol dm-3 sodium acetate solution and 5 rnol dm-3 acetic acid.Apparatus Model UV-300 (Shimadzu) and Model 722 (Shanghai Ana- lytical Instruments Factory No. 3, China) spectrophotometers were used for all absorbance measurements. A Model S-3 pH meter (Shanghai Analytical Instruments Factory No. 2, China) was used for pH measurements. General Procedure Add 1.5 ml of 2% VC solution to a 25 ml calibrated flask containing 10 pg of PtIV and 10 pg of Pd" and mix thoroughly. After 1 min, add 5.0 ml of buffer solution (pH 3.3) and mix thoroughly, and after a further 3 min, add 2.0 ml of 0.02% TMK solution and 3.0 ml of 5% TX-100 solution. Dilute the 0.70 0.50 T 0.30 0.10 300 400 500 600 700 h/nm Fig. 1 Absorption spectra. 1, PdI1 + TMK + VC + TX-100 against reagent blank; 2, PtIV + TMK + VC + TX-100 against reagent blank; 3, PtIV + PdI1 f TMK + VC + TX-100 against reagent blank; and 4, reagent blank (VC + TMK + TX-100) against water.Pt, 10 pg; Pd, 10 pg; VC, 4.5 x 10-3 mol dm-3; TMK, 2.1 x 10-5 rnol dm-3; TX-100, 3.3 x 10-3 mol dm-3; and pH, 3.3214 ANALYST, FEBRUARY 1991, VOL. 116 mixture to the mark with de-ionized water, mix thoroughly and measure the absorbance at 530 nm against a reagent blank. Results Spectral Characteristics The absorbance spectra of the Pd"-TMK-VC-TX-100, PtIV -TMK-VC-TX-100 and Pt*V-PdII-TMK-VC-TX-lOO systems and the reagent blank (TMK-VC-TX-100) are shown in Fig. 1. The complex resulting from the PtIV-PdII-TMK- VC-TX-100 system exhibits maximum absorbance at 530 nm.Under similar conditions, the other systems do not absorb appreciably at this wavelength. All subsequent studies were therefore made at 530 nm. Effect of Acidity The effect of pH on the absorbance of the complex was studied using acetic acid-sodium acetate buffer solution. Fig. 2 shows the optimum pH range to be between 2.8 and 4.5. A 5.0 ml volume of buffer solution of pH 3.3 was used for subsequent studies. Effect of Ascorbic Acid Solution A total of 1.0-2.0 ml of 2% ascorbic acid solution in a final volume of 25 ml sufficed for a solution containing less than 10 pg of PtIV and 10 pg of PdI* (Fig. 3). A 1.5 ml volume of 2% VC solution was selected for subsequent studies. I I T0.40 1 2.0 3.0 4.0 5.0 6.0 PH Fig. 2 TMK, 7.03 x 10-4 rnol dm-3; and TX-100, 3.3 X Effect of pH.Pt, 10 pg; Pd, 10 pg; VC, 4.5 X rnol dm-3; rnol dm-3 I 1 T 0.40 - 0.20 0 2.0 4.0 6.0 V/m I Fig. 3 Effect of VC on 1, the colour reaction of Pd" with TMK; and 2, the colour reaction of PtIV with TMK q 0.40 j 0.20 r ; 0 2.0 4.0 6.0 V/ml Effect of TMK concentration. Pt, 10 pg; Pd, 10 pg; VC, 4.5 X Fig. 4 1 0 - 3 rnol dm-3; TX-100, 3.3 X rnol dm-3; and pH, 3.3 Effect of TMK Concentration The effect of TMK concentration was investigated by measur- ing the absorbance at 530 nm of solutions containing 10 pg of PtIV and 10 pg of Pd" with various amounts of the reagent in a solution of pH 3.3 (Fig. 4). A total of more than 1.5 ml of the 0.02% m/v reagent solution in a final volume of 25 ml is sufficient for solutions containing less than 10 pg of PtIV and 10 pg of Pd".A 2.0 ml volume of 0.02% m/v TMK solution was used for subsequent studies. Effect of TX-100 In the absence of TX-100 the solution is turbid, which makes the determination of the absorbance of the complex very difficult. Triton X-100 is able to increase the solubility, sensitivity and stability of the complex. A total of 3.0 ml of 5% TX-100 solution was chosen for subsequent investigations. 0.80 1 0 20 40 60 80 100 120 Atls Fig. 5 TMK; and 2, the co-colour reaction of PtIV and PdI1 with TMK Effect of VC stop-time on 1, the colour reaction of Pd" with Table 1 Effect of foreign ions on the determination of platinum (10 pg) and palladium (10 pg) Tolerance Tolerance Ion added limitlpg Ion added limi t/pg Al"' 100,500* MoVI 175 AsV 20 Nil1 100,250" Au"' 0.5 0sv111 15 BaII 500 Rh"' 27 BiIII 25 RulI1 30 CalI 1200 Tb"' 15 Cd" 100,250* Zn" 4000 CeIV 500 ZrIV 4000 CO" 300,2950* c1- 80 000 Cr"' 20 I- 80 000 CU" 5,300* F- 80 000 Fe"' 50,100* Br- 80 000 La111 60 sop 80 000 Ag' 10 Mn" loo, 2000* MgI1 1000,1500* * In the presence of 6.0 ml of 1.0 X 10-2 rnol dm-3 EDTA.Table 2 Determination of Pd" and PtIV in a synthetic sample of composition Mg" 2000, Zn" 800, A1111 100, Nil1 100, CO" 50, FelI1 50 and Rh"' 5.0 pg per 25 ml Determination of palladium *- PtIV/ Pd" added/ 10.0 5.0 5.0 100 10.0 10.0 10.0 100 Average Pd" recovery? pg per 25 ml pg per 25 rnl pg per 25 ml % Determination of platinum*- Average PtIV recovery? PdIV PtIV added/ 10.0 5 .O 5.2 104 10.0 10.0 9.6 96 * In the presence of 6.0 ml of 1.0 x 10-2 mol dm-3 EDTA.t Average of six experiments. pg per 25 ml pg per 25 ml pg per ml YoANALYST. FEBRUARY 1991, VOL. 116 Pt 215 Calibration Graph and Limit of Determination Under the optimum conditions described above, Beer's law was obeyed in the ranges 04.8 ppm of PdII in the presence of 10 pg of Pt1V and 0-0.48 ppm of PtIV in the presence of 10 pg of Pd". The molar absorptivities for Pd" (in the presence of 10 pg of PtIV) and for PtIV (in the presence of 10 pg of Pd") are 1.96 x 105 and 2.98 x 105 dm3 mol-1 cm-1, respectively. A straight-line calibration graph passing through the origin was obtained using the recommended procedure. The correlation coefficients were 0.9999 for Pd" and 0.9994 for PtIV. Pd TMK Fig. 6 Three-component phase diagram R I lo Fig. 7 Structure of the complex Effect of Order of Addition of Reagents Reagents should be added to the PtIV and Pd" solution in the following order: 2% VC, buffer solution, 0.02% TMK solution and 5% TX-100.A change in this order had an adverse effect on the development of the colour of the Pt*V-PdII-TMK complex. Effect of VC Stop-time The stop-time is defined as the period between the addition of VC and the addition of buffer solution. For the colour reaction of Pd" with TMK, the absorbance decreased as the VC stop-time increased [Fig. 5(1)]. If the stop-time is more than 50 s, the absorbance decreases to a negligible level. For the co-colour reaction of the PtIV-Pd"-TMK-VC- TX-100 system, after subtracting the absorbance of the colour reaction of Pd*1 with TMK, the graph shown in Fig.5(2) was obtained, indicating that the optimum VC stop-time was 55-75 s. A VC stop-time of 60 s was chosen for subsequent studies. Effect of Foreign Ions Interferences caused by foreign ions in the system were studied using solutions containing 10 pg of PtIV, 10 pg of Pd" and the foreign ion. The tolerance limit was taken as the concentration that caused an error of not more than -+5% in the determination of platinum and palladium. The results are given in Table 1. It is interesting to note that EDTA does not interfere in the co-colour reaction of PtIV and Pd" with TMK, hence it can be used as a complexing agent to mask the interfering metals. Applications The proposed method was used for the determination of platinum and palladium in a synthetic sample. The composi- tion of the sample and the results are given in Table 2.Discussion The TMK reacted with neither palladium nor platinum alone, hence the reaction between TMK and noble metal ions probably occurs through bonding to sulphur rather than bonding to nitrogen. These noble metals are known to be the sulphide group metals. The reaction between metal ions and TMK has been described,6 and the molar ratio of metals in the complex with TMK was determined by experiments for palladium [atomic radius (Y) = 1.28 A] and platinum (Y = 1.30 A), which have similar atomic radii owing to the lanthanide contraction in the Periodic Table. It is probable that PtIVis first reduced to PtIl in the presence of ascorbic acid, then platinum and palladium react with TMK proportionally to form a heteropolynuclear complex; this may be the essence of the co-colour reaction.In order to establish the composition of the complex, the continuous variation, the molar ratio and the three-com- ponent phase diagram (Fig. 6) methods were applied. The Pt : Pd : TMK molar ratio was found to be 1 : 2 : 12. It was also found that TX-100 was able to increase the solubility, sensitivity and stability of the complex; this indicates that the complex is electroneutral . According to the properties of the complex and the molar ratio of metals and reagent in the complex, the structure shown in Fig. 7 for the complex formed by the co-colour reaction is proposed. 1 2 3 4 5 6 7 8 9 10 References Cheng, K. L. Mircochern. J., 1966, 10, 158. Tsukahara, I., Talanta, 1977, 24, 633. Liu, Q . - Z . , Li, H.-R., and Chen, S.-Q., Anal. Chern. (China), 1979, 7,43. Wei, D.-X., Metall. Anal., 1982, 4, 34. Li, L.-Y., Yu, S.-K., Li, N.-X., and Qu, Y.-L., Chern. Reagents (China), 1984, 6, 357. K. L. Cheng, Microchirn. Acta, 1967,5, 820. Chang, W.-B., Li, X.-P., Kang, R.-Y., and Ci, Y.-X., Rare Metal, 1986,5, 122. Li, L.-Y., Sun, Y.-M., and Jin, G., Anal. Chern. (China), 1985, 4, 285. Yao, F.-J., Xu, S.-J., and Ci, Y.-X.. Rare Metal, 1987, 6, 145. Chang, W.-B., and Ci, Y.-X.. Microchern. J . , 1989, 39, 149. Paper 0101649G Received April 12th, 1990 Accepted August 13th, 1990

ISSN:0003-2654    

DOI:10.1039/AN9911600213

出版商:RSC    

年代:1991

数据来源: RSC

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ANALYST. FEBRUARY 1991, VOL. 116 217 BOOK REVIEWS Pharmaceutical Thermal Analysis. Techniques and Appli- cations J. L. Ford and P. Timmins. Ellis Horwood Series in Pharmaceutical Technology. Pp. 31 3. Ellis Horwood. 1989. Price f55.00. ISBN 0 7458 0346 6 (Ellis Horwood); 0 470 21219 5 (Halsted Press). ‘Thermal analysis has long been the Cinderella of the instrumental techniques available to the pharmaceutical researcher’. So begins the author’s preface and it might turn out that by their efforts this situation changes. This is a highly readable account of the theory and practice of thermal analysis for pharmaceutical applications. The 14 chapters cover: ( i ) instrumentation, theory and practice, and information content of data (ch. 1-3); (ii) thermogravimetry and kinetics (ch.4); (iii) purity of materials (ch. 5 ) ; (iv) applications to solids, solid dispersions, polymeric drug delivery systems and solid dosage forms (ch. 6-9); ( v ) compatibility studies (ch. 10); and (vi) applications to semi-solid systems, liposomes, freeze drying and other applications (ch. 11-14). The work is thoroughly referenced and will appeal both to the pharmaceutical development worker and to the student, as a valuable work of reference. C. Burgess Selective Sample Handling and Detection in High- performance Liquid Chromatography- Part B Edited by K. Zech and R. W. Frei. Elsevier. 1989. Journalof Chromatography Library. Volume 39B. Pp. xi + 394. Price $1 29.95; Df1265.00. ISBN 0 444 88327 4. This book is the second of a two-volume series, published in the Journal of Chromatography Library series, on selective sample handling and detection in high-performance liquid chromatography (HPLC).The book consists of seven chap- ters written as reviews by experts in their fields, with the late Professor Roland Frei contributing, not only as an editor, but also as an author of two of the chapters. In Chapter I the authors review the applications of chelating silica as solid phases for pre-concentration, metal-ion chro- matography and enantiomer separation. By highlighting this single but important area of chemically modified silicas, a fitting illustration is provided of the potential of solid-phase chemistries in sample handling and detection, an important theme in the book. Sample handling techniques for ion chromatography are described in Chapter 11, covering sample collection and dissolution, contamination effects and the use of pre-concentration columns.Whole-blood sample clean-up for chromatographic analysis is detailed in Chapter I11 with the determination of cyclosporine chosen as an illustrative example. The following three chapters are concerned with selective detection methods. Chapter IV deals with radio- column liquid chromatography with sections on the use of radioisotopes in chemical analysis and on off-line and flow- through liquid-scintillation counting. The uses of immobilized enzymes and solid-phase chemistries for post-column reaction detection are described in Chapter V, including examples of their use with miniaturized HPLC systems. New selective luminescence detection techniques based on immobilized fluorophores, liquid-phase phosphorescence and alternative phosphorophores/luminophores are covered in Chapter VI.The book concludes with a review of the use of continuous separation techniques in flow-injection analysis, of particular relevance and potential in clinical, food and environmental analysis. The book is very well produced, is full of detailed examples and instrumental configurations and is aimed at investigators specializing in the areas outlined. There is a strong emphasis on the chemical principles involved in the analysis of complex samples, an approach which adds further to the quality and value of the production. J. D. Glennon ASTM Standards on Chromatography. Second Edition American Society for Testing and Materials.Pp. xvii + 805. American Technical Publishers. 1989. Price f60.00. ISBN 0 8031 1219 X. This book is a compendium of all of the ASTM chromato- graphic methods, and as such must be a heterogeneous collection of applications. Most analysts using chromato- graphy in any of its forms will find something of use and interest in its covers; very few will find all of it useful. Some of the standards included have little to do with chromatography in real terms, such as the first in the book. This deals with the sampling and analysis of alkaline detergents, and although a very valuable collection of methods is given, most of them refer to volumetric and gravimetric procedures for the normal detergent constituents. Chromatography is used in an ion-exchange method for separating and measuring the different phosphates, or with an alternative paper-chromatographic method.These two methods between them account for 6 pages out of a total of 25. There are standards relating to 115 matrices, ranging from acetate esters through atmospheres, detergents, oils, poly- mers, waxes, petroleum products, rubbers, solvents, etc. There are 128 components or properties measured in this range of sample matrices, and it is apparent that the number of combinations and permutations is fairly high. All told, there are 141 standards including four proposed standards. The quality of the methodology presented and the clarity of presentation are uniformly high as one would expect; equally predictable is the absence of any use of recently introduced procedures such as solid-phase extraction or chiral columns. Any publication of this type will always suffer from both the time taken in its preparation and its origin as a committee generated document in this respect, but at least any method found in this book can be taken and used in the certain knowledge that it should work! Overall, I think it a very useful work; it is a pity that the binding does not match the quality of the contents. The book is a large paperback, and in my copy the first two leaves are already coming adrift. I believe that a book, such as this one, intended for use on the bench, should be presented in a form that allows it to lie flat and not to lose pages too readily. My ideal format would be a three-ring binder, which would even allow ready updating. R. C. Rooney

ISSN:0003-2654    

DOI:10.1039/AN9911600217

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

218 ERRATUM ANALYST, FEBRUARY 1991, VOL. 116 Determination of Trace Amounts of Fluorine, Boron and Chlorine From a Single Sodium Carbonate Fusion of Small Geological Sample Masses Alfons Hofstetter, Georg Troll and Dietmar Matthies Analyst, 1991, 116, 65 Page 66, Reagents Standard sodium chloride solution: for ‘Sodium chloride (0.8242 g) (dry) is dissolved in 1 1 of doubly distilled water’. Read ‘Sodium chloride (0.8242 g ) (dry) is dissolved in 0.5 1 of doubly distilled water’.

ISSN:0003-2654    

DOI:10.1039/AN9911600218

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1991, VOL. 116 219 Al-Tamrah, S. A., 183 Alarie, Jean Pierre, 117 Alfassi, Zeev B., 35 Altesor, Carmen, 69 Alwarthan, A. A . , 183 Anderson, Fiona, 165 Apak, Regat, 89 Asselt. Kees van, 77 Baba, Jun-ichi, 45 Balasubramanian, N., 207 Baykut, Fikret, 89 BiCaniC, Dane, 77 Birch, Brian J., 323 Bisagni, E., 159 Blais, J . , 159 Bowyer, James R., 117 Cepeda, A.. 159 Chan, Wing Hong, 39 Chang, Wen-Bao, 213 Chen, Danhua, 171 Cheung, Yu Man. 39 Ci. Yun-Xiang, 213 Ciesielski, Witold, 85 Cohen. Arnold L., 15 Costa-Bauza, A . , 59 Covington, Arthur K., 135 Cresser. Malcolm, 141 de la Torre, M., 81 Dol. Isabel, 69 Donnelly, Garret, 165 Elagin, Anatoly. 145 Evans, Otis, 15 CUMULATIVE AUTHOR INDEX JANUARY-FEBRUARY 1991 Favier, Jan-Paul, 77 Fernandez-Gamez, F., 81 Fernandez-Romero, J.M., 167 Fleming, Paddy, 195 Gaind, Virindar S . , 21 Grases. F., 59 Harper, Alexander, 149 Hart, John P . , 123 Hendrix, James L., 49 Hofstetter, Alfons, 65 Hong, Jian, 213 Ishida, Ryoei, 199 Jacobs, Betty J . , 15 Jqdrzejewski, Wlodzimierz, 85 Jerrow, Mohammad, 141 Kakizaki, Teiji, 31 Kataky, Ritu, 135 Keating, Paula, 165 Kielbasinski, Piotr, 85 Knochen, Moisks, 69 Kudzin. Zbigniew H., 85 Kumar, B. S. M., 207 Lan, Chi-Ren, 35 Lazaro, F., 81 Lee, Albert Wai Ming, 39 Liu, Shaopu, 95 Liu. Zhao-Lan, 213 Liu, Zhongfan, 95 Lubbers, Marcel, 77 Luque de Castro, M. D., 81, Lyons. David J . , 153 167, 171 McCallum, Leith E . , 153 Mahuzier, G., 159 March, J. G., 59 Marr, Iain, 141 Matthies, Dietmar, 65 Mattusch, Juergen, 53 Mikolajezyk, Marian, 85 Miller, James N., 3 MilosavljeviC, Emil B., 49 Morimoto, Kazuhiro, 27 Mueller, Helmut, 53 Nakagawa, Genkichi, 45 Nelson, John H ., 49 Nicholson, Patrick E.. 135 NikoliC, Sneiana D., 49 Nobbs, Peter E., 153 Nukatsuka, Ishoshi, 199 O’Dea, John, 195 Ohzeki, Kunio, 199 O’Kennedy, Richard, 165 Osborne, William J., 153 Parker, David, 135 Prognon, P., 159 Prownpuntu, Anuchit, 191 Rios, Angel, 171 Ruan, Chuanmin, 99 Sakai, Tadao, 187 Sakurada, Osamu, 31 Sargi, L., 159 Sepaniak, Michael J., 117 Shijo, Yoshio, 27 Stoyanoff, Robert E., 21 Strauss, Eugen, 77 Sugawara, Kazuharu, 131 Sultan, Salah M., 177, 183 Taga, Mitsuhiko, 31. 131 Tanaka, Shunitz, 31, 131 Tatehana, Miyoko, 199 Thompson, Robert Q., 117 Tikhomirov, Sergei, 145 Titapiwatanakun, Umaporn, Troll, Georg, 65 Tseng, Chia-Liang, 35 Tutem, Esma, 89 Uehara, Nobuo, 27 Valcarcel, Miguel, 81, 171 Vazquez, M. L., 159 Vo-Dinh, Tuan, 117 Volynsky, Anatoly, 145 Wada, Hiroko, 45 Werner, Gerhard, 53 Wring, Stephen A., 123 Wu, Weh S . , 21 Xu, Qiheng, 99 Yang, Mo-Hsiung, 35 Yuchi, Akio. 45 191

ISSN:0003-2654    

DOI:10.1039/AN9911600219

出版商:RSC    

年代:1991

数据来源: RSC