to be devised if any of these interfering ions were known to be present. For example, Table I1 shows that small amounts of platinum(1V) may be determined in the presence of large quantities of cadmium(I1) by first precipitating the cadmium as its oxalate. Separation of large quantities of lead(I1) by precipitation as the sulfate was tried but the subsequent platinum determinations were low. indicating some loss of platinum during the precipitation of lead. Additional interference studies (Table 11) on mixtures of platinum(1V) or gold(II1) with copper(II), iron(ID), nickel(II), cobalt(II), and chromium(I1) showed that these base metal ions exert no influence on the gravimetric determination of platinum(IV) or gold(II1). The
separation of gold(II1) from the other platinum group metals could probably be accomplished by a solvent extraction procedure which has been mentioned in a recent paper describing the gravimetric determination of gold with tetraphenylarsonium chloride (9). ACKNOWLEDGMENT The author wishes to acknowledge the assistance of Kristine Chadwick in the preparation of the manuscript. Received for review January 8, 1974. Accepted April 8, 1974. The author wishes to thank Union Sugar Division/ Consolidated Foods Corporation for generous support of this project.
Loss of Lead from Aqueous Solutions during Storage Haleern J. Issaq' and Walter L. Zielinski, Jr. NCI Frederick Cancer Research Center, Frederick, Md. 2 170 1
T h e development of flameless atomic absorption and atomic fluorescence has enabled the detection of trace metals in the ppb and sub-ppb range. I t is expected t h a t measurements a t such low levels generate unique problems which do not confront workers concerned with the analyses of concentrated samples. One fundamental problem is the loss of metal ions due to adsorption by container surfaces which can strongly influence the accuracy and reproducibility of data. Rosain and Wai ( I ) discussed the loss rate of mercury from aqueous solutions stored in polyethylene, polyvinyl chloride, and soft glass. Robertson ( 2 )studied the adsorption behavior of trace metals in sea water and concluded that serious losses of indium, scandium, iron, silver, uranium, and cobalt can occur by adsorption onto container surfaces. Schultz and Turekian ( 3 ) on the other hand, reported that no significant adsorption of selenium, silver, cobalt, cesium, zinc, chromium, and antimony occurred when sea water was stored in Pyrex bottles for periods of up to six months. Others (4-6) have studied the adsorption of mercury by the surfaces of various containers from aqueous solutions. Riley ( 7 ) has suggested several reasons for the adsorptive properties of glass and plastic surfaces. Using flameless atomic absorption, we observed decreases in lead concentrations with time. This paper describes the time dependence of such losses as a function of container material. T h e inhibition of this phenomenon by nitric acid and hydrogen peroxide is discussed. EXPERIMENTAL A p p a r a t u s . A Perkin-Elmer Model 403 atomic absorption spectrophotometer equipped with a deuterium background corrector. a rapid response strip chart recorder, and a Westinghouse lead holAuthor to whom correspondence should be sent. ( 1 ) R. M. Rosain and C. M. Wai, Anal. Chim. Acta, 65, 279 (1973). ( 2 ) D. E. Robertson, Anal. Chim. Acta, 42, 533 (1968). (3) D. F. Schuitz and K . K . Turekian, Geochim. Cosmochim. Acta, 29, 259 (1965). (4) P. Benes and I . Rajman, Collect Czech. Chem. Commun., 34, 1375 (1969). 15) P. Benes, Collect. Czech. Chem. Commun., 35, 1349 (1970). ( 6 ) R. V. Coyne and J. A . Collins, Anal. Chem.. 44, 1093 (1972). (7) J. P. Riley, and G. Skirrow, "Chemical Oceanography," Vol. 2, Academic Press, London and New York, 1965, p 303.
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low cathode lamp was used for this study. T h e burner assembly was replaced by a Perkin-Elmer HGA-2000 graphite furnace without further modification. T h e containers were precleaned with concentrated nitric acid followed by five rinses with deionized water. Eppendorf microliter pipets having disposable plastic tips were used for sample introduction. R e a g e n t s a n d Materials. All reagents were of analytical grade. Deionized water was used for all sample preparations. T h e stock lead nitrate solution was a 1000-ppm (1000 bg/ml) certified atomic absorption standard obtained from Fisher Scientific. I n t e r m e d i a t e S t a n d a r d . A 400-ppb solution was prepared bv pipetting 10.0 bl of the stock solution into the appropriate container which was then accurately brought t o volume with deionized water. T h e sample containers were 25-ml volumetric flasks of Pyrex and Kimax brands (Fisher Scientific), and 125-ml polyethylene bottles (Sprayon Prod. Inc., Cleveland, Ohio). P r o c e d u r e . T h e spectrometer was operated a t an absorption line of 283.3 n m with a lamp current of 5 mA and a 1-mm slit width. A 10.O-pl aliquot of t h e test sample was introduced into the graphite tube furnace. T h e sample was then dried for 30 sec a t 140 O C , heated for 30 sec a t 400 "C, and atomized for 5 sec a t 2100 O C . T h e graphite furnace was operated with a nitrogen purge and a water flow of approximately 1.5 and 3.0 l i t e d m i n , respectively. Standard graphite tubes were employed.
RESULTS AND DISCUSSION
Tables I and I1 show the percent of loss of lead due to adsorption from 400-ppb lead aqueous solutions which were stored in five containers each of Pyrex, Kimax. and polyethylene. It is clear from the range of values that adsorptive properties differ in degree from container to container. It was noteworthy that 30% of the lead in solutions stored in Pyrex and Kimax was adsorbed after five minutes and that approximately 50% loss occurred after one hour. The loss of lead in polyethylene containers, although not as great, was similarly observed. T h e rates of loss roughly approximated a semi-log relationship of L = r log t + Lo, where L is the % of loss, r the slope, and t the time (min). Values o f t < 1 represent trivial solutions. This relationship gave the highest correlation coefficients of six equation forms attempted (Pyrex: 0.995; Kimax: 0.988; polyethylene: 0.965). T h e explicit equation for each container type gave values for L of 38, 40, and 12, a t 15 minutes; and 42. 47, and 20, a t 30 minutes, for Pyrex, Kimax, and polyethylene, respectively. These results clearly dictate the necessity for in-
A U G U S T 1974
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Table I. Loss of Lead from 400-ppb Aqueous Solutions Stored in Pyrex and Kimax Containers Pyrex
Time, min
5 15 30
60
________ Loss, 'I Rangeb
30 38
43 46
21 26 20 27
1 Mean of five different containers. container-to-container \ ariations.
Table 11. Loss of Lead from 400-ppb Aqueous Solutions Stored i n Polyethylene Containers
Kiinax
Time, min
Loss,
5'U
Hangeh
Rangeb
15
10
11
29
20
23
10
38 49 53
25
30 90
31
8
Loss,
c;a
35 35
Mean of five containers. to-container variation. tL
Range of loss ('/c j values illustrating container-
' Range of loss ( ' 6 ) values illustrating
hibiting such losses, if meaningful quantitative analyses of lead a t trace levels are to be realized. Rosain and Wai ( 1 ) had reported t h a t losses of mercury were not observed after 110 hours of storage when solutions were acidified to p H 0.5 with nitric acid. Robertson (Z), employing hydrochloric acid for the preservation of trace metals in sea water, still found adsorption of silver on Pyrex and polyethylene surfaces, reporting losses of 1020% cobalt and 10% rubidium on polyethylene and Pyrex, respectively, after 10 days, and 5-10% cobalt on Pyrex after 20 days. Coyne and Collins (6) concluded t h a t nitric acid was the only effective acid preservative for mercury. While the use of nitric acid has been suggested as the acid preservative of choice, its removal of metallic impurities from container walls (8-11) can present an additional concern. It was observed t h a t hydrogen peroxide, is, as nitric acid, a good preservative for aqueous solutions containing trace lead ions. A volume of 100 ml containing 400 ppb of lead was introduced into each of two containers of each type (Pyrex, Kimax, and polyethylene). A volume of 1.00 ml of concd "03 was added to one container of each type, while 1.00 ml of H202 was added to the second of the containers. After a one-week storage period, no detectable loss of lead was observed in any of the solutions. An advantage to the choice of H202 over H N 0 3 as a preservative for trace Lead solutions lies in the poor desorptive (8) D. E. Robertson, Anal. Chem.. 40, 1067 (1968). (9) E. C. Kuehner and D. H. Freeman, "Purification of Inorganic and Organic Materials." M. Zief. Ed., Marcel Dekker. Inc.. New York, N.Y., 1969, p 297. ( l o ) D. H. Freeman and W. L. Zielinski, Jr., Ed., Nat. Bur. Stand. (U.S.) Tech. Note, 549, 60 (1970). ( 1 1 ) E. C. Kuehner, R. Alvarez. P. J. Pauisen, and T. J. Murphy, Ana/. Chem., 44, 2050 (1972).
properties of H202. A 1.00-ppm P b solution was stored for 24 hours in Pyrex, Kimax, and polyethylene containers, after which the solution was discarded. The containers were washed with a detergent solution, rinsed thoroughly with deionized water, and finally filled with 4% HNO:3. Analysis of the nitric acid solutions after 5 minutes readily produced lead signals. When H202 was used under the same conditions, no P b signal was observed after the addition of H202 to the rinsed containers, even after holding for two days. While glass surfaces such as Pyrex and Kimax are known to exhibit ion-exchange behavior, and plastics are known to imbibe solutions due to their permeability, the inhibition phenomena reported here appear different for the HNO:I and H 2 0 ~cases. An ion-exchange mechanism for the glass surfaces based upon proton competition with lead ions does not explain why the weaker acid H202 ( K = 1.5 x IO-'?) (12), is a comparable preservative. Furthermore, the removal of adsorbed lead from both glass and plastic containers is greater with "03. The stabilization of lead solutions by H202 appears to involve some solution mechanism which prevents adsorption from occurring, while HNO:{ merely alters the lead adsorption equilibirum by compel itive inhibition. Further studies are needed for a clear understanding of the actual mechanisms involved. Preliminary results have shown that H202 is also an effective preservative for trace mercury solutions. Received for review October 25, 1973. Accepted April 11, 1974. This Research was sponsored by the National Cancer Institute under contract NIH-NCI-E-72-3294 with Litton Bionetics, Inc. (12) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry." 2nd ed., lnterscience Publishers, London, 1966, p 373.
Improved Methylthymol Blue Procedure for Automated Sulfate Determinations Michael R. McSwain and Russell J. Watrous Eastern Deciduous Forest B i o m e . U S - l B P University of Georgia Athens G a 30602
James E. Douglass Southeastern Forest Experiment Station USFS Coweeta Hydrologic Laboratory Franklin N C 28734
Recent technical data published by Central Laboratories of the U.S.Geological Survey ( 2 ) and other laboratories have pointed t o the need for a more sensitive and reli( 1 ) R. L. McAvoy, ' Automation in a Water Quality Laboratory Scheme. ' "Advances In Automated Analysis,' Mediad Incorporated, Tarrytown, N . Y . , 1972, Vol. 8 , p . 4 0
able automated sulfate determination. The Central Laboratories' statement concerning their sulfate determination is typical: ". . , . . . . . it is unstable from day to day, requiring much attention to denote shifting of the calibration curve. Additional research of the AutoAnalyzer I1 sulfate method is needed to increase its stability during operation and hence its accuracy."
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