I20
Anal. Chem. 1986, 58,120-124
Flow Injection Analysis of Binary and Ternary Mixtures of Arsenite, Arsenate, and Phosphate Pilar Linares, M.D. Luque de Castro, a n d Miguel Valclrcel* Department of Analytical Chemistry, Faculty of Sciences, University of CBrdoba, Avda. Medina Azahara, CBrdoba, Spain
Several photametrlc methods are suggested for the sequentlal AsO,~--PO:-, and AsO,--AsO?--PO?mlxtures based on the formation of heteropoly aclds wlth Moot-. The presence of a Selecting valve In the FIA system allows obtaining a sultable medlum for the lndlcator reactlon to develop wlth one, two, or the three specles of the mlxture. The methods proposed hereln are slmple and reliable and cover wlde ratlo ranges for mlxture resolution, and the errors encountered In thelr appllcatlon to real samples are relatlvely small.
flow Injection analysls (FIA) of AsO,-AsO?-,
The speciation of elements is of great interest, especially
to environmental analysis, taking into account that the toxicity of an element depends on its chemical form. This is the root of the enormous development of analytical methods which allow discerning between the different forms under which the substance can be found in the medium investigated. The interest in this topic shows in the number of papers and reviews (1-3) that appeared in the last few years, in addition to a monograph on the subject ( 4 ) . The influence of the chemical form of a given compound on its toxicity has a representative example in arsenic; thus, As(II1) is more toxic than As(V), the latter exhibiting higher toxicity as arsenate than as monomethylarsonic acid (and this, in turn, exhibiting higher toxicity than dimethylarsonic acid). This gradation of toxicity accounts for the interest in the speciation of these compounds (5);there are some conventional methods for Speciation of arsenic in the literature (e.g., that based on its selective transformation into hydride and subsequent measurement by AAS (1, 6-9). The simultaneous presence of arsenic, generally as AsO4'-, and PO?- in natural water as well as in wastewater (the P043-/As02- ratio being usually high) has given rise to the development of manual conventional methods for determination of these species with (10-12) or without (13-15) a prior separation process. The reaction on which these determinations are based is in most cases the formation of a heteropolyacid with MOO?-, with photometric detection of Mo(V), molybdenum blue, generated with the aid of a suitable reductant (12-16), or voltammetric detection, taking advantage in this case of the modification of the redox potential of Mo(V1) caused by the formation of the heteropoly acid. Flow injection analysis, FIA, has proved to be a suitable technique for speciation (17)as well as for simultaneous determinations (18);however-and surprisingly-it has been used only once for determination of PO?- and A s O ~ (volt~ammetric detection (19)),but not for the resolution of ternary mixtures of this type or for arsenic speciation. In this paper are suggested several photometric FIA methods for arsenic speciation (as AsOz- and and resolution of binary ( A s O ~ ~ - - P O ~and ~ - ) ternary (AsOz-A s O ~ ~ - - P O ~mixtures ~-) based on heteropoly acid formation and on the use of a simple FIA configuration which, by means of a selecting valve, allows fixing the most suitable conditions to this end.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Lambda 1spectrophotometer, Radiometer REC 80 recorder, Selecta S-382 thermostat, Gilson Minipuls-2 and Ismatec S-840 peristaltic pumps, Hellma 178.12 QS flow cell (inner volume 18 IL), Tecator L 100-1and Rheodyne 5041 injection valves, and Tecator TM I11 Chemifold were used. Reagents. Stock solutions included aqueous solutions of 0.024 M (NHJ2Mo04in 1.36 M HN03 containing 10% of glycerin, 6.3% ascorbic acid containing 10% of glycerin, 2.7 X M KIO,, 3 M H2S04,and 1.0 x 10-2M standard aqueous solutions of KH2P04 and Na2HAs0 and NaAsOz. More dilute standard solutions were prepared by appropriate dilution. Procedure. Determination of AsOz- and AsOt-. The sample is injected into a distilled water stream (Figure la) that merges with another one carrying an IO3- solution which oxidizes AsO; to AsO4%in reactor L1. The subsequent confluence with a MOO^^+ ascorbic acid channel gives rise to the formation of the heteropoly acid in reactor L2and subsequently reduction of complexed Mo(V1) to Mo(V), which is monitored at 820 nm. The signal obtained corresponds to the sum of the concentration of both ions in the sample. The selecting valve is turned prior to the following injection, thereby making possible the confluence of the sample with the distilled water stream, followed by that of the MOO^^+ ascorbic acid mixture; thus, the signal obtained at the detector is that of As043-alone. Determination of - 4 ~ 0and ~~The sample, a mixture of the two anions, is sequentially injected into a distilled water stream with the selecting valve in position 1 (confluence with an acidic stream that results in a signal corresponding to PO4%alone) and in position 2 (confluence with the distilled water stream that yields a signal resulting from the contribution of both As02- and Pod3-). Determination of AsOz-, and PO?-. After the sample is injected into the distilled water stream, it merges with streams of (1)sulfuric acid (determination of Pot-), (2) distilled water and (3) potassium iodate (determination of PO?- and AsO~~-), (determination of PO:-, and As02-). The multiple calibration curves allow resolution of the mixture in every case. RESULTS AND DISCUSSION All the determinations suggested here rely on the same indicator reaction (formation of a heteropoly acid and reduction of complexed molybdenum) using a Rheodyne 5041 injectian valve acting as selecting valve to obtain suitable conditions for the reaction of one, two, or all three species. Preliminary experiments showed the following: (a) The maximum absorption wavelength for the monitored product is 820 nm (20-23). (b) The mixing of MOO^^- and ascorbic acid prior to their confluence with the sample yields higher peaks than two sequential confluences. (c) It is necessary to work at temperatures above 50 "C to achieve a suitable reaction rate for the FIA technique (23-26). (d) The presence of glycerin (10% w/v) hinders the formation of the precipitate, which disturbs the stability of the base line. (e) The use of nitric acid instead sulfuric acid results in a remarkable enhancement of the analytical signal. Speciation of Arsenic. The method is based on the use of a configuration such as that outlined in Figure la. The sample, AsO2--As0?- mixture, merges, depending on the
0003-2700/86/0358-0120$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
121
Table I SAHPlE
sion coeff
re1 std dev, %
0.999
Ml21
0.999
k0.40
0.999
*0.95
regres-
5.'C i.$2nnT
\ I
I:
/
I
I
equation
,
linear range, M
hhs043-w,n = -0.0048 8.0 39i?.07[As0,3-] hAa04"rn.-= -0.0083 + 3972.73[As0,3-1 hAs02"ro3-= -0.OOO1 8.0 3243.45[AsOz-]
t
+ +
X
X
104-2,0
X
104-2.5
lo-'
X
Table 11. Resolution of AsO; and A s O l - Mixtures molar A~(v)/
9-
amt found,
PPm
PPm
ratio
re1 error, % As(V) As(II1) As(V) As(II1) As(V) As(II1)
20:l 3:l 3:2 1:l 1:l 1:l 2:3 1:3 1:15 1:20
14.98 11.24 11.24 11.24 3.75 0.75 7.49 3.75 0.75 0.75
As(II1)
Figure 1. Manifokl and optimum workin conditions for the determination of (a) As0 - and AsO,", (b) AsO, and PO4%,and (c) AsO,-, A s O P , and PO!-. I V and SV are injection and selecting valves,
amt added,
0.75 3.75 7.49 11.24 3.75 0.75 11.24 11.24 11.24 14.98
respectively. position of the selecting valve, SV, with an oxidant or distilled water stream; therefore, in merging with the Mo0,2--ascorbic acid mixture, AsOz- will be present in this form or will have been oxidized to As043- (thus allowing the determination of As043- present in the sample alone in the first position and the sum of both species in the second one). Optimization of Variables. This study has been performed using the modified simplex method (MSM) for interdependent variables (chemical and FIA), while the temperature has been optimized by the univariant method. The procedure followed involves the separate and sequential injection of As02- and As02- in each position of the selecting valve. The injection of AsOz- yields the transient signal (FIA peak whose height is denoted by hhoz-) only when the sample plug merges with the oxidant stream, while in the injection of As02- the signal obtained in both media is the same height, hAso4>.The selected response function takes into account the increase in the height of both peaks (their product) and a not very excessive difference between both (second term); thus, this function adopts the form
(hA802' - hhO4")
F = hAsOz-hA804" -
4
(1)
The MSM has been applied twice; once for optimization of chemical variables MOO^^-, HN03, IO3-, and ascorbic acid concentrations) and once for FIA variables (flow rates, injection volume, VI,and reactor lengths L1 and L J . The values corresponding to the centroid point in each application are shown in Figure 1. The evolution of the simplex imposed restrictive conditions in such a manner that the response function was considered nil for reagent concentrations giving rise to the formation of precipitate in the system, for negative reactor lengths, and for very long residence times, T (low sampling rate), taking T = 65 s as the limit. The study of the temperature shows an exponential increase in peak height with this variable, which compels the selection of the highest value at which air bubbles have not yet formed in the system, 55 O C . Determination of AsO; and As04? The calibration curves were run by separately injecting the As02- and AsO,~-solu-
14.83 11.31 11.09 10.79 3.66 0.76 7.44 3.76 0.74 0.72
0.74 3.56 7.42 11.39 3.58 0.75 11.01 11.16 10.71 14.88
-1.00 0.62 -1.33 -4.00 -2.40 1.33 -0.67 0.27 -1.33 -4.00
-1.33 -5.07 -0.93 1.33 -4.53 0.00 -2.05 -0.71 -4.71 -0.67
hod3-
tions. Owing to the fact that when the concentration of is changed, th: height of the peaks obtained in an oxidant medium, h$fb , and without oxidant, hf$" are slightly different, three calibration curves have been obtained: two corresponding to As043-and the one provided by AsO; (oxidant medium). The features of the linear range of these curves are given in Table I. The statistical study of the reproducibility of the method has been performed utilizing 11different samples in each case (triplicate injection), the arsenic concentration being 1.0 X M for both species. The sampling frequency referred to the sample injection was 36 h-' in each case. Mixture resolution was performed with the aid of the calibration curve and with alternating injection of the mixture in both media, using the expressions
+ 3947.07[A~0,3-1 hI03-= 0.8840 + 3912.73[A~04~-] + 3243.45[AsOZ-] hHzO = -0.0048
(2) (3)
where h is the peak height provided by the mixture in each medium. The results listed in Table I1 show the error of these determinations to be quite small; in fact it does not exceed the average value of 2% for concentration ratios of 1:20. Sequential Method for Determination of As043-and Po43-. The determination of these species is based on the use of an agent that prevents As043-from forming its corresponding heteropoly acid. The use of a selecting valve in a configuration similar to that described above (Figure Ib) allows selecting the stream of this agent or a medium in which both anions react with MOO^^-. Preliminary studies showed that, although in a previous paper (13)a HzS04-NazS205-Na2Sz03 reductant mixture was used to achieve the inhibition of the formation of the As02heteropoly acid, the use of a sulfuric acid solution at a concentration of 3 M or higher drastically hinders the formation, thus making As(V) reduction unnecessary. This acid concentration has therefore been adopted for its mixture with the sample prior to the confluence with the chromogenic reagent (position 1of SV), whilst when the valve is in position 2 it works under the conditions optimized in the above-de-
122
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table I11
Table V sion coeff
re1 std dev, %
0,999
f0.47
0.999
f0.57
0.999
f0.40
regres-
equation
linear range, M
hP04k~2904 = -0.0010 2.0 X 10-"5.0 + 1095.69[P0,3-] = -0.0057 hP04"~2~ + 1673.31[PO,3-] hA6043-~20 = -0.0032 8.0 X 104-2.0 + 3638.00[A~O~~-]
X
X
lo-' lo4
Table IV. Resolution of AsOf and Poi-Mixtures molar
amt added,
amt found,
P(V)/As(V)
ratio
PPm P(V) As(V)
PPm P(V) As(V)
re1 error, % P(V) As(V)
50:l 40:l 30:1 20:l 1O:l 41 1:l 1:2 1:3 1:4
15.48 12.39 9.26 6.19 3.10 6.19 1.55 1.55 1.55 1.55
15.79 12.36 9.26 6.20 3.15 6.07 1.54 1.64 1.60 1.51
2.00 -0.26 0.00 0.16 1.61 -1.94 -0.64 5.81 3.22 -2.58
0.75 0.75 0.75 0.75 0.75 3.75 3.75 7.49 11.24 14.98
0.73 0.74 0.72 0.72 0.70 3.55 3.55 7.48 11.46 14.91
equation
regression coeff
re1 std dev, 70
X
0.999
0.95
X
0.999
0.21
0.999
0.40
0.999
0.33
0.999
0.57
0.999
0.47
linear range, M
hAs02-10a= -0.0039 8.0 X 104-2.5 + 3020.97[A~02-1 hA6043-~03= -0.0027 8.0 X 104-2.0 + 3855.65[AsO:-] hA6043-H20= -0.0012 + 3886,20[AsO:-] hP04"~03-= -0.0016 2.0 X 10-'-5.0 + 1491.53[Po43-] hPo43-H20= +o.ooii + 1731.69[POt-] hP04" H2SO4 +0.0014 + 12O5.08[POd3-1
X
lo-'
-2.13 -1.07 -3.60 -3.87 -6.66 -5.39 -5.23 -0.16 1.97 -0.49
scribed method for determination of Taking into account that the indicator reaction is faster and more sensitive for Po43-than for AsO~~-, the method has not been optimized for the determination of PO:-, but for that of AS^^^-, using the previously established conditions. Determination of A s O ~ and ~ - PO:-. Since PO>- yields an analytical signal in the two media provided by the selecting valve (3 M HzSO, and H 2 0 streams) and As0:- only reacts in the aqueous medium, three calibration curves are necessary: two for Po43(one in each medium) and one for AsO:-. The equations corresponding to the linear ranges of these curves, the concentration ranges covered and the relative standard deviation (for As0:and Pod3-concentrations of 1.0 X lom4 M and 2.5 X M, respectively) are given in Table 111. (Note the remarkably good linearity of these straight lines (r = 0.999 in all cases.) The statistical study of the method, performed under similar conditions to those mentioned above, shows good reproducibility, with a relative standard deviation less than 0.6%. The sampling frequency was 36 h-l. With the aid of the calibration curves previously run the resolution of a series of mixtures covering P:As ratios from 5 0 1 to 1:4 has been carried out (Table IV). The procedure is very simple, involving thes-direct calculation of the Po43concentration (equation hL:&04) and substitution of this value into equation h$:( to calculate this height, which is subtracted from the peak height provided by the mixture in the same medium. Method for Sequential Determination of AsO2; As04*, and Pod3-.The determination of these three species in mixtures requires a working scheme such as that shown in Figure IC,in which SV can select three different streams which make possible the development of the indicator reaction via one, two, or the three species occurring in the sample. The principle of the resolution of the ternary mixture is as follows: The presence of 3 M H2S04in the medium inhibits the formation of the As04* heteropoly acid (position 1of the SV). The signal obtained corresponds only to PO:- present in the unknown. Position 2 of the SV (HzO stream) corresponds to the optimized conditions for determination of As04* in which PO-: also reacts, so that the signal obtained is due to the contribution of both.
t
t
t
Recordings obtained with the different carriers (depending and on the selecting valve position) for determination of P043-; AsO,~-; and PO:-, AsO,~-, and AsO,-, respectively. Figure 2.
When valve SV is turned to position 3 (oxidant stream), suitable conditions for determination of AsO; optimized in the first method are achieved (PO:- also reacts). Therefore, the peak height is a function of the concentrations of the three species in the mixture. The optimization of the system (both chemical and FIA variables) is performed as in the first suggested method, since As02- and As0:- are generally of lower concentration in relation to Po43and optimum conditions are established for the first two anions, to the detriment of the third. Determination of As02-,AsOd3-,and Po43-.Running the calibration curves in this case is a more laborious process because each species requires one curve per reaction medium. Therefore, only a single curve is necessary for As02- (confluence with an IO3- stream, the sole medium in which the species responsible for indicator reaction is obtained). Two calibration curves are necessary for As043- (in the media provided by positions 2 and 3 of SV). Finally, three curves are necessary for Po43-because this species yields a signal in all three media (Table V). It is noteworthy that while the signals provided by As043in the two media in which this species reacts are almost identical, those of are very different from each other (the one corresponding to the confluence with distilled water is higher). The straight segments of these curves are perfectly linear in every case. The statistical study of this method shows excellent reproducibility. Logically, the resolution of the ternary mixture is more complex since it entails the direct calculation of the Pod3concentration from the peak obtained in a H2S04medium and the transformation of this concentration into the peak height corresponding to the H20medium, which provides the concentration. Finally, both concentrations expressed as the heights of the peak obtained in the IO3- medium are subtracted from the height of the signal provided by the mixture
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
123
Table VI. Resolution of AsOz, AsOt-, and PO?- Mixtures molar P(V):As(V):As(111) ratio 1:1:1 1:l:l 1:1:2
1:2:1 2:l:l 1Ol:l 302:15 202:15 5:15:2 5:2:15
P(V)
amt found, ppm AsW) As(II1)
amt added, ppm AsW) As(II1)
P(V)
3.75 7.49 7.49 3.75 3.75 3.75 11.24 11.24 1.50 11.24
1.56 3.15 1.52 1.54 3.19 15.36 9.17 6.41 1.56 1.54
1.55 3.10 1.55 1.55 3.10 15.48 9.29 6.19 1.55 1.55
3.75 7.49 3.75 7.49 3.75 3.75 1.50 1.50 11.24 1.50
3.72 7.64 3.81 7.27 3.70 3.70 1.56 1.48 11.46 1.61
3.92 7.27 7.19 4.03 3.65 4.06 10.26 11.09 1.46 11.16
P(V)
re1 error, % AsW)
As(II1)
0.64 1.61 -1.93 0.65 2.90 -0.77 -1.29 3.55 0.65 -0.65
-0.80 2.00 1.60 -2.93 -1.33 -1.33 1.20 1.33 1.96 7.33
4.53 -2.93 -4.00 7.46 -2.66 8.26 -8.72 -1.33 -2.66 -0.71
Table VII. Sample Composition for the Study of InterferentsO sample no.
composition 10 ppm Si03z-,25 ppm NO,, 10 ppm NO,, 25 ppm C1-, 10 ppm SO4'-, 25 ppm CO?-, 5 ppm F-, 1 ppm phenol, 10 ppm Fe3+, 5 ppm Cuz+,50 ppm Cazt, 50 ppm Mgz+, 10 ppm NH4+ 5 ppm Si032-,5 ppm NO3-, 5 ppm NOz-, 20 ppm CL, 5 ppm SO4'-, 10 ppm C03'-, 5 ppm F-, 0.5 ppm phenol, 1 ppm Fe3+,2 ppm C U . ~50 ~ ' ,ppm CaZt, 100 ppm Mgzt, 5 ppm NH4+ 2.5 ppm S I O ~ ~10 - , ppm NO,-, 25 ppm NOz-, 20 ppm C1-, 10 ppm S04z-, 5 ppm CO?-, 10 ppm F-, 0.5 ppm phenol, 5 ppm Fe3+,5 ppm Cuzt, 50 ppm Cazt, 25 ppm Mgzt, 2 ppm NH4+ 5 ppm Si03z-,5 ppm NO,, 5 ppm NO,, 10 ppm C1-, 2 ppm SOf, 5 ppm CO:-, 2 ppm F-, 0.2 ppm phenol, 5 ppm Fe3+,10 ppm Cuzt, 50 ppm Cazt, 100 ppm Mgzt, 10 ppm NH4+ 5 ppm CO?-, 2.5 ppm F-, 0.5 ppm phenol, 5 ppm Fe3+, 20 ppm Si03z-,2.5 ppm NO3-, 10 ppm NO,, 25 ppm C1-, 5 ppm SO-,: 2.5 ppm Cuzt, 25 ppm Caz+,25 ppm Mgzt, 50 ppm NH4+ 12.5 ppm C o t - , 12.5 ppm F-, 1 ppm phenol, 2 25 ppm Si03z-,12.5 ppm NO3-, 5 ppm NOz-, 12.5 ppm C1-, 12.5 ppm SO-,: ppm Fe3+,10 ppm Cuzt, 12.5 ppm Caz+,25 ppm Mgzt, 5 ppm NH4+ 12.5 ppm Si032-,25 ppm NO,-,12.5 ppm NOT, 10 ppm C1-, 5 ppm SO4", 25 ppm CO?-, 12.5 ppm F-, 2 ppm phenol, 2.5 ppmFe3+,5 ppm Cuzt, 25 ppm Cazt, 50 ppm Mgzt, 12.5 ppm NH4+ 50 ppm Si032-,50 ppm NO,, 10 ppm NOz-, 25 ppm C1-, 12.5 pprn 25 ppm C03z-, 5 ppm F,0.5 ppm phenol, 10 ppm Fe3+,10 ppm Cuzt, 50 ppm Cazt, 25 ppm Mgzt, 25 ppm NH,+ 10 ppm SiO?-, 12.5 ppm NO,, 10 ppm NOz-, 10 ppm, C1-, 50 ppm SO4'-, 50 ppm C032-,20 ppm F-, 1.5 ppm phenol, 5 ppm Fe3+,25 ppm Cuzt, 25 ppm Cazt, 50 ppm Mgzt, 50 ppm NH4+ 25 ppm Si032-,25 ppm NO,-, 12.5 ppm NOT, 25 ppm C1-, 25 ppm SO?-, 25 ppm C032-,5 ppm F-, 0.2 ppm phenol, 5 ppm Fe3+,5 ppm Cuz+,10 ppm Cazt, 25 ppm Mg2+,12.5 ppm NH4+
1 2 3 4 5 6 7 8 9 10
"The concentrations of As(III), As(V), and P(V) are 8.32, 6.48, and 3.91 ppm, respectively.
Table VIII. Concentrations and Errors Found in the Determination of P(V), As(V), and As(II1) in the Presence of Interferents According to Table VI1 sample no.
amt of WV), ppm
error,
amt of As(V), ppm
error, %
amt of As(III), ppm
error,
%
1 2 3 4 5 6 7 8 9 10
3.89 3.91 3.91 3.94 3.88 3.98 3.96 3.97 3.87 3.93
-0.51 0.00 0.00 0.77 -0.77 1.79 1.28 1.53 -1.02 0.51
6.56 6.21 6.66 6.60 6.92 6.55 6.29 6.98 6.65 6.58
1.23 -2.93 2.78 1.85 6.79 1.08 -2.93 7.71 2.62 1.54
8.45 7.71 7.92 8.14 7.78 8.74 8.91 8.45 8.31 8.21
2.53 -8.43 -5.94 -3.33 -7.60 3.80 5.82 0.36 -1.31 -2.49
%
Table IX. Percent Recovery for P(V), As(V), and As(II1) from Natural Waters natural water from La Rambla Santa Eufemia Lucena Po20 CBrdoba
P(V) 3.91 5.22 3.91 5.22 3.91 5.22 3.91 5.22 3.91 5.22
amt added, ppm As(V) As(II1) 6.48 8.64 6.48 8.64 6.48 8.64 6.48 8.64 6.48 8.64
8.42 11.22 8.42 11.22 8.42 11.22 8.42 11.22 8.42 11.22
P(V) 3.98 5.30 3.90 5.27 3.95 5.27 3.87 5.20 3.94 5.16
atm found, ppm As(V) As(II1) 6.66 8.82 6.35 8.61 6.48 8.67 6.29 8.53 6.55 8.40
7.99 10.88 8.53 10.91 8.31 11.12 8.50 11.55 8.49 11.55
P(V)
recovery, % As(V)
As(II1)
101.8 101.5 99.7 101.0 101.0 101.0 99.0 99.6 100.7 98.9
102.8 102.1 98.0 99.7 100.0 100.3 97.1 98.7 101.1 97.2
94.9 97.0 101.3 97.3 98.7 99.1 101.0 102.9 100.8 102.9
124
Anal. chem. 1986, 58, 124-127
in this medium to obtain the AsOf concentration (see Figure 2). These calculations by difference imply an increasing error accumulation from As043- to AsOz-, as shown in Table VI, from which it can be also observed that the mixtures studied are from P(V):As(V):As(III) molar ratio of 1:l:l to 30:2:15, 5215, and 5152. Higher ratios and concentrations than those listed in Tables V and VI have not been used owing to the fact that the signal obtained in the IO3- medium (in which all three species react) falls out of the range in which reliable absorbance measurements can be made. As can be observed from the above-mentioned tables, the errors in the determination do not exceed 3% in any case for PO-: (and only in one of the mixtures for AsO4%),while for AsOf the percentage error amounts to 8.7. The average errors of these mixtures are 1.5%, 2.20%, and 4.33%, respectively. If sodium tartrate (concentration 1%)is added to the samples, silicate is tolerated up to 400-fold concentration that of each of the three analytes. Determination of AsO;, A s O ~ ~ and - , P043-in Real Samples. Prior to determining these anions in real samples, we have performed a study of interferenta by preparing a series of samples containing a fixed amount of As02-, and PO4+ (1% sodium tartrate to overcome the interference from Si032-)and variable concentrations of cationic and anionic species as shown in Table VII. The errors made (Table VIII) are similar to those found in Table VI for samples containing no foreign species. The determination of the ternary mixture of these anions in real samples has been carried out on five different natural waters from five towns and villages in the province of Cordoba which contained none of these anions and to which two different amounts of AsOz-, AsO~~-, and PO:(1% sodium tartrate) were added. The percent recoveries found are shown
in Table IX. These recoveries are between 95 and 103%, which shows the validity of the suggested method.
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RECEIVED for review April 9, 1985. Accepted July 19, 1985.
Determination of Trace-Level Chromium(V1) in the Presence of Chromium(I11) and Iron(III) by Flow Injection Amperometry Kenneth W. Pratt* and William F. Koch Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Chromlum(V1) Is determined by flow InJectionamperometry at Au and Iodized Pd electrodes without prlor chromatographic or other separation. Dissolved 0, and Cr( I I I ) do not Interfere. Use of H,PO, as the supportlng electrolyte suppresses the interference f r m Fe( I I I). Chloride Ion Interferes in the detemlnatlon at Au electrodes but not at Pd electrodes. Decay In sensltlvlty of the electrodes wlth tlme has been eliminated by contlnuous precondltionlng of the electrode wlth a pulsed-potentlal wave form In place of constant-potentlal amperometry. The detectlon limit for Cr(V1) is 5 ng/mL.
The toxicity of Cr depends on its oxidation state, Cr(V1) being significantly more toxic than Cr(II1) (1-3). Hence, oxidation-state-specificdeterminations of Cr are of particular interest. Element-specific techniques, such as atomic absorption spectrometry, require a preliminary chemical separation of Cr(V1) from Cr(II1) for the selective determination
of Cr(V1). This separation is generally achieved by liquidliquid extraction ( 4 , 5 ) or ion exchange (6-9), requiring additional sample preparation prior to the actual determination. Amperometric (electrochemical) determination of Cr(V1) inherently discriminates against Cr(II1) without preliminary chemical separation. However, amperometric techniques are not element-specific, and other species that are reduced at the potential used for reduction of Cr(V1) interfere with its determination. One particularly significant interference in environmental samples is Fe(II1). Previous workers have used polymer-modified electrodes (10, 11) or liquid chromatographic separation (12) to eliminate the interference of Fe(II1) in amperometric determinations of Cr(V1). Here we report an alternative procedure: the use of H3P04as the supporting electrolyte for the trace-level amperometric determination of Cr(VI), at Au or Pd electrodes. This procedure suppresses the interference from Fe(II1) since the complex species Fe(P04)36and Fe(HP04)33-(13-15) are formed and are not reduced at the potential used for the amperometric detection
This article not subject to U S . Copyright. Published 1985 by the American Chemical Society
