1498
Anal. Chem. 1986, 58, 1498-1501
Differential Pulse Polarographic Determination of Trace Levels of Platinum Zaofan Zhao' and Henry Freiser* Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721
By use of differential pulse polarography (DPP) in formaldehyde-hydrazine-sulfurlc acid medium, a very sensltlve catalytlc hydrogen polarographic wave is observed for Pt( I I ) or Pt(1V) In the range of 6 X lo-' to 1 X IO-'' M. Except for Rh3+ and Ir4+, In which the tolerated amounts are only 10 and 2 times that of platinum, respectively, there are no serious Interferences from other piatlnum group metal Ions. The course of the reactlon appears to be that formarone, which forms from formaldehyde and hydrazine, reacts with Pt( I I ) to give a Pt(CH,=NNH,);+ complex. This polarographically active complex can be adsorbed strongly at the surface of an electrode to cause a high Catalytic hydrogen current.
There are only a few polarographic methods for the determination of platinum. These include the reduction of Pt(1V) in NaCl (I), NaCI-NaN03 (2), ethylenediamineKSCN-KNOB (3), and ethylenediamine-citrate (4). Small amounts of platinum (10-6-10-7 M) can be determined from catalytic hydrogen currents arising on the dropping mercury electrode in hydrochloric acid solution in the presence of platinum chlorides (5, 6). In this medium, ions of all the platinum group metals display catalytic hydrogen waves having different sensitivities. The most sensitive one is caused by ruthenium, which can be detected down to -5 X 10-lo M. The catalytic hydrogen waves presumably arise by the decrease in the hydrogen overpotential on the active centers of metallic platinum group metals reduced and deposited on the surface of the mercury electrode. In EDTA-NaC1 medium (pH 4.5), there is also a catalytic hydrogen wave for Pt(IV), but the sensitivity of the method is not so high, the detection limit being only about 2 X M (7). There is also a catalytic hydrogen wave of Pt(I1) in ethylenediamine medium with a detection limit of 3 X loWsM (8). Since many nitrogen- and sulfur-containing organic compounds can cause catalytic hydrogen waves (9),it is interesting to study the catalytic hydrogen waves caused by platinum group metal complexes using such compounds as ligands. Many exciting results have been reported (10). In pyridinepyridinium chloride buffer solution (pH 3.7), by using the catalytic hydrogen wave formation, trace amounts of platinum as low as 2 x lo-* M can be determined (11). The detection limit for the platinum-1,2-diaminobenzene system is about 5 X lo+' M (IO). A very sensitive catalytic hydrogen wave for platinum can be obtained from hexamethylenetetraaminesulfuric acid medium; the detection limit is down to 1 X M (12). In this paper, detailed information about the catalytic hydrogen wave of platinum in formaldehyde-hydrazine (or hydroxylamine) medium is given. This procedure is potentially useful for determination of trace platinum, because the detection limit is as low as 1 X M. We have successfully used this method to determine trace amounts of Pt(I1) or Visiting Professor from Wuhan University, People's Republic of
China.
Pt(1V) in aqueous solution in studying the extraction of Pt(I1) or Pt(1V) by phenylthiourea and 8-mercaptoquinoline. EXPERIMENTAL SECTION Chemicals and Reagents. All salts of platinum group metals were purchased from Alfa Analytical Laboratories, Inc., and used without further purification. All other chemicals used in this work were ACS certified reagent grade. Doubly deionized water was used throughout this study. Stock solutions of Pt(I1) and Pt(1V) (lom2M in 1 M H2S04) were prepared from K2PtC14and hydrated Na2PtC1, (Pt, 34.0%) M, which were used as and diluted stepwise to and working solutions. Solutions of formaldehyde and hydrazine must be prepared separately and may be mixed freshly daily. Formaldehyde-H2S04 and hydrazine sulfate solutions are stable for at least 3 months. Apparatus. A polarograph (EG&G Princeton Applied Research Model 174A) with a dropping mercury electrode (DME) time assembly (Model 170/70) and a linear sweep module accessory (Model 174/51) and an X-Y recorder (Hewlett-Packard mode 7040A) were used for the polarographic measurements. For differential pulse polarographic (DPP) operation, the parameters were as follows: 1-s drop time, 5 mV/s scan rate, 50-mV modulation amplitude, positive display direction, negative scanning direction, 1.5-V range. A three-electrode system was used with a dropping mercury counter electrode as the working electrode, a glassy carbon counter electrode, and a commercial fiber plug saturated calomel electrode (SCE), or a Ag-AgC1 electrode, as the reference electrode. All electrode potentials are quoted vs. the SCE. The electrolytic cell was a 10-mL beaker. With the mercury column height at 40 cm, the DME had a mass flow rate of 0.42 mg/s for the capillary used. It was not necessary to remove oxygen from the solution as it does not affect the polarographic wave at all. The pHs of solutions were measured with an Orion Model 701A Digital Ionalyzer. Procedure. Sample solutions were kept to 10 mL in which the concentration of platinum (either Pt(I1) or Pt(1V)) was varied M. The concentrations over a range from 6 X to 1 X of hydrazine, formaldehyde, and sulfuric acid in the sample solutions were 0.002% (w/v), 0.02% (w/v), and 0.75 M, respectively. Higher concentrations of chloride, bromide, iodide, thiocyanate, nitrate, etc., must be avoided. The sample solutions are stable for at least 4 h. The DPPs were recorded from -0.8 to -1.1 V (vs. SCE). Peak currents (ZJ of the DPP were measured in the usual way. RESULTS AND DISCUSSION The direct current (dc), differential pulse (DPP), and linear sweep (LSP) polarograms of Pt(I1) in 0.02% (w/v) formaldehyde-0.002 % (w/v) hydrazine-0.75 M H2S04medium (Figure 1) exhibit a well-defined polarographic wave that appeared at ca. -1.04 V (vs. SCE) with a characteristic peak shape. As the concentration of Pt(I1) increases to ca. 6 X M or more, hydrogen gas bubbles are generated vigorously at the surface of the DME, leaving little doubt that the polarographic wave is a catalytic hydrogen wave. The catalytic wave occurs only in the presence of formaldehyde and hydrazine (or hydroxylamine) in sufficiently high acid concentrations. The sensitivity of the wave depends on the nature of the acid present, achieving its highest value in sulfuric acid medium (Figure 2). In hydrochloric acid me-
0003-2700/86/035S1498$0 1.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58,
1.0
0.0
NO.7, JUNE 1986
2.0 -Log
1499
4.0
3.0
c
Figure 3. Effects of some salts on DPP of platinum. Conditions are the same as in Figure 1.
-1.0
-0.9
-0.8
E
(VS.
-1.1
v
SCE)
Figure 1. Dc, DPP, and LSP of Pt(I1) in formaldehyde-hydrazineH2S0, medium (Pt, 5 X M): electrolyte, 0.02% (w/v) HCHO/ 0.002% (w/v) N,H4/0.75 M H,SO,; initial potential, -0.8 V; scan rate, dc and DPP, -5 mV/s, LSP, 100 mV/s; B = blank of dc polarography.
1.5
1.0
.
P
i l
HCHO
,
I/'
or:
'
1.0
:
'
2.0
:
3.0
'
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Flgure 4. Effects of formaldehyde and hydrazine on DPP of platinum. Condltions are the same as in Figure 1.
0.5
- %O O
0.01 INCHO) 0.01 l N 2 f 1 4 )
0.5
1.0
c1.5 (PO
Figure 2. Effects of different kinds of acids on DPP of platinum. Condltions are the same as in Figure 1, except for the change of concentration of different kinds of acids.
dium, however, high concentration of chloride decreases the catalytic current probably because of platinum chloride complex formation (Figure 3). Similar interference effects are exhibited by bromide, iodide, and thiocyanate. Nitric acid cannot be used as a supporting electrolyte because of its oxidation action on formaldehyde and hydrazine. This, in turn, causes the appearance of a polarographic wave in the background at the same potential range as the catalytic wave. Under these conditions,the catalytic wave cannot develop well and disappears at last (Figure 3). Perchloric acid also cannot
be used as a supporting electrolyte, because its presence causes the catalytic wave to split into two waves and decreases the catalytic current. There are no interferences from less than 1 M sulfate, phosphate, acetate, or perchlorate salts (Figure 3). The effects of formaldehyde and hydrazine on the catalytic current a r e shown in Figure 4. As the concentration of formaldehyde or hydrazine increases, the catalytic current increases rapidly at first and then levels off. The optimum concentrations for formaldehyde and hydrazine were found to be 0.02% (w/v) and 0.002% (w/v), respectively. As will be described in detail below, formaldehyde can react with hydrazine, forming formazone (CH=NNH2). The catalytic current is caused by the platinum-formazone complex. Formazone can be irreversibly reduced at the DME and gives rise to a polarographic wave ( E l j zN -0.85 V vs. SCE) just preceding the catalytic wave (E, -1.04 V vs. SCE). For this reason, higher concentrations of formaldehyde or hydrazine should be avoided to minimize high background currents. The detection limit (SIN = 3) of the method is 1 X M, making it one of the most sensitive methods for deter-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
Table I. Relationship between I , of Catalytic Current and Concentration of Pt(I1) C,
M
Zp,
0 x 10-10
IIA
C,
Ma
: -
2.0+
re1 dev, %
I
0
1.00 x 10-10 2.00 x 10-10 4.00 X 6.00 X 8.00 x
1 x 10-9 2.00 x 10-9 4.00 X 10* 6.00 x 10-9 8.00 x 10-9 1x 10-8 2.00 x 10-8 4.00 x 6.00 X 10" 8.00 x 1 x 10-7 2.00 x 10-7 4.00 x 10-7 6.00 X lo-'
0.02 0.04 0.08 0.14 0.19 0.30 0.60 1.25 1.87 2.37 3.00 6.00 11.80 17.50 22.30 28.50 60.00 122.00 175.00
1.06 X 1.90 X 3.57 X 6.08 X 8.17 0.83 X 1.84 X 4.04 X 10" 6.14 X 7.83 X 1.00 X 2.01 X 3.97 x 5.90 X 7.86 X 0.96 X 2.02 x 10-7 4.13 x 10-7 5.92 X
6.0 5.0 11.0 1.3 2.1 17.0 7.0 1.0 2.3 2.1 0 0.5 0.8 1.7 1.7 4.0 1.0 3.0 1.3
"Calculated; Zp = [(2.389 f 0.076) X 108]C - (0.0053 f 0.0034) for M order, and Zp = [(2.956 f 0.019) X 10B]C+ (0.056 f 0.363) for others from least-sauares calculation. Table 11. Effects of Some Metal Ions on the Determination of Platinum"
metal ions
recovery Pt concn, of added M (found) Pt, %
, ' K Na+, eazt, Mg2+, erst,Ala+, NH4+ Ni2+,ZnZt, UOz2+,WOf, VOg-, Sn4+, PbZt Fez+,Fe3+,eo2+,Mn2+,Hg2+, Cu2+, Cd2+,Pd2+,Au3+, Ag+ Ru3', Os(1V) Rh3+,Ir4+ CrzO?-, Mn04-, Ce4+,Sn2+ 'Based on 5 X
10-3
10-4 10-6 10-7 5 X lo-'
10-3
I
.o
n
-
a
w
a. 0.51
I
'
' /
/
6
1'0
2'0
\ 3'0
1
1
0
4'0
5'0
$0
7%
H(cm) TPC)
Figure 5. Effectsof temperature and height of mercury column on M, LSP polarography of platinum: (1) temperature, Pt(II), 5 X and (2)height of mercury column, Pt(II), 5 X lo-' M, dc.
4-01
99.7-102.5 98-102 97.5-103 98.5-102 97.4-103 102-118 0-43
M Pt(I1).
mination of trace platinum. The catalytic behavior of Pt(I1) and Pt(1V) is identical, since Pt(1V) is reduced quantitatively to Pt(I1) by hydrazine (1). As shown in Table I, there is a linear relationship between peak current and concentration of platinum between and 6 X lo-' M Pt. The catalytic wave becomes irregular and is not reproducible at higher concentrations because of vigorous evolution of hydrogen bubbles at the surface of the DME. For practical use, a calibration curve is prepared for each concentration range of platinum. The effects of other metal ions on catalytic current are shown in Table 11. Among these metal ions, Rh3+and Ir4+ seriously interfere with the determination of platinum. Their concentrations must be kept under 10 and 2 times the concentration of platinum, respectively. I++ gives a catalytic wave at the same potential as platinum, but is less sensitive. Rh3+ gives a catalytic wave at -1.2 V vs. SCE, just following the catalytic wave of platinum. Hence, this method may be used for the simultaneous determination of platinum and rhodium. Further work on It+and Rh3+is under way. Strong oxidizing ions react with formaldehyde or hydrazine and decrease or even eliminate the catalytic current. Stannous ion reduces Pt(I1) to Pt(0) and also decreases or eliminates the catalytic current. That the polarographic wave arises from the electrode reaction of an adsorbed species may be seen from the proportionality of the Ipof dc polarographic wave to the height, h,
I .o
i
Figure 6. Effects of the (1) duration time of the DME and (2)potential scanning rate on LSP of platinum (Pt(II), 5 X lo-' M).
of mercury column of the DME and its negative temperature coefficient (Figure 5) (13). Furthermore, surfactants decrease the current seriously. Gelatine (0.01%) or Triton X-100 decreases the current to -60% of its original value. The adsorption results in a higher concentration of depolarizer at the surface of electrode, enhancing method sensitivity. The system was also examined by linear sweep polarography. In one set of experiments, the potential scan rate was maintained constant (200 mV/s). The drop time of the DME changed from 1to 9 s before scanning the applied potential; the Ipof the catalytic current (Figure 6, curve 1) increased with drop time, another indication of the adsorptive nature of the depolarizer. But the Ipdecreased with increasing potential scanning rate (Figure 6, curve 2), which suggests that there are one or more chemical reactions coupled with the electrode reactions (CE process) (13). This complicated electrode process will be reported elsewhere. In a formaldehyde-hydrazine-H2S04 medium, the catalytic current reaches its maximum value quickly and remains stable for at least 4 h (Figure 7). In a formaldehyde-hydroxylamine-H2SO4 medium, however, the catalytic current is very small at first, then increases slowly with time, requiring almost
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1988 1
*
t
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I .I*
*-
*
- - .
.
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z
V v
I
/
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I
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0
I
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Tlmr(min) Flgure 7. Formation and stability of catalytic current of Pt(I1) in NpH4 and NHPOHsystem: Pt(II), 5 X lo-’ M; HCHO, 0.02% (wlv); H2SO4, 0.75 M; DPP; N2H4and ”,OH, 0.002%.
4 h to reach its maximum value. Once formed, there seems to be no significant difference in the catalytic waves for both systems. If the mixture of hydroxylamine and formaldehyde and sulfuric acid is boiled for 1-2 min before adding the platinum, however, or if the hydroxylamine is allowed to react with formaldehyde in neutral or slightly alkaline solution for 1 or 2 min before adding the sulfuric acid and platinum, the catalytic current reaches B, its maximum value, very soon and remains stable. Furthermore, an irreversible reduction wave of the product of hydroxylamine and formaldehyde appears just prior to the catalytic wave and is very similar to that observed with the hydrazine system. These phenomena indicate that with either hydroxylamine or hydrazine, the catalytic current can only be attributed to the interaction of platinum with the reaction product of formaldehyde with either hydroxylamine or hydrazine. It is well-known that formaldehyde can react with hydrazine and hydroxylamineforming formazone and formaldoxime (14) HCHO NH2NH2 + CH2=N-NH2
+
HCHO
+ NHZOH + CH,=N-OH
In sulfuric acid solution, NH20H is protonated to “,+OH, pK, = 6.09, which consequently decreases the rate of the reaction between formaldehyde and hydroxylamine. In the case of hydrazine, also protonated, forming NH,-NH,+ (pKal = 7.99), there is another -NH2 group capable of reacting with formaldehyde, so the reaction can be very fast. Therefore, hydrazine is superior to hydroxylamine for the determination of platinum. With a plot of log Ipvs. log CL (L, hydrazine of hydroxylamine) as shown in Figure 8, the mole ratios of Pt(I1): hydrazine and Pt(I1):hydroxylamine are found to be 1:1.96 and 1:2.16, respectively, indicating that Pt(I1) might form a 1:2 complex with either formazone or formaldoxime, Pt(CHz=NHZ)$+ or Pt(CH2=N-OH)22+. These are electro-
L--+-J4
2.2
log
c
Flgure 8. Determination of mole ratio of Pt(I1) to hydrazine and M; H2S04, 0.75 M DPP. formaldehyde: Pt(II), 5 X
chemically active complexes which are strongly adsorbed at the surface of the DME and cause a catalytic hydrogen current similar to that in the Co(I1)-RS system (facilitating the discharge of protonated hydrogen ion within the complex molecule) (161, and meanwhile, the Pt(I1) reduces to Pt(0). The electrode process is very complex, and further research is under way.
ACKNOWLEDGMENT We acknowledge the assistance of Larry Cunningham and G. S. Wilson. Registry No. Pt, 7440-06-4;HCHO,50-00-0; NH2NH2,30201-2.
LITERATURE CITED Pats, R. G.; Arefeva, T. V. I n Analytlcal Chemlstry of Ratlnum Metals; Wiley: New York, 1975; p 246. English, F. Anal. Chem. 1850, 22, 1501. Beran, P.; Dolezal, J. Collect. Czech. Chem . Commun . 1956, 21, 808. Stankoviansky, S.;Podany, V.; Kalusova, A. Collect. Czech. Chem. Commun. 1860, 25, 3173. Slendyk, J.; Collect. Czech. Chem. Commun. 1832, 4 , 335. Slenkyk, J.; Herasymenko, P. 2. Phys. Chem. Abt. A 1932, A162, 223. Ezerskaya, N. A.; Kiseleva, I. N. Zh. Anal. Khim. 1868, 2 4 , 1684. Alexander, P. W.; Hoh, R.; Smythe, L. E. Talanta 1877, 2 4 , 543. Malranovskll, S. 0. Catalytlc and Kinetic Waves in Polarography; Plenum: New York, 1968; pp 241 and 269. b o , X. X.; Yao, S. R. Polarographlc Catalytlc Waves of Platinum Group Elements ; Science Publlshlng: Beijing, 1977 (Chinese). Zhao, Z. F.; Shlng, A. H. Proc. Annu. Meet. Anal. Chem. China 1864, 125. Shu, B. C.; Zheng, R. Y. Huaxue Xuebao 1@63,4 1 , 418. Bond, A. M. Modern Polarographlc Methods h Analytical Chemistry; Marcel Dekker: New York, 1980; pp 117 and 195. Millar, I.T.; Springall, H. D. A . Shorter SMgwlck’s Organic Chemlstry of Nitrogen; Clerendon: Oxford, 1969; pp 246 and 378.
RECEIVED for review December 20,1984. Resubmitted January 21,1986. Accepted February 13, 1986. This research was supported by a grant from the Engineering Division of the National Science Foundation.
