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A mathematical model for continuous hydride generation with inductively coupled plasma spectrometry—II. pH dependence of hydride forming elements
.Spem&m~ca Acta. Vol. 425. Nos l/2. pi Pnnled in Great Bnlaln
139-M.
1987.
0
0584~854?f%?s3.00+ 0.00 1987PngamonJournals Ltd.
A mathematical model for continuous hydride generation with inductively coupled plasma spectrometry-II. pH dependence of hydride forming elements XIAORU
WANGand RAMONM.
BARNES
Department of Chemistry, GRC Towers, University of Massachusetts, Amherst, MA OlOO3OO35, U.S.A. (Receiued 5 Fehary
1986; in reuisedfarnr 16 June 1986)
Abstract-The solution pH dependencies of four hydride forming elements, As, Pb, Se and Sn, have been studied theoretically and experimentelly. All of the main chemical reactions related to the hydride forming systems are considered, and detailed mathematical calculations are experimentally verified with ICP spectrometry.
1, INTRODUCTION HYDRIDE generation based on the volatile hydrides of As, Bi, Ge, Pb, Sb, Se, Sn and Te has been used widely in atomic absorption (AAS) and inductively coupled plasma (ICP) s~trometry to determine trace levels of these elements. Ex~riment~ly, the technique is well established for almost all of the hydride forming elements, but theoretically, hydride generation is less well characterized. Following a previous study of a mathematical model for hydride transfer [l J, we shall evaluate the pH dependence of hydride generation theoretically and experimentally in the present study. The solution pH has a significant effect on the hydride generation process, and various acids have been applied in hydride generation [2-4]. The optimum pH regions for all hydride forming elements have been reported [S J; however, those are almost exclusively experimental studies. A theoretical prediction of the effect of pH on hydride generation has not yet been performed. In the present study, four hydride forming elements, As, Pb, Se and Sn, are investigated both experimentally and theoretically. Consideration of the main chemical reactions related to hydride formation are included in detailed mathematical ~lculations. Experimental verifi~tion using ICP s~trometry provides comparisons of the theoretical description.
2. EXPERIMENTAL 2.1. Apparatus ICP atomic emission
spectrometry (AES) operating conditions for As, Pb, Se. and Sn with a 40.68 MHz rf generator, l-m Czerny Turner monochromator (slit height 5 mm, slit widths 0.05 mm) at 15 mm observation height above the induction coil are summarized in Table 1. A commercial continuous hydride generator (Plasma-Therm. Ltd) based upon the work of THOMPSON et al. [4-S] was applied in the amine and stannane generation study. A commercial flow injection inst~ment (FIAtron, SHS 200) was adapted for plum~e and selenium hydride generation. This apparatus included an all-Teflon, dual channel sample injector consisting of e~~troni~ly driven, microminiature seienoid valves. A multiroller, variable-speed pump situated before the valves can be operated in continuous-flow or in stop-flow modes. The former was applied for plumbane generation, and the latter was applied to the study of selenium hydride generation after modification of the manifold. A schematic diagram of the FIA modification is shown in Fig. 1. [l] X. R. WANGand R. M. BARNES,Spectrochim. Acta 41B, 967 (1986). [2] Y. YAMAMOTO and T. KUMAMARU. Fresenius 2. Anal. Gem. 281, 355 (1976). [3] P. W. V~JANand G. D. WOOD,Analyst (London) 101,966 (1976). [4] H. D. FLEMING and R. G. IDE, Anal. Ckim. Acta 83,67 (1976). [S] T. NAKAHARA, Prog. Anal. Atom. Spectrosc. 6, 163 (1983). [6] M. THOMPSON, B. PAHLAVANPOUR, S. J. WATSONand G. F. KIRKBRIGHT, AMlyst (.&&ox) 103,568 (1978). [7] M. THOMPSON, 8. PAH~AVAN~UR,S. J. W~rxt~ and G. F. K~RKBRIGHT, Analyst (London) 103,705 (197X). [8] M. THOMPSON and J. N. WALSH,A quick of l~~tiee~y Coupled Plasma Spectrometry, pp. 152-155. Blackie, Glasgow (1983). s142:1/2IhJ
139
X~hoauWANGand RAMON M. BARNES
140
Table 1. ICP-AESoperatingconditions As
Pb
Se
Sn
193.7 15 0.28 0.42 0.95
220.1 15 0.4 0.4 1.1
196.1 15 0.1 0.5 1.05
X90.3 15 0.25 0.4 0.8
Parameter Wavelength(nm) Qltm gas (I/b) Intermediategas (l/min) Carriergas (i/tin) Power (i&V)
ICP
Borshydrida
Fig. 1. Schematicd&mm of FIA controlledhydride generation.
The glass 8ow cell for gas-liquid separation is similar to that ofTHOMPSON et al. [6-g] or T~OHet al. [?I. The argon carrier gas Bushed the vapors through the upper outlet side tube directly into the base of the plasma torch.
Certified reagents were used to prepare standard stock solutions of X000#&ml As, Pb, Se and Sn. Various solutions of sodium borohydride (OS-5 %) in 0.2 M sodium hydroxide were prepared. The effects of the concentrations of borohydride and hydrochloric acid on the hydride generation of four
hydride-forming elements, and the effect of nitric acid on selenium hydride generation were studied. The optimum hydride generation conditions for four elements are summarized in Table 2. 3, THEORYAND EXPERIMENTAL
For this ~v~~~~on~ interfering elements [7,8-J were assumed to he absent. Only ~~dr~h~or~~ acid was considered except for selenium hydride generation, for which both hydrochloric and nitric acids were compared The activity coe&ients were assumed to he equal to 1, and the apparent concentrations were used for ah theoretical calculations. The pH in these calculations refers to the value after the reaction. 3.1. pH dependence of amine generation 3,l. 1. Predicted pH dependence. ‘The main chemical reactions related to pH in this system include the reduction of As+~ to As’ 3 and As+~ to As’, and the dissociation of borohydride. The reduction of As” to As’s may be represented as ~AsO;~+BH;+H+
[9] N. H. TIOH,
Y. ISRAEL
and R. M. BARNES,
= 4AsU3 ’ -k H3 30s + H2 0.
RR?&.
CMn.
Acta,
in press.
(1)
Continuous hydride generation
141
Table 2. Hydride generation conditions Element Technique As Pb se
caltinuous FIA-on-line FIA control
Sn
continuous
BHi (%)
Acid (M) HCI, 1 HCI, 0.3 HCI, 2 HNO,, 1.25 HCl, 0.1
Oxidant (M)
1 5 2.5 2.5 0.5
none H,02,0.8 none none none
The half reduction reaction can be represented as As0i3+2e-+2H+
= AsO;~+H~O.
(2)
This latter reaction can be represented in terms of an equilibrium constant K, [lo]. [As0i3 ] Kr =
[A&;“]
Ce-]2
[H’]’
=
(3)
l”*‘l’
Based upon Eqn (l), electrons are produced by the oxidation of borohydride. Stoichiometrically, this yields [e-] = 8[BH;]. In the present experiments, [BH; ] = 0.25 M, which gives the concentration of electrons, [e-] = 8(0.25) = 2M. Log K, of the reaction is 8.1 [lo]. When Eqn (3) is rearranged, and 1 is added to both sides, Eqn (4) is obtained. [AsO; “1 [As0i3] + [AsO;j ] = 10m8.’ [e-l-
[H’]”
+ 1’
(4)
If 100 % of the available electrons were consumed in arsine generation, then substitution of [e-] = 2M into Eqn (4) yields
CASO;~I = [AsIt
1
lo-8.12-2[H+]-2
+
1’
(5)
[AsI is the sum of [AsO;“] and [AsO;~]. A curve with the fraction of reduced As+’ is plotted in Fig. 2 as a function of PH. Above pH 3.5 the reduction of As+’ decreases, and no reduction occurs at pH > 5.6. The reduction of As+~ to As0 was also studied. The reaction is HAsOz+3H’
+3e-
= As”+2H20.
(6)
The standard potential of the reaction E, = 0.2475 V 183, hence, the free energy of the reaction, AGO,can be calculated as AGO = - nFEo = - RT 1nK = - 3(96500) (0.2475) (J/mol) = - 17.14 (kcal/mol)
(7)
n is the number of electrons transferred and F is the Faraday constant. The calculated log K is 12.6, which gives
CAs”l
logK = log
pAso
1
CH
+
13
=
12.6.
(8)
[As”] is the concentration of intermediate state of arsenic in the solution. -Rearranging Eqn (8) and adding 1 to both sides results in I; 1 + [H + ]” 10i2+j
[As”] + [HAsO The fraction of [HAsO,]
[AsO1 t-H+ I” 10’2*6reduced in the system may be represented as [HAsO~-J,~~
Pslt [lo] J. INCRWDY, (1976).
AMlyrical Application
of Compkx
=
[H’]’ 10’2*6 1+ [H’]” 1012.6’ Equilibria
(9)
(10)
(Translation Ed. J. TYSON).Wiley, New York
XIAORUWANG and RAMON M. BARNES
142
1.00 +
-1.00
3. 40
1.20
5. 60
7.80
1 *00
PH Fig. 2. Theoretical reduetion of As+’ to As+~ as a function of pH with the assumption that 100% of theelectronsareeonsumed (l).Theoretical pH profilesofAs+s to As+j reduction with less than 100 % electron consumption. (2) loo/,, (3) 1 “/ (4) 0.1%, (5) 0.05 %.
The theoretical pH dependent profile based on this equation is illustrated in Fig. 3 with the pH profile of As(II1) hydride generation reported in Ref. [i2]. Both curves generally agree well except the slope of the falling side of the curve from Ref. [ 121 is smaller than that of the theoreti~l profile. This is attributed to buffered reaction solutions in Ref. [12]. Solution pH also strongly affects the dissociation of borohydride in the presence of hydrochloric acid. The accepted dissociation of borohydride is [5-j BH; +3Hz0+H+ a-x
+H,BO,+gH’. b-x
(11) x
Since this reaction has a large negative AGO,it proceeds spontaneously, as has been proven experimentally. Here the reaction is considered only kinetically. The original concentrations of borohydride and hydrochloric acid are a and b, respectively; x is the concentration of dissociated borohydride at time t, and k is the reaction rate constant. In this case, the change of ~on~ntration of dissociated ~rohydride with time can be represented as dx -= dt
-d(a-x) dt
Upon integration of the above equation with A = kt and rearrangement, borohydride dissociated, x/a, is then given as
x/a =
w
= kfa-x)(b-x).
the fraction of
1 l-- eAta_-b) 1 (a/b) - A . &@-bl
(13)
When a G b, the original concentration of borohydride is less than or equal to the con~ntration of hydrochloric acid, A(a - b) B 0. This results in eAfuebf Q 0. Then the above equation can be approximated as x/a = 1, which means that borohydride has been completely dissociated in an excess of hydrochloric acid.
[ll] J. G. STARKand H. G. WALLACE,Chemistry Date Book, 2nd Edn. John Murray, London (1982). 112) J. AGGETTand A. C. ASPELL,Analyst (London) 101, 341 (1976).
Continuous hydride generation
143
0. 80 As(III)
to As(B)
0. 20
0. 00
l-
.l. 00
I-
1.00
3. 00
5. 00
7. 00
9. 00
PH Fig. 3. Theoretical pH profile of the fraction of As+’ reduced to As0 (solid line). Experimental results [12] for As +3 hydride generation (0).
On the other hand, when a > b, for example, the original concentration of borohydride is greater than that of hydrochloric acid, A(a -b) 9 1, which results in eA(‘-‘)% 1. Therefore, the above equation can be approximated as x/a = b/u. A formal representation may be written as
P-L ldiss. _ w 3 PHBIwig. - [BHi I orig.’
(14)
For which [BH,],i,, is the concentration of dissociated borohydride, and [BH; ]Orig,is the original concentration of borohydride. The curve of the fraction of BH, dissociated as a function of pH based on the above equation is given in Fig. 4. These results predict that BH, does not dissociate above pH 2. In a higher pH range (over pH 7), several monomeric species such as ASP , As(O and several polymeric species such as Asg (OH), , Ass (OH),, may exist. Because they are not related to the pH range of amine generation, they are not considered. 3.1.2. Experimental amine generation. Under the optimum ICP-AES operating and hydride generation conditions summarized in Tables 1 and 2,2 &ml of As(V) solution with pH values ranging from - 1 to 2 were examined. An experimental profile of pH dependence of arsenic hydride generation is illustrated in Fig. 5 and is compared with the dissociation of borohydride and the reduction of As+5 to As+’ in Fig. 6. These theoretical and experimental studies demonstrate that the most important reaction affecting As(V) hydride generation appears to be the dissociation of borohydride, even though a small deviation from the experimental result is observed. However, since the concentration of borohydride is far greater than that of As+ ’ in the system, and the standard reduction potential of As(V) to As(II1) is relatively high ( + 0.58 V, [ll]), it might be reasonable to assume that < 100% electrons participate in the reduction of As(V) to As(II1). Therefore, further calculations with 10 %, lx, 0.1% and 0.05 % of electrons used in As + ’ to As + 3 reduction reaction, respectively, were made, and the results also are illustrated in Fig. 2. With the assumption that only 0.05 % of the electrons are consumed, the model matched very well with the experimental results (Fig. 7). The theoretical and experimental studies of pH dependence of both As(V) and As(II1) hydride generation indicate that both reactions are pH dependent. However, As(V) reduction occurs in a lower pH region which is restricted mainly by dissociation of the borohydride. The reaction condition of As(II1) is relatively broad, as has been verified in many experiments
XIAORUWANG and RAMONM. BARNES
,j
0.00
f -1.00
r
1. 20
5. 60
3. 40
7. 80
10.00
PH Fig. 4. Theoretical dissociation curves of BH; at different concentrations as a function of pH. Individual curves correspond to borohydride concentrations of (1) 1.32 M, (2) 0.66 M, (3) 0.25 M, and (4) 0.13 M.
Experlmsntal
400.00
‘: 1
300.00
: o-l m” 200.00
0.00i -1.00
1.00
5. 00
3. 00
7. 00
9. 00
PH Fig. 5. Experimental pH profile of As+’ hydride generation with 2 &ml As(V).
[13, 145, Only a little dissociation of borohydrideis necessary for As(II1) hydride generation. For instance, if the dissociation of borohydride is assumed to be only 1% of that in the case of As(V) hydride generation, the theoretical pH proflle of borohydride dissociation will match well with the experimental one (Fig. 8). The selective determination of As(V), As(II1) and the total arsenic by hydride generation, therefore, may be performed by the control of pH value of the solution. [13] A. U. SHAIKHand D. E. TALIMAN,And. Chim. Acra 98,251 (1978). [14] M. H. ARBAB-ZAVAR,Analyst (London) 105,744 (1980).
Continuous hydride geuemtion
145
1.00
-1.00
I. 20
3. 40
5. 60
7.80
J
fH
Fig. 6. Corner
of ~~~ results of AS+’ hy~de lotion (2) witb tbeoreticaI As+’ to As+~ reduction curve (3) and the dissociation of 0.25 M SW; (1).
0. 80
0. 20
0. 00 00
1. 20
3. 40
5.60
7. 80
18.00
PH
Fig. 7. Comparison of normalized experimental pH profile (3) witb theoretical reduction o~As+~ to As+’ assuming 0.05 % efectrons are consumed (2), and theoretical d&o&&on curve of BH; (1).
3.2. pH e&ct on tin hydride generation 3.2.1. Predicted gH dependence. The following reactions are considered to be related mainly to the concentration of hydrochloric acid during tin hydride generation. In high concentration of HCl solution, the main reaction occurring is the complexation of Sn +‘. &Cl, -I-Cl- 4 SnCl; .
(15)
The equilibrium can be represented as [ll, 15, 167 [SnCl; ] log K = [SnC1,] [Cl_ 1 = 1.5. [IS] F. A. COTTON, Adtmzced Inorgan& Chenrisry, 2nd Edn. Wiley, New York (1978). [l&j A. S. DOUGLAS,F~~~nt~s ofAnaly~ka1 Chemistry, 4th Fidn. SaGtide@,P~~dcip~~
(1982).
XIAORUWANGand RAMONM. BARNES
146
oc
‘”
ct
0. 60
: It
0.40
0. 20
0.00
L I.20
-1.00
3. 40
5. 60
7. 80
18.00
PH Fig. 8. ~rn~~n of normaliked theoretical results of the reduction of AsC3 to As’, and ex~~rn~~ results of Asi hydride generation [lZ] (a) with theoretical dissociation of BH; assuming only 1 yOdissocktioa of that in the case of As(V) hydride generation.
Apparently, [Cl-] = [H’] h ere, therefore, [Cl-] in the above equation can be replaced by [H+ J and rearranged to obtain the fraction of SnCl, uncomplexed by Cl-.
isnc121 _
CW
0.032 [H+] + 0.032’
(17)
For which, [Sn& = [SnClz] + [S&l; ]. The curve calculated from Eqn (17) with [SnC12]f[S n ] t as a function of pH, illustrated in Fig. 9, shows that the complexation of %-I+’ has a very strong influence on stannane generation in the lower pH region. The hydrolysis of Sn f2 also produces a strong influence on tin hydride generation. Sn2+ t-H,0
i-H+.
= Sn(OH)+
(18)
The calculated K is 0.2.
iswH)+I CH’I = [Sn2+] The fraction of unhydrolyzed
O2
(19)
’ *
Sn2 + can be expressed as -ul_
2i-
is4
[Wf 0.2+[H+]’
Again, [Sn],is the sum of [Sn2 ’ ] and [Sn(OH)' ] in this system. The curve for [Sn2+]/[Sn], as a function of pH also is shown in Fig. 9. This curve predicts that most of the SnZf is hydrolyzed above pH 3. Another hydrolysis reaction of Sn2+ in higher pH values is 3Sn2+ +4H20 = Sns(OH)i+ +4H+. (21) The pK value of SnJ (OH)$+ dissociation constant is 6.77 [15]. Therefore, the equilibrium can be expressed as
[SIP+]3
-l”g[Sn3(OH);+] [H']' Based on Eqn (21), stoichiometri~ly,
[Sn2’]
(22)
= 6*77*
= 3[Sn3(OH)i+],
[Sn2+],is, = [Sn”‘] + 3[Sn, (OH)i’].
which yields (23)
Continuous hydride generation
147
0.00\
-1.00
1. 20
3. 40
5. 60
7.90
1
100
PH Fig. 9. The theoretical curves including the dissociation of borohydride (l), thecomplexation of Sri+++ as SnCI; (2) and the hydrolysis of tin, Sn(OH)+ (3) and Sns (OH)12 (4). as a function of pH.
If the original concentration of tin is 10m5M (in this work), the concentration hydrolyzed Sn2 + may be represented as [Sn, (OH):+]
k 1/3([Sn2+],,,is,-
[Sn2+])
of (24)
= l/3(10-’ - [Sn “1).
l/4 IH+l= [l()-7(l()-I_ 1
Substitution of [Sri3 (OH);+]
in Eqn (22) with Eqn (24) yields upon rearranging 3[Sn2+13
[Sn2+])
(25)
The fraction [Sn2+]/[Snlr as a function of pH based on this relationship is illustrated in Fig. 9. This hydrolysis reaction is important only at pH > 3. Other tin complexation with chloride may occur in hydrochloric acid. However, they were not considered here owing to insufficient data. The dissociation of borohydride (also plotted in Fig. 9), of course, strongly affects stannane generation, as discussed in the previous section. 3.2.2. Experimental stannane generation. Under the optimum ICP operating and hydride generation conditions, 2 &ml of Sn2 + solutions with different pH values were pumped into a commercial continuous hydride generator, and the stannane was analyzed by ICP-AES. The experimental profile of pH effect on stannane generation is illustrated in Fig. 10, and the comparison of experimental and theoretical results is given in Fig. 11. The pH dependence of tin hydride generation might be restricted by both dissociation of the borohydride and the complexation of Sn2 + or controlled by the complexation reaction of tin and its first order hydrolysis reaction. Since the normalized results of the latter are closer to the experimental results, this hypothesis is believed to be more reasonable. 3.3. pH dependence of lead hydride generation 3.3.1. Predicted pH dependence. The hydride generation efficiency of Pb’+ to plumbane and detection limits of lead are very poor with the straightforward method [17,18]. Adding some oxidant to the lead hydride generation system has proved to be an effective technique for determination of trace lead [ 19,201. In the present study, hydrogen peroxide was adopted [17] J. F. CHAPMAN and L. S. LALE,Anal. Chim. Acta 111, 137 (1979). [18] K. C. THOMPSON and D. R. THOMPSON, Analyst (bttdon) 99, 595 (1974). [19] J. KAZUOand T. MITSUHIKO, Anal. Chim. Acza 143,229 (1982). 1203J. R. CASTILLO, J. M. MIR, C. MARTINEZ, J. V~~and M. P. COU)N, Mikrochim. Acta [Wien]
1,253 (1985).
148
XIAORUWANGand RAMONM. BARNES 40.00
32.00 > :
24.00
0-I ; cot
16.00
-1.00
1. 20
3. 40
5. 60
7.80
1
00
fH
Fig. 10. The experimental profile (o)pH effect on stannane generation with 2 &ml of Sn+’ solutions.
1.00
;
i
0.80 -
! i LL 0.60 -
-G E 2 0.40:
O.-O0 7 -2.00
Fig. 11. The comparison
-. 60
0. 80
2. 20
3. 60
5‘.
00
1
normalized experimental (a) and theoretical profiles of tin hydride generation as a function of pH.
as the oxidant. The chemical reactions related to pH in the HzO,-HCl lead hydride generation system involve the complexation of lead with chloride ion, the hydrolysis of Pb2 +, and the dissociations of borohydride and hydrogen peroxide.,For each of the following equilibrium calculations, the assumption is made that no other complexes are formed simultaneously, so that only one equilibrium reaction is treated at a time. The complexations of lead with Cl- may be represented as Pb+‘fCl-
= PbCl+
[21] A. J. RUBIN, Ed., Aquous Environmental Chemistry
1ogK = 1.6[21].
of Metals.Ann
(26)
Arbor Press, Ann Arbor, MI (1974).
Continuous
The fraction of uncomplexed represented as
149
hydride generation
Pb2+ in the original concentration [Pb* + -J 1 - P-w = 1+ 101*6[H+]’
of lead can then be
(27)
Similarly, Pb* + + 2Cl- = PbC12 and Also, and Finally, and
1ogK = 1.8 [21]
CPbZ+I= cwt
1
(2%
1 + lo’.8 [H+]*’
Pb*+ + 3Cl- = PbCl; [Pb* + ] ____ = 1+
t-Pblt
IogK = 1.7 [21]
(31)
[H+]3.
1 1+ 10’.4[H+]4’
IBIt
(30)
1 101.7
Pb2 + c 4Cl- = PbCl; * log K = 1.4 [21] [Pb*+] -=
(28)
(32) (33)
[PbJ corresponds to the sum of [Pb*+] and the individual complex, respectively. The pH effects on those complexation reactions are illustrated in Fig. 12. The hydrolysis reaction of Pb* + can be represented as Pb*+ +H,O
= Pb(OH)+ +-H+.
The calculated K is 1O-8.15, and the fraction of unhydrolyzed [Pb* ‘1
___ [Pblt Similarly,
w+1 = [H+]+ 10-8.15’
(34) Pb*+ is represented as (35)
Pb* + + 2H2 0 = Pb(OH)* + 2H +.
(36)
The calculated K is 10-r7.*. This yields
[pbz+l= Pw
[H+]2+
P-I+I’
(37)
10-17.2’
The third order hydrolysis reaction is given as Pb*+ +3H,O
= Pb(OH);
+3H+.
(38)
The calculated K is lo- 28.1. Therefore, the fraction of unhydrolyzed Pb* + can be obtained from
L-*+1 _ CPblt
[H+]3
CH+ 1” +
10-28.1
*
(3%
The pH dependencies of lead hydrolysis based on the above equations are illustrated in Fig. 13, and the combined pH effects on both lead complexation and hydrolysis are given in Fig. 14. The study predicts that pH ranging from 4 to 6 seems good for lead hydride generation because 100% free Pb * + should be expected in this pH range. However, in practical lead hydride generation experiments, the optimum pH range is relatively narrow, and the maximum signal could be expected at pH about 0.5 (Fig. 15). The major deviation of the theoretical from experimental results appeared to be due to the hydrogen by-product which exhibited a large overpotential [22,23] on lead. This overpotential significantly inhibits the dissociation of sodium borohydride; therefore, the further. reaction of Pb to plumbane is prevented. In order to resolve this problem several oxidants such as potassium [22] M. D. ZHOULDERand V. V. STENDER,Zh. Prid. Khim. 31, 711 (1958). [23] W. A. KOEHLER,Principles 2nd Edn, Vol. 2, p. 47. Wiley, New York (1944).
150
XIAORUWANG and RAMONM. BARNES
Fig. 12. Theoretical pH dependencies of lead complexes (1 PbCI+, 2 PbC&, 3 PbCl; ,4 PbCl;‘), and the dissociation of 1.32 M borohydride (5) in the presence of 0.8 M hydrogen peroxide.
cc 0
0. 60
k ;: 0
0.40
: L 0. 20
0. 00 + -1.00
1.20
5. 60
3. 40
7. 60
10.00
PH Fig. 13. Theoretical hydrolysis curves of lead (1 Pb(OH)+, 2 Pb(OH),, 3 Pb(OH); ) as a function of PH.
dichromate, hydrogen peroxide, or peroxidisulfate were applied as depolarizing agents. In the presence of oxidants in the reaction system, the pH dependence of lead hydride generation varies wth the actual reaction system (i.e. selection of oxidant, concentration of oxidants, concentration of borohydride). At this stage, the HCl-H202 system is adopted in the present work; therefore, the pH dependence of lead hydride generation was considered with the addition of only hydrogen peroxide. The redox reactions included in the system are the following: (1) The reduction of borohydride Eqn (11). (2) The reduction of lead ion Pb’+ + 2e- +Pb.
(40)
Continuous hydride generation
151
PH Fig. 14. Combined lead chloride complexes (1 PbCI+, 2 PbCl,, 3 PbCl;, 4 PbC1;2) and first lead hydrolysis (5 Pb(OH)+).
(3) The dissociation of hydrogen peroxide H,Oz + 2H+ + 2e- 42&O.
(41)
Stoichiomet~~lly, one BH; is equivalent to 4Hz02. If the con~ntration of H202 % Pbzf, then the concentration of electrons consumed by Pb2+ may be ignored. In other words [HzOt] = 4[BH;]. Also, [BH;] = [H’], and 2[H,Os] = [H’]. The total concentration of [H ‘1 which should be consumed for quantitative lead hydride generation, [H+],, is the sum of [BH;] and 2[H,O,] which equals 9[BH;] or 9/4[H,O,], depending on which concentration is lower. The fraction of [H+lcurrent to [H+], which should be consumed in the reaction may be used to represent the actual fraction of dissociated borohydride. For instance, in the present study, [BH;] = 1.35 M (5 %), [HzO,] = 0.8 M, the actual fraction of dissociated borohydride to the original concentration of borohydride could be represented as (42) The effect of pH on the actual dissociation of borohydride in the presence of hydrogen peroxide is shown in Fig. 12 along with pH dependence of all of the reactions described above. The optimum pH region of lead hydride generation, therefore, is restricted mainly by the fourth order of the complexation of Pb’+, and the dissociation of borohydride in the presence of 0.8 M hydrogen peroxide. 3.3.2. Experimental plumbane generation. Various 2 fig/ml Pb* + solutions with different pH values were analyzed with a FIA on-line hydride generation technique. The experimental pH dependence of lead hydride generation is represented in Fig. 15. The normalized experimental and theoretical results are compared in Fig. 16. The optimum pH region in both studies matches well. 3.4. pH dependence
ofselenium hydride generation
Solution pH dependence of selenium hydride generation has been studied in both hydrochloric acid and nitric acid. Various 0.2 @g/ml Se solution with different pH values in both hydrochloric and nitric acids systems were examined with FIA controlled on-line hydride generation technique (Fig. 1). The results obtained from the two are compared in Fig. 17. The experimental profiles of pH dependence of the selenium hydride generation in the two acids are different. The optimum hydride signal should be expected with almost the
152
XIAORU WANG and RAMONM. BARNES
a
6. 40
k --i
4.80
: m c;;
3.20
1. 60
-1.00
1. 20
3. 48
5. 60
7. 80
IW. ww
PH Fig. 15. Experimental pH dependence of lead hydride generation with 2pgJml Pb”
00
Fig. 16. The comparison of normalized experimental (o)and theoretical (solid line) pH profiles of lead hydride generation.
same pH region in both acids. However, a plateau in HCl was observed before the pH inflection point. In contrast, a maximum was observed in the pH dependence in HN03. Therefore, the reaction mechanisms in both acids were examined theoretically in order to rationalize these differences. 3.4.1. Hydrochloric acid. After consideration of all possible chemical reactions related to selenium hydride generation, we found the main effect that restricts the optimum pH region of selenium hydride generation is the dissociation of borohydride, as discussed in Section 3.2. The theoretical results for borohydride dissociation are compared with the experimental pH effects on selenium hydride generation in both acids in Figs 18 and 19, respectively. These comparisons indicate that calculated results of borohydride dissociation agree well with the experimental results in the HCl system. They also match well with the falling side of pH profile obtained from the HNOJ reaction system. The unmatched part of the latter is attributed to the oxidizing property of nitric acid.
Continuoushydridegeneration
$
153
0. 60
-1.00
1. 00
3. 00
5 00
7. 00
9. 00
Fig. 17. Experimental pH dependence.of Se hydride generation with O.Z&ml Se+* in hydrochloric acid (0) and nitric acid ( x ).
0. 80 -
0.20
-
Fig. 18. Comparison of normalixed experimental results of Se +* hydride generation in HCI with the dissociation of borohydride pH profile.
3.4.2. Nitric acid. The effect of nitric acid on H$eO, H#eO, b-x
has been studied as
+ NO; 4 SeOt - f H + + HN02. a-x
(43)
x
Similar to the studies in the dissociation of borohydride, represented kinetically as
the above reaction can be
dx = k(u--x)(b-x). dt
-
If the concentration
of nitric acid is assumed to be far greater than that of oxidized
154
XIAORU WANG and RAMONM. BARNES 1.00’
I.
\ -BH4-dissoc:1atlon
0.80
8 ‘-i 4
-
--In
HNOJ
0. 60
8 L:
0.40.
0.20
-
Fig. 19. Comparison of normalized experimental results of Se+’ hydride generation in HN03 with the pH profile of borohydride dissociation.
SeO$-, x, then the (a - x) might be approximated as a. Equation (44) can then be simplified, integrated, and if kt = A, this yields X
- = l-e-*.
(45)
b
If y = b-x ted as
= [H,Se03],,,,,,,
the fraction of unoxidized H$eO,
P-WO&msentY [H2Se03]origina,
=
$ =
e-As = e-ACH”
can then be represen-
(46)
Because a was defined as the concentration of nitric acid, a = [H’]. Since the A value (A = kt) of this reaction is not available from any data source, it was estimated from limited experimental results of the present work, as summarized in Table 3. Based on the above calculations and estimations, the pH dependence of the oxidation of SeO$- to SeO:- with HNOJ, the normalized combination effects of this reaction, and the dissociation of borohydride are illustrated in Fig. 20. The theoretical results were then compared with experimental results obtained in HN03 system (Fig. 21). A small shift in optimum pH appears between the two curves, and the pH range is smaller for the experimental result than predicted. However, the trends basically correspond well. 4. CONCLUSION The pH dependencies of arsenic, lead, selenium, and tin hydride generation have been studied both theoretically and experimentally. Generally, excellent agreement between the theoretical and experimental results was obtained, and the main contributing reactions appear to be included. The observed deviations may result from the following: (i) Experimental pH reading error, especially in the higher pH regions. (ii) Experimental pH change resulting from the reaction. (iii) Heterogeneity of the reactants. (iv) Interferent components resulting from the impurities of the reagents. (v) Insufficient experimental points, especially around the optimum pH region. (vi) Estimation errors in the theoretical calculations. (vii) Experimental errors.
15.5
Continuous hydride generation Table 3. Estimated A value
CH+IW
exp.y/b
A
1.25 2.5 5.0
0.95 0.89 0.62
0.04 0.05 0.09
A (m-d 0.06 -
t 1. 2.
0.
0.
-J.
00
1. 00
BH [ diasoclatlon Ss
3. 00
5. 00
7. 00
I9. 00
PH Fig. 20. Norma&d theoretical pH profile of selenium hydride in nitric acid consisting of the (1) dissociation of borohydride and (2) oxidation of Se+* by HNOs. 1.
Fig. 21. Comparison of normalized experimental pH profile of Se+* hydride generation in HNO, and the theoretical results.
The significance of this study demonstrates that critical hydride generation parameters can be evaluated theoretically. Similar procedures may be applied to the study of pH dependence
156
XIAORUWANGand RAMONM. BARNES
of the remaining hydride forming elements although this has not been attempted. Characterization of hydride generation in the presence of interferent elements also may be feasible. For example, the optimum pH regions for each hydride forming element in the presence of concomitants could be theoretically calculated and deviations detected. Therefore, serious interferences might be eliminated by careful selection of pH region, proper separation procedure, or masking techniques depending on the actual sample system and the contents of the interferent elements. With the definition of the reaction mechanisms, the critical reactions of the hydride generation may be controlled effectively toward the improvement of hydride generation efficiency and the sensitivity of the detection. For instance, the theoretical study and experimental verification predict that the optimum pH regions for most of hydride generation mostly are limited by the dissociation of borohydride; therefore, if other techniques can be adapted to generate in situ the free hydrogen radical (Ho) required for hydride generation, then the hydride generation efficiency might be improved, the optimum pH region might be controlled, the technique may be operated more automatically, and the reaction mechanism might be proven. Acknowledgements-Research was supported by the ICP Information Newsfettg.
139-M.
1987.
0
0584~854?f%?s3.00+ 0.00 1987PngamonJournals Ltd.
A mathematical model for continuous hydride generation with inductively coupled plasma spectrometry-II. pH dependence of hydride forming elements XIAORU
WANGand RAMONM.
BARNES
Department of Chemistry, GRC Towers, University of Massachusetts, Amherst, MA OlOO3OO35, U.S.A. (Receiued 5 Fehary
1986; in reuisedfarnr 16 June 1986)
Abstract-The solution pH dependencies of four hydride forming elements, As, Pb, Se and Sn, have been studied theoretically and experimentelly. All of the main chemical reactions related to the hydride forming systems are considered, and detailed mathematical calculations are experimentally verified with ICP spectrometry.
1, INTRODUCTION HYDRIDE generation based on the volatile hydrides of As, Bi, Ge, Pb, Sb, Se, Sn and Te has been used widely in atomic absorption (AAS) and inductively coupled plasma (ICP) s~trometry to determine trace levels of these elements. Ex~riment~ly, the technique is well established for almost all of the hydride forming elements, but theoretically, hydride generation is less well characterized. Following a previous study of a mathematical model for hydride transfer [l J, we shall evaluate the pH dependence of hydride generation theoretically and experimentally in the present study. The solution pH has a significant effect on the hydride generation process, and various acids have been applied in hydride generation [2-4]. The optimum pH regions for all hydride forming elements have been reported [S J; however, those are almost exclusively experimental studies. A theoretical prediction of the effect of pH on hydride generation has not yet been performed. In the present study, four hydride forming elements, As, Pb, Se and Sn, are investigated both experimentally and theoretically. Consideration of the main chemical reactions related to hydride formation are included in detailed mathematical ~lculations. Experimental verifi~tion using ICP s~trometry provides comparisons of the theoretical description.
2. EXPERIMENTAL 2.1. Apparatus ICP atomic emission
spectrometry (AES) operating conditions for As, Pb, Se. and Sn with a 40.68 MHz rf generator, l-m Czerny Turner monochromator (slit height 5 mm, slit widths 0.05 mm) at 15 mm observation height above the induction coil are summarized in Table 1. A commercial continuous hydride generator (Plasma-Therm. Ltd) based upon the work of THOMPSON et al. [4-S] was applied in the amine and stannane generation study. A commercial flow injection inst~ment (FIAtron, SHS 200) was adapted for plum~e and selenium hydride generation. This apparatus included an all-Teflon, dual channel sample injector consisting of e~~troni~ly driven, microminiature seienoid valves. A multiroller, variable-speed pump situated before the valves can be operated in continuous-flow or in stop-flow modes. The former was applied for plumbane generation, and the latter was applied to the study of selenium hydride generation after modification of the manifold. A schematic diagram of the FIA modification is shown in Fig. 1. [l] X. R. WANGand R. M. BARNES,Spectrochim. Acta 41B, 967 (1986). [2] Y. YAMAMOTO and T. KUMAMARU. Fresenius 2. Anal. Gem. 281, 355 (1976). [3] P. W. V~JANand G. D. WOOD,Analyst (London) 101,966 (1976). [4] H. D. FLEMING and R. G. IDE, Anal. Ckim. Acta 83,67 (1976). [S] T. NAKAHARA, Prog. Anal. Atom. Spectrosc. 6, 163 (1983). [6] M. THOMPSON, B. PAHLAVANPOUR, S. J. WATSONand G. F. KIRKBRIGHT, AMlyst (.&&ox) 103,568 (1978). [7] M. THOMPSON, 8. PAH~AVAN~UR,S. J. W~rxt~ and G. F. K~RKBRIGHT, Analyst (London) 103,705 (197X). [8] M. THOMPSON and J. N. WALSH,A quick of l~~tiee~y Coupled Plasma Spectrometry, pp. 152-155. Blackie, Glasgow (1983). s142:1/2IhJ
139
X~hoauWANGand RAMON M. BARNES
140
Table 1. ICP-AESoperatingconditions As
Pb
Se
Sn
193.7 15 0.28 0.42 0.95
220.1 15 0.4 0.4 1.1
196.1 15 0.1 0.5 1.05
X90.3 15 0.25 0.4 0.8
Parameter Wavelength(nm) Qltm gas (I/b) Intermediategas (l/min) Carriergas (i/tin) Power (i&V)
ICP
Borshydrida
Fig. 1. Schematicd&mm of FIA controlledhydride generation.
The glass 8ow cell for gas-liquid separation is similar to that ofTHOMPSON et al. [6-g] or T~OHet al. [?I. The argon carrier gas Bushed the vapors through the upper outlet side tube directly into the base of the plasma torch.
Certified reagents were used to prepare standard stock solutions of X000#&ml As, Pb, Se and Sn. Various solutions of sodium borohydride (OS-5 %) in 0.2 M sodium hydroxide were prepared. The effects of the concentrations of borohydride and hydrochloric acid on the hydride generation of four
hydride-forming elements, and the effect of nitric acid on selenium hydride generation were studied. The optimum hydride generation conditions for four elements are summarized in Table 2. 3, THEORYAND EXPERIMENTAL
For this ~v~~~~on~ interfering elements [7,8-J were assumed to he absent. Only ~~dr~h~or~~ acid was considered except for selenium hydride generation, for which both hydrochloric and nitric acids were compared The activity coe&ients were assumed to he equal to 1, and the apparent concentrations were used for ah theoretical calculations. The pH in these calculations refers to the value after the reaction. 3.1. pH dependence of amine generation 3,l. 1. Predicted pH dependence. ‘The main chemical reactions related to pH in this system include the reduction of As+~ to As’ 3 and As+~ to As’, and the dissociation of borohydride. The reduction of As” to As’s may be represented as ~AsO;~+BH;+H+
[9] N. H. TIOH,
Y. ISRAEL
and R. M. BARNES,
= 4AsU3 ’ -k H3 30s + H2 0.
RR?&.
CMn.
Acta,
in press.
(1)
Continuous hydride generation
141
Table 2. Hydride generation conditions Element Technique As Pb se
caltinuous FIA-on-line FIA control
Sn
continuous
BHi (%)
Acid (M) HCI, 1 HCI, 0.3 HCI, 2 HNO,, 1.25 HCl, 0.1
Oxidant (M)
1 5 2.5 2.5 0.5
none H,02,0.8 none none none
The half reduction reaction can be represented as As0i3+2e-+2H+
= AsO;~+H~O.
(2)
This latter reaction can be represented in terms of an equilibrium constant K, [lo]. [As0i3 ] Kr =
[A&;“]
Ce-]2
[H’]’
=
(3)
l”*‘l’
Based upon Eqn (l), electrons are produced by the oxidation of borohydride. Stoichiometrically, this yields [e-] = 8[BH;]. In the present experiments, [BH; ] = 0.25 M, which gives the concentration of electrons, [e-] = 8(0.25) = 2M. Log K, of the reaction is 8.1 [lo]. When Eqn (3) is rearranged, and 1 is added to both sides, Eqn (4) is obtained. [AsO; “1 [As0i3] + [AsO;j ] = 10m8.’ [e-l-
[H’]”
+ 1’
(4)
If 100 % of the available electrons were consumed in arsine generation, then substitution of [e-] = 2M into Eqn (4) yields
CASO;~I = [AsIt
1
lo-8.12-2[H+]-2
+
1’
(5)
[AsI is the sum of [AsO;“] and [AsO;~]. A curve with the fraction of reduced As+’ is plotted in Fig. 2 as a function of PH. Above pH 3.5 the reduction of As+’ decreases, and no reduction occurs at pH > 5.6. The reduction of As+~ to As0 was also studied. The reaction is HAsOz+3H’
+3e-
= As”+2H20.
(6)
The standard potential of the reaction E, = 0.2475 V 183, hence, the free energy of the reaction, AGO,can be calculated as AGO = - nFEo = - RT 1nK = - 3(96500) (0.2475) (J/mol) = - 17.14 (kcal/mol)
(7)
n is the number of electrons transferred and F is the Faraday constant. The calculated log K is 12.6, which gives
CAs”l
logK = log
pAso
1
CH
+
13
=
12.6.
(8)
[As”] is the concentration of intermediate state of arsenic in the solution. -Rearranging Eqn (8) and adding 1 to both sides results in I; 1 + [H + ]” 10i2+j
[As”] + [HAsO The fraction of [HAsO,]
[AsO1 t-H+ I” 10’2*6reduced in the system may be represented as [HAsO~-J,~~
Pslt [lo] J. INCRWDY, (1976).
AMlyrical Application
of Compkx
=
[H’]’ 10’2*6 1+ [H’]” 1012.6’ Equilibria
(9)
(10)
(Translation Ed. J. TYSON).Wiley, New York
XIAORUWANG and RAMON M. BARNES
142
1.00 +
-1.00
3. 40
1.20
5. 60
7.80
1 *00
PH Fig. 2. Theoretical reduetion of As+’ to As+~ as a function of pH with the assumption that 100% of theelectronsareeonsumed (l).Theoretical pH profilesofAs+s to As+j reduction with less than 100 % electron consumption. (2) loo/,, (3) 1 “/ (4) 0.1%, (5) 0.05 %.
The theoretical pH dependent profile based on this equation is illustrated in Fig. 3 with the pH profile of As(II1) hydride generation reported in Ref. [i2]. Both curves generally agree well except the slope of the falling side of the curve from Ref. [ 121 is smaller than that of the theoreti~l profile. This is attributed to buffered reaction solutions in Ref. [12]. Solution pH also strongly affects the dissociation of borohydride in the presence of hydrochloric acid. The accepted dissociation of borohydride is [5-j BH; +3Hz0+H+ a-x
+H,BO,+gH’. b-x
(11) x
Since this reaction has a large negative AGO,it proceeds spontaneously, as has been proven experimentally. Here the reaction is considered only kinetically. The original concentrations of borohydride and hydrochloric acid are a and b, respectively; x is the concentration of dissociated borohydride at time t, and k is the reaction rate constant. In this case, the change of ~on~ntration of dissociated ~rohydride with time can be represented as dx -= dt
-d(a-x) dt
Upon integration of the above equation with A = kt and rearrangement, borohydride dissociated, x/a, is then given as
x/a =
w
= kfa-x)(b-x).
the fraction of
1 l-- eAta_-b) 1 (a/b) - A . &@-bl
(13)
When a G b, the original concentration of borohydride is less than or equal to the con~ntration of hydrochloric acid, A(a - b) B 0. This results in eAfuebf Q 0. Then the above equation can be approximated as x/a = 1, which means that borohydride has been completely dissociated in an excess of hydrochloric acid.
[ll] J. G. STARKand H. G. WALLACE,Chemistry Date Book, 2nd Edn. John Murray, London (1982). 112) J. AGGETTand A. C. ASPELL,Analyst (London) 101, 341 (1976).
Continuous hydride generation
143
0. 80 As(III)
to As(B)
0. 20
0. 00
l-
.l. 00
I-
1.00
3. 00
5. 00
7. 00
9. 00
PH Fig. 3. Theoretical pH profile of the fraction of As+’ reduced to As0 (solid line). Experimental results [12] for As +3 hydride generation (0).
On the other hand, when a > b, for example, the original concentration of borohydride is greater than that of hydrochloric acid, A(a -b) 9 1, which results in eA(‘-‘)% 1. Therefore, the above equation can be approximated as x/a = b/u. A formal representation may be written as
P-L ldiss. _ w 3 PHBIwig. - [BHi I orig.’
(14)
For which [BH,],i,, is the concentration of dissociated borohydride, and [BH; ]Orig,is the original concentration of borohydride. The curve of the fraction of BH, dissociated as a function of pH based on the above equation is given in Fig. 4. These results predict that BH, does not dissociate above pH 2. In a higher pH range (over pH 7), several monomeric species such as ASP , As(O and several polymeric species such as Asg (OH), , Ass (OH),, may exist. Because they are not related to the pH range of amine generation, they are not considered. 3.1.2. Experimental amine generation. Under the optimum ICP-AES operating and hydride generation conditions summarized in Tables 1 and 2,2 &ml of As(V) solution with pH values ranging from - 1 to 2 were examined. An experimental profile of pH dependence of arsenic hydride generation is illustrated in Fig. 5 and is compared with the dissociation of borohydride and the reduction of As+5 to As+’ in Fig. 6. These theoretical and experimental studies demonstrate that the most important reaction affecting As(V) hydride generation appears to be the dissociation of borohydride, even though a small deviation from the experimental result is observed. However, since the concentration of borohydride is far greater than that of As+ ’ in the system, and the standard reduction potential of As(V) to As(II1) is relatively high ( + 0.58 V, [ll]), it might be reasonable to assume that < 100% electrons participate in the reduction of As(V) to As(II1). Therefore, further calculations with 10 %, lx, 0.1% and 0.05 % of electrons used in As + ’ to As + 3 reduction reaction, respectively, were made, and the results also are illustrated in Fig. 2. With the assumption that only 0.05 % of the electrons are consumed, the model matched very well with the experimental results (Fig. 7). The theoretical and experimental studies of pH dependence of both As(V) and As(II1) hydride generation indicate that both reactions are pH dependent. However, As(V) reduction occurs in a lower pH region which is restricted mainly by dissociation of the borohydride. The reaction condition of As(II1) is relatively broad, as has been verified in many experiments
XIAORUWANG and RAMONM. BARNES
,j
0.00
f -1.00
r
1. 20
5. 60
3. 40
7. 80
10.00
PH Fig. 4. Theoretical dissociation curves of BH; at different concentrations as a function of pH. Individual curves correspond to borohydride concentrations of (1) 1.32 M, (2) 0.66 M, (3) 0.25 M, and (4) 0.13 M.
Experlmsntal
400.00
‘: 1
300.00
: o-l m” 200.00
0.00i -1.00
1.00
5. 00
3. 00
7. 00
9. 00
PH Fig. 5. Experimental pH profile of As+’ hydride generation with 2 &ml As(V).
[13, 145, Only a little dissociation of borohydrideis necessary for As(II1) hydride generation. For instance, if the dissociation of borohydride is assumed to be only 1% of that in the case of As(V) hydride generation, the theoretical pH proflle of borohydride dissociation will match well with the experimental one (Fig. 8). The selective determination of As(V), As(II1) and the total arsenic by hydride generation, therefore, may be performed by the control of pH value of the solution. [13] A. U. SHAIKHand D. E. TALIMAN,And. Chim. Acra 98,251 (1978). [14] M. H. ARBAB-ZAVAR,Analyst (London) 105,744 (1980).
Continuous hydride geuemtion
145
1.00
-1.00
I. 20
3. 40
5. 60
7.80
J
fH
Fig. 6. Corner
of ~~~ results of AS+’ hy~de lotion (2) witb tbeoreticaI As+’ to As+~ reduction curve (3) and the dissociation of 0.25 M SW; (1).
0. 80
0. 20
0. 00 00
1. 20
3. 40
5.60
7. 80
18.00
PH
Fig. 7. Comparison of normalized experimental pH profile (3) witb theoretical reduction o~As+~ to As+’ assuming 0.05 % efectrons are consumed (2), and theoretical d&o&&on curve of BH; (1).
3.2. pH e&ct on tin hydride generation 3.2.1. Predicted gH dependence. The following reactions are considered to be related mainly to the concentration of hydrochloric acid during tin hydride generation. In high concentration of HCl solution, the main reaction occurring is the complexation of Sn +‘. &Cl, -I-Cl- 4 SnCl; .
(15)
The equilibrium can be represented as [ll, 15, 167 [SnCl; ] log K = [SnC1,] [Cl_ 1 = 1.5. [IS] F. A. COTTON, Adtmzced Inorgan& Chenrisry, 2nd Edn. Wiley, New York (1978). [l&j A. S. DOUGLAS,F~~~nt~s ofAnaly~ka1 Chemistry, 4th Fidn. SaGtide@,P~~dcip~~
(1982).
XIAORUWANGand RAMONM. BARNES
146
oc
‘”
ct
0. 60
: It
0.40
0. 20
0.00
L I.20
-1.00
3. 40
5. 60
7. 80
18.00
PH Fig. 8. ~rn~~n of normaliked theoretical results of the reduction of AsC3 to As’, and ex~~rn~~ results of Asi hydride generation [lZ] (a) with theoretical dissociation of BH; assuming only 1 yOdissocktioa of that in the case of As(V) hydride generation.
Apparently, [Cl-] = [H’] h ere, therefore, [Cl-] in the above equation can be replaced by [H+ J and rearranged to obtain the fraction of SnCl, uncomplexed by Cl-.
isnc121 _
CW
0.032 [H+] + 0.032’
(17)
For which, [Sn& = [SnClz] + [S&l; ]. The curve calculated from Eqn (17) with [SnC12]f[S n ] t as a function of pH, illustrated in Fig. 9, shows that the complexation of %-I+’ has a very strong influence on stannane generation in the lower pH region. The hydrolysis of Sn f2 also produces a strong influence on tin hydride generation. Sn2+ t-H,0
i-H+.
= Sn(OH)+
(18)
The calculated K is 0.2.
iswH)+I CH’I = [Sn2+] The fraction of unhydrolyzed
O2
(19)
’ *
Sn2 + can be expressed as -ul_
2i-
is4
[Wf 0.2+[H+]’
Again, [Sn],is the sum of [Sn2 ’ ] and [Sn(OH)' ] in this system. The curve for [Sn2+]/[Sn], as a function of pH also is shown in Fig. 9. This curve predicts that most of the SnZf is hydrolyzed above pH 3. Another hydrolysis reaction of Sn2+ in higher pH values is 3Sn2+ +4H20 = Sns(OH)i+ +4H+. (21) The pK value of SnJ (OH)$+ dissociation constant is 6.77 [15]. Therefore, the equilibrium can be expressed as
[SIP+]3
-l”g[Sn3(OH);+] [H']' Based on Eqn (21), stoichiometri~ly,
[Sn2’]
(22)
= 6*77*
= 3[Sn3(OH)i+],
[Sn2+],is, = [Sn”‘] + 3[Sn, (OH)i’].
which yields (23)
Continuous hydride generation
147
0.00\
-1.00
1. 20
3. 40
5. 60
7.90
1
100
PH Fig. 9. The theoretical curves including the dissociation of borohydride (l), thecomplexation of Sri+++ as SnCI; (2) and the hydrolysis of tin, Sn(OH)+ (3) and Sns (OH)12 (4). as a function of pH.
If the original concentration of tin is 10m5M (in this work), the concentration hydrolyzed Sn2 + may be represented as [Sn, (OH):+]
k 1/3([Sn2+],,,is,-
[Sn2+])
of (24)
= l/3(10-’ - [Sn “1).
l/4 IH+l= [l()-7(l()-I_ 1
Substitution of [Sri3 (OH);+]
in Eqn (22) with Eqn (24) yields upon rearranging 3[Sn2+13
[Sn2+])
(25)
The fraction [Sn2+]/[Snlr as a function of pH based on this relationship is illustrated in Fig. 9. This hydrolysis reaction is important only at pH > 3. Other tin complexation with chloride may occur in hydrochloric acid. However, they were not considered here owing to insufficient data. The dissociation of borohydride (also plotted in Fig. 9), of course, strongly affects stannane generation, as discussed in the previous section. 3.2.2. Experimental stannane generation. Under the optimum ICP operating and hydride generation conditions, 2 &ml of Sn2 + solutions with different pH values were pumped into a commercial continuous hydride generator, and the stannane was analyzed by ICP-AES. The experimental profile of pH effect on stannane generation is illustrated in Fig. 10, and the comparison of experimental and theoretical results is given in Fig. 11. The pH dependence of tin hydride generation might be restricted by both dissociation of the borohydride and the complexation of Sn2 + or controlled by the complexation reaction of tin and its first order hydrolysis reaction. Since the normalized results of the latter are closer to the experimental results, this hypothesis is believed to be more reasonable. 3.3. pH dependence of lead hydride generation 3.3.1. Predicted pH dependence. The hydride generation efficiency of Pb’+ to plumbane and detection limits of lead are very poor with the straightforward method [17,18]. Adding some oxidant to the lead hydride generation system has proved to be an effective technique for determination of trace lead [ 19,201. In the present study, hydrogen peroxide was adopted [17] J. F. CHAPMAN and L. S. LALE,Anal. Chim. Acta 111, 137 (1979). [18] K. C. THOMPSON and D. R. THOMPSON, Analyst (bttdon) 99, 595 (1974). [19] J. KAZUOand T. MITSUHIKO, Anal. Chim. Acza 143,229 (1982). 1203J. R. CASTILLO, J. M. MIR, C. MARTINEZ, J. V~~and M. P. COU)N, Mikrochim. Acta [Wien]
1,253 (1985).
148
XIAORUWANGand RAMONM. BARNES 40.00
32.00 > :
24.00
0-I ; cot
16.00
-1.00
1. 20
3. 40
5. 60
7.80
1
00
fH
Fig. 10. The experimental profile (o)pH effect on stannane generation with 2 &ml of Sn+’ solutions.
1.00
;
i
0.80 -
! i LL 0.60 -
-G E 2 0.40:
O.-O0 7 -2.00
Fig. 11. The comparison
-. 60
0. 80
2. 20
3. 60
5‘.
00
1
normalized experimental (a) and theoretical profiles of tin hydride generation as a function of pH.
as the oxidant. The chemical reactions related to pH in the HzO,-HCl lead hydride generation system involve the complexation of lead with chloride ion, the hydrolysis of Pb2 +, and the dissociations of borohydride and hydrogen peroxide.,For each of the following equilibrium calculations, the assumption is made that no other complexes are formed simultaneously, so that only one equilibrium reaction is treated at a time. The complexations of lead with Cl- may be represented as Pb+‘fCl-
= PbCl+
[21] A. J. RUBIN, Ed., Aquous Environmental Chemistry
1ogK = 1.6[21].
of Metals.Ann
(26)
Arbor Press, Ann Arbor, MI (1974).
Continuous
The fraction of uncomplexed represented as
149
hydride generation
Pb2+ in the original concentration [Pb* + -J 1 - P-w = 1+ 101*6[H+]’
of lead can then be
(27)
Similarly, Pb* + + 2Cl- = PbC12 and Also, and Finally, and
1ogK = 1.8 [21]
CPbZ+I= cwt
1
(2%
1 + lo’.8 [H+]*’
Pb*+ + 3Cl- = PbCl; [Pb* + ] ____ = 1+
t-Pblt
IogK = 1.7 [21]
(31)
[H+]3.
1 1+ 10’.4[H+]4’
IBIt
(30)
1 101.7
Pb2 + c 4Cl- = PbCl; * log K = 1.4 [21] [Pb*+] -=
(28)
(32) (33)
[PbJ corresponds to the sum of [Pb*+] and the individual complex, respectively. The pH effects on those complexation reactions are illustrated in Fig. 12. The hydrolysis reaction of Pb* + can be represented as Pb*+ +H,O
= Pb(OH)+ +-H+.
The calculated K is 1O-8.15, and the fraction of unhydrolyzed [Pb* ‘1
___ [Pblt Similarly,
w+1 = [H+]+ 10-8.15’
(34) Pb*+ is represented as (35)
Pb* + + 2H2 0 = Pb(OH)* + 2H +.
(36)
The calculated K is 10-r7.*. This yields
[pbz+l= Pw
[H+]2+
P-I+I’
(37)
10-17.2’
The third order hydrolysis reaction is given as Pb*+ +3H,O
= Pb(OH);
+3H+.
(38)
The calculated K is lo- 28.1. Therefore, the fraction of unhydrolyzed Pb* + can be obtained from
L-*+1 _ CPblt
[H+]3
CH+ 1” +
10-28.1
*
(3%
The pH dependencies of lead hydrolysis based on the above equations are illustrated in Fig. 13, and the combined pH effects on both lead complexation and hydrolysis are given in Fig. 14. The study predicts that pH ranging from 4 to 6 seems good for lead hydride generation because 100% free Pb * + should be expected in this pH range. However, in practical lead hydride generation experiments, the optimum pH range is relatively narrow, and the maximum signal could be expected at pH about 0.5 (Fig. 15). The major deviation of the theoretical from experimental results appeared to be due to the hydrogen by-product which exhibited a large overpotential [22,23] on lead. This overpotential significantly inhibits the dissociation of sodium borohydride; therefore, the further. reaction of Pb to plumbane is prevented. In order to resolve this problem several oxidants such as potassium [22] M. D. ZHOULDERand V. V. STENDER,Zh. Prid. Khim. 31, 711 (1958). [23] W. A. KOEHLER,Principles 2nd Edn, Vol. 2, p. 47. Wiley, New York (1944).
150
XIAORUWANG and RAMONM. BARNES
Fig. 12. Theoretical pH dependencies of lead complexes (1 PbCI+, 2 PbC&, 3 PbCl; ,4 PbCl;‘), and the dissociation of 1.32 M borohydride (5) in the presence of 0.8 M hydrogen peroxide.
cc 0
0. 60
k ;: 0
0.40
: L 0. 20
0. 00 + -1.00
1.20
5. 60
3. 40
7. 60
10.00
PH Fig. 13. Theoretical hydrolysis curves of lead (1 Pb(OH)+, 2 Pb(OH),, 3 Pb(OH); ) as a function of PH.
dichromate, hydrogen peroxide, or peroxidisulfate were applied as depolarizing agents. In the presence of oxidants in the reaction system, the pH dependence of lead hydride generation varies wth the actual reaction system (i.e. selection of oxidant, concentration of oxidants, concentration of borohydride). At this stage, the HCl-H202 system is adopted in the present work; therefore, the pH dependence of lead hydride generation was considered with the addition of only hydrogen peroxide. The redox reactions included in the system are the following: (1) The reduction of borohydride Eqn (11). (2) The reduction of lead ion Pb’+ + 2e- +Pb.
(40)
Continuous hydride generation
151
PH Fig. 14. Combined lead chloride complexes (1 PbCI+, 2 PbCl,, 3 PbCl;, 4 PbC1;2) and first lead hydrolysis (5 Pb(OH)+).
(3) The dissociation of hydrogen peroxide H,Oz + 2H+ + 2e- 42&O.
(41)
Stoichiomet~~lly, one BH; is equivalent to 4Hz02. If the con~ntration of H202 % Pbzf, then the concentration of electrons consumed by Pb2+ may be ignored. In other words [HzOt] = 4[BH;]. Also, [BH;] = [H’], and 2[H,Os] = [H’]. The total concentration of [H ‘1 which should be consumed for quantitative lead hydride generation, [H+],, is the sum of [BH;] and 2[H,O,] which equals 9[BH;] or 9/4[H,O,], depending on which concentration is lower. The fraction of [H+lcurrent to [H+], which should be consumed in the reaction may be used to represent the actual fraction of dissociated borohydride. For instance, in the present study, [BH;] = 1.35 M (5 %), [HzO,] = 0.8 M, the actual fraction of dissociated borohydride to the original concentration of borohydride could be represented as (42) The effect of pH on the actual dissociation of borohydride in the presence of hydrogen peroxide is shown in Fig. 12 along with pH dependence of all of the reactions described above. The optimum pH region of lead hydride generation, therefore, is restricted mainly by the fourth order of the complexation of Pb’+, and the dissociation of borohydride in the presence of 0.8 M hydrogen peroxide. 3.3.2. Experimental plumbane generation. Various 2 fig/ml Pb* + solutions with different pH values were analyzed with a FIA on-line hydride generation technique. The experimental pH dependence of lead hydride generation is represented in Fig. 15. The normalized experimental and theoretical results are compared in Fig. 16. The optimum pH region in both studies matches well. 3.4. pH dependence
ofselenium hydride generation
Solution pH dependence of selenium hydride generation has been studied in both hydrochloric acid and nitric acid. Various 0.2 @g/ml Se solution with different pH values in both hydrochloric and nitric acids systems were examined with FIA controlled on-line hydride generation technique (Fig. 1). The results obtained from the two are compared in Fig. 17. The experimental profiles of pH dependence of the selenium hydride generation in the two acids are different. The optimum hydride signal should be expected with almost the
152
XIAORU WANG and RAMONM. BARNES
a
6. 40
k --i
4.80
: m c;;
3.20
1. 60
-1.00
1. 20
3. 48
5. 60
7. 80
IW. ww
PH Fig. 15. Experimental pH dependence of lead hydride generation with 2pgJml Pb”
00
Fig. 16. The comparison of normalized experimental (o)and theoretical (solid line) pH profiles of lead hydride generation.
same pH region in both acids. However, a plateau in HCl was observed before the pH inflection point. In contrast, a maximum was observed in the pH dependence in HN03. Therefore, the reaction mechanisms in both acids were examined theoretically in order to rationalize these differences. 3.4.1. Hydrochloric acid. After consideration of all possible chemical reactions related to selenium hydride generation, we found the main effect that restricts the optimum pH region of selenium hydride generation is the dissociation of borohydride, as discussed in Section 3.2. The theoretical results for borohydride dissociation are compared with the experimental pH effects on selenium hydride generation in both acids in Figs 18 and 19, respectively. These comparisons indicate that calculated results of borohydride dissociation agree well with the experimental results in the HCl system. They also match well with the falling side of pH profile obtained from the HNOJ reaction system. The unmatched part of the latter is attributed to the oxidizing property of nitric acid.
Continuoushydridegeneration
$
153
0. 60
-1.00
1. 00
3. 00
5 00
7. 00
9. 00
Fig. 17. Experimental pH dependence.of Se hydride generation with O.Z&ml Se+* in hydrochloric acid (0) and nitric acid ( x ).
0. 80 -
0.20
-
Fig. 18. Comparison of normalixed experimental results of Se +* hydride generation in HCI with the dissociation of borohydride pH profile.
3.4.2. Nitric acid. The effect of nitric acid on H$eO, H#eO, b-x
has been studied as
+ NO; 4 SeOt - f H + + HN02. a-x
(43)
x
Similar to the studies in the dissociation of borohydride, represented kinetically as
the above reaction can be
dx = k(u--x)(b-x). dt
-
If the concentration
of nitric acid is assumed to be far greater than that of oxidized
154
XIAORU WANG and RAMONM. BARNES 1.00’
I.
\ -BH4-dissoc:1atlon
0.80
8 ‘-i 4
-
--In
HNOJ
0. 60
8 L:
0.40.
0.20
-
Fig. 19. Comparison of normalized experimental results of Se+’ hydride generation in HN03 with the pH profile of borohydride dissociation.
SeO$-, x, then the (a - x) might be approximated as a. Equation (44) can then be simplified, integrated, and if kt = A, this yields X
- = l-e-*.
(45)
b
If y = b-x ted as
= [H,Se03],,,,,,,
the fraction of unoxidized H$eO,
P-WO&msentY [H2Se03]origina,
=
$ =
e-As = e-ACH”
can then be represen-
(46)
Because a was defined as the concentration of nitric acid, a = [H’]. Since the A value (A = kt) of this reaction is not available from any data source, it was estimated from limited experimental results of the present work, as summarized in Table 3. Based on the above calculations and estimations, the pH dependence of the oxidation of SeO$- to SeO:- with HNOJ, the normalized combination effects of this reaction, and the dissociation of borohydride are illustrated in Fig. 20. The theoretical results were then compared with experimental results obtained in HN03 system (Fig. 21). A small shift in optimum pH appears between the two curves, and the pH range is smaller for the experimental result than predicted. However, the trends basically correspond well. 4. CONCLUSION The pH dependencies of arsenic, lead, selenium, and tin hydride generation have been studied both theoretically and experimentally. Generally, excellent agreement between the theoretical and experimental results was obtained, and the main contributing reactions appear to be included. The observed deviations may result from the following: (i) Experimental pH reading error, especially in the higher pH regions. (ii) Experimental pH change resulting from the reaction. (iii) Heterogeneity of the reactants. (iv) Interferent components resulting from the impurities of the reagents. (v) Insufficient experimental points, especially around the optimum pH region. (vi) Estimation errors in the theoretical calculations. (vii) Experimental errors.
15.5
Continuous hydride generation Table 3. Estimated A value
CH+IW
exp.y/b
A
1.25 2.5 5.0
0.95 0.89 0.62
0.04 0.05 0.09
A (m-d 0.06 -
t 1. 2.
0.
0.
-J.
00
1. 00
BH [ diasoclatlon Ss
3. 00
5. 00
7. 00
I9. 00
PH Fig. 20. Norma&d theoretical pH profile of selenium hydride in nitric acid consisting of the (1) dissociation of borohydride and (2) oxidation of Se+* by HNOs. 1.
Fig. 21. Comparison of normalized experimental pH profile of Se+* hydride generation in HNO, and the theoretical results.
The significance of this study demonstrates that critical hydride generation parameters can be evaluated theoretically. Similar procedures may be applied to the study of pH dependence
156
XIAORUWANGand RAMONM. BARNES
of the remaining hydride forming elements although this has not been attempted. Characterization of hydride generation in the presence of interferent elements also may be feasible. For example, the optimum pH regions for each hydride forming element in the presence of concomitants could be theoretically calculated and deviations detected. Therefore, serious interferences might be eliminated by careful selection of pH region, proper separation procedure, or masking techniques depending on the actual sample system and the contents of the interferent elements. With the definition of the reaction mechanisms, the critical reactions of the hydride generation may be controlled effectively toward the improvement of hydride generation efficiency and the sensitivity of the detection. For instance, the theoretical study and experimental verification predict that the optimum pH regions for most of hydride generation mostly are limited by the dissociation of borohydride; therefore, if other techniques can be adapted to generate in situ the free hydrogen radical (Ho) required for hydride generation, then the hydride generation efficiency might be improved, the optimum pH region might be controlled, the technique may be operated more automatically, and the reaction mechanism might be proven. Acknowledgements-Research was supported by the ICP Information Newsfettg.