Anal. Chem. 1980, 52, 591-592
-
0
,
I
40
20
591
30
50
40
60
70
245
265
80
90
205
305
!00
TIME ( m i n i I
I
125
445
165
I
I
185
205
~-
I
225
325
TEMPERATURE ( " C )
Figure 4. Direct injection o f a coal liquefaction product. Column: 50 m X 0.2 mm i.d. Dexsil-400 glass capillary. Conditions as in Figure 1
fractionation techniques in current use, Dexsil-400 glass capillary columns should find increasing utility in the analyses of complex organic materials, especially polycyclic aromatic hydrocarbons.
ACKNOWLEDGMENT The authors thank M. L. Lee for supplying the carbon black extract.
LITERATURE CITED (1) Schultz, R. V.: Joraenson. J. W.; Maskarinec, M. P.; Novotnv, M.; Todd, L. J. Fuel1979, 52, 783. (2) Blomberg, L.; Wannman, Th. J . Chromafogr. 1978, 748, 379. (3) Borwitzky, H.; Schomburg, G. J . Chromatogr. 1979, 170, 99. (4) Grob, K.; Grob. G. High Resolution Chromatogr., Chromafogr. Commun. 1979, 3 . 109. (5) Lee, M. L., unpublished results. (6) Novotny, M.; Segura, R.: Zlatkis, A. Anal. Chem. 1972, 4 4 , 9. (7) Blomberg, L.; Wannman, Th. J . Chromatogr. 1979, 168, 81. (8) Lee, M. L.; Novotny, M.; Bartle, K. D. Anal. Chem. 1976, 4 8 , 1566. (9) Lee, M. L.; Hites, R . A. Anal. Chem. 1976, 4 8 , 1890. (10) Giger, W.; Schaffner, C. Anal. Chem. 1978, 5 0 , 243.
(11) Pollock, G. E. Anal. Cbem. 1972, 4 4 , 634. (12) Pollock, G. E. Anal. Chem. 1972, 4 4 , 2368. (13) . . Kubota. H.; Griest, W. H.; Guerin, M. R. Trace Subsf. Envson Health-IX, 1975, 281. (14) Lao, R. C.; Thomas, R. S.; Monkman J. L. J . Chromatogr. 1975, 112, 68 1 (15) Alexander, G.; Rutten, G. A. F. M. Cbromatographia 1973, 6 , 231. (16) Lee. M. L.: Vassilaros, D. L.; Phillips, L. V.; Hercules, D. M.; Azumaya, H.; Jorgenson. J. W.; Maskarinec, M. P.; Novotny, M. Anal. Lett. 1979, 12, 191. (17) Bouche, J.; Verzele, M. J . Gas Chromafogr. 1968, 6, 501. (18) Novotny, M.; Tesarik, K. Chromatographia 1968, I , 332. (19) Hair, M. L.; Hertl. W. J . Phys. Chem. 1973, 7 7 , 1965. (20) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley: New York, 1966; p 334. (21) Horning, M. G.; Moss, A. M.; Horning, E!. C. Biochim. Biopbys. Acta 1967, 4 8 , 597. (22) Lee, M. L., personal communication (23) "Polycyclic Aromatic Hydrocarbons and Cancer", Vol. 1, Gelboin. H. V.. Tso, P. O.,Eds., Academic Press: New York. 1978.
RECEIVED for review June 19, 1979. Accepted November 27. 1976. Research sponsored by the US.Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation.
Ethylenediaminetetraacetic Acid Titration by End-Point pH Measurements for Determination of Lead C.
H. Culberson" and C. L. Washburne
College of Marine Studies, University of Delaware, Newark, Delaware
In a study of lead iodate solubility, we required a simple b u t precise method for the determination of lead a t concentrations of approximately 1 x 10-4 molar in synthetic seawater. Hydrogen ions are released when metals are complexed by ethylenediaminetetraacetic acid (EDTA),and P o p w et al. ( 1 ) showed that plots Of 'H the concentration Of EDTA added could be used to locate the end point in EDTA titrations. In this paper we evaluate the precision of this method for the determination of lead in the presence of sodium, magnesium, and calcium at concentrations comparable to those in seawater.
""
1971 1
and measuring the change in pH that occurred upon addition of EDTA. Five mL of Pb(NOd2 (1 X :LOP M) and five mL of salt solution (0.505 M NaCI, 0.505 M NaC1 + 0.065 M CaCl,, 0.605 M NaCl + 0.065 M MgC12, or seawater) were added to a 1-inch diameter test tube at room temperature and adjusted to pH 5.0 with dilute NaOH, The initihl pH c,f the natural Seawater was adjusted to pH 4.4 with HC], The mixtures were titrated with EDTA using a j . , ~ Metrohm automatic buret readable to fO,ml m ~and , the p~ was measured to & 0 , ( ~p~ 1 using an orion7 0 1 ~ digital pH meter and a Corning semi-micro combination electrode (~476050)with 4 M KC1 filling solution.
EXPERIMENTAL
RESULTS A N D DISCUSSION
Lead was determined by titrating a lead nitrate plus salt mixture with dilute disodium ethylenediaminetetraacetate solution
Typical experimental titration curves are shown in Figure 1. T h e decrease in pH that occcurs upon addition of EDTA
0003-2700/80/0352-0591$01.00/0
C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
592
Table I. Results of Computer Simulated Titrations of 5 mL of 1 x M Pb(NO,), plus 5 m L of Salt Solution with 2 x M Disodium EDTAa end point 10 0
salt solution r
/
/p D
4
90
0.505 M NaCl 0.505 M NaCl/ 0.065 M MgCl, 0.505 M NaCl/ 0.065 M CaCl,
a
2
4 a,,
d
uncorrected a H vs. volume, mL
2.499 2.499
8.493 2.433
2.507
2.547
2.505
2.489
synthetic seawater
d
105
corrected aH vs. volume, mL
0.505 M NaCl/ 0.055 M MgCl,/ 0.010 M CaCl,
a 0
Corrected a H is the quantity a H ( 1 0
+
V ) / l 0 , where
a H is the uncorrected hydrogen ion activity and V i s the volume of EDTA.
7 0
-
Table 11. Results of Triplicate Titrations of 5 m L of M Pb(NO,), plus 5 m L of Salt Solution with 1x 2 x 1 W 4 M Disodium EDTA
d I 20
22
- 1 24
26
2a
30
salt solution
Volume o f E D T A (rnll
M) plus 5 mL Figure 1. Typical titration of 5 mL of Pb(NO,), (1 X of salt solution (0.505 M NaCI, 0.505 M NaCl 0.065 M CaCI,, 0.505 M NaCl 0.065 M MgCI,, or seawater)with 2 X M disodium EDTA. The values of a H have been corrected for dilution
+
+
+
is due t o t h e reaction: Pb2++ H2Y2- = PbY2 2H+. T h e end point for a given curve is t h e point of intersection of t h e two straight lines. Least-square fits for each line determine simultaneous linear equations which are solved to give the end point. For linear regression, the same number of d a t a points on either side of the end point was used and the point closest t o t h e intersect was neglected. Computer simulated titrations (Table I) of lead in different salt media were performed in which the theoretical end point was 2.500 mL. T h e initial p H of each titration was 5.0, and the dissociation constants for EDTA and the formation constants for the lead, calcium, and magnesium EDTA complexes were taken from Kolthoff et al. (2). Sodium was assumed not t o complex with EDTA. T h e computer-simulated titrations showed that graphs of u H , corrected for dilution, vs. the volume of EDTA added yielded more accurate end points than plots of uncorrected u H vs. volume, although the difference was small in the sodium, sodium/magnesium, and seawater systems. Owing to calcium complexation with t h e titrant, t h e end point break in the sodium/calcium system is not sharp, and the calculated end point is sensitive t o the dilution correction. T h e theoretical results in Table I show t h a t the method is accurate and t h a t magnesium and calcium a t seawater concentrations do not interfere significantly with t h e lead titration. Typical titration curves determined with the experimental procedure outlined above are shown in Figure 1. T h e end point for t h e seawater titration in Figure 1 differs from t h e
0.505 M NaCl 0.505 M NaCli 0.065 M MgC1, 0.505 M NaC1/ 0.065 M CaC1,
seawater (salinity = 30.22 ppt)
end point, corrected aH vs. std. dev., volume, mL mL 2.393 8.397
0.007 0.007
2.405
0.014
2.498
0.008
end points of the other three curves because the seawater titration was performed with different lead and EDTA solutions. T h e results of triplicate analyses of each salt solution (Table 11) show t h a t the method is capable of a precision of 0.6% or better in the presence of large excesses of calcium or magnesium. The above procedure was used to measure lead in a study of lead iodate solubility in NaCl/NaClO, mixtures a t ionic strength 0.7. At a lead concentration of 6 X lo-' M (after dilution), the relative standard deviation calculated from 28 triplicate analyses was 0.2%. This method yields precise results a t lead concentrations considerably lower than those found in our solubility studies. In three test titrations we obtained a relative standard deviation of kO.870 at a lead concentration of 5 x 10 M in 0.25 M NaC1.
'
LITERATURE CITED (1) H. Poppe, G. Den Boef, and F. Freese, Anal. Chim. Acta. 51, 199 ( 1970).
(2) I. M. Kolthoff, E. 6. Sandell, E. J. Meehan, and S. Bruckenstein, "Quantitative Chemical Analysis", 4th ed., Macmillan. New York, 1969.
RECEIVED for review May 21,1979. Accepted October 19, 1979. Research supported by the National Science Foundation grant OCE78-07488.
