~~
Table I. Loss of Lead from 400-ppb Aqueous Solutions Stored i n 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 that 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 that 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 that 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."
A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 9 , AUGUST 1974
1329
" 1200lWATER
WASH R E C E P T
~
10321AlR 1141n.of Z m m I D l
61
WASTE
-
lot","
20 t","
157-0370
nnnM
4 -
~
IO.421NoOH
NOTE: FIGURES I N P A R E N T H E S E S R E P R E S E N T FLOW R A T E S IN M L ' S I M I N . I*- S I L I C O N E 1
Figure 1. Sulfate
manifold Figure 2. Two
Three somewhat different automated sulfate procedures are currently in general use; all utilize the barium sulfate suspension by either measuring turbidity or color reduction of methylthymol blue. Time-consuming concentration of samples is often required to put the anion in the 10-200 mg/l. range. Barium sulfate is relatively insoluble in water and tends to coat the colorimeter flow cell and other glassware. Plugging of small diameter tubing and connectors also constitutes a problem. These difficulties are compounded if gelatin is used to form a barium sulfate suspension for turbidimetric determinations. The resulting determination is typically noisy, unstable, and generally difficult to maintain. Ethylenediaminetetraacetic acid (EDTA) is sometimes used between samples to clean the manifold and flow cell, but is unsatisfactory because it restricts scale expansion. Technicon's procedure KO.226-72W is a color-reducing reaction using methylthymol blue and for Coweeta analysis is preferable to the turbidimetric methods. The detection limit with this determination is poor in the 0-10 mg of sulfate/l. range. The procedure is also noisy and plagued with a shifting base line. Coweeta's improved procedure is a n adaptation of Technicon method 226-72W. The method is accurate, easily maintained in operation, and sensitive to concentrations in the 0-10 mg/l. range. The improved sensitivity eliminates the need t o concentrate samples.
calibration plots for the improved sulfate proce-
dure A . Alternating water and 0.3 mg/l. standards demonstrating reproducibility and base-line return at 40 samples/hour. The fifth 0.3 mg/l. standard was hand-timed to demonstrate signal steadiness. B. Paired 1.0, 0.3, and 0.04 mgil, standards demonstrating linearity of the procedure at 30 samplesihour with a 640-1 sample-to-wash ratio
grey. T h e barium sulfate reaction must take place a t p H 2.5-3.0 and the barium-methylthymol blue reaction a t p H 12.5-13.0 ( 2 ) . T h e diagram for t h e mixing manifold is shown in Figure 1.
RESULTS
A typical recorder response with a sampling rate of 30 samples/hour and a 6-to-1 sample-to-wash ratio is shown in Figure 2B. Note that the first 0.3 mg/l. sample after the 1.0 mg/l. samples did not stabilize because of the short wash time between samples. Low concentration samples following high concentration samples must be rerun. Reproducibility is excellent in the 0-1.0 mg/l. range (Figure 2). Detection limits for the determination are better than 0.020 mg/l., and the per cent accuracy of the determination is 1.7% error. Samples may be analyzed continuously from 6-8 hours with only minor corrections for base-line shifting. DISCUSSION
The increase in sensitivity is achieved largely by using a 50-mm flow cell. However, the longer flow cell also amplifies noise in the system and in itself will not yield totally satisfactory results. The longer flow cell is sensitive EXPERIMENTAL to inconsistencies in bubbling and small variation in reApparatus. A Technicon AutoAnalyzer I1 is used to colorimetagent addition or mixing. These problems are reduced by rically determine sulfate. using a high flow rate to the colorimeter and a low F/C Reagents. B a r i u m Chloride Dihydrate Stock. Dissolve 1.526 extraction rate. The increased flow rate aids in moving grams of barium chloride dihydrate in 1000 ml distilled water and particulate matter through the manifold and in preventstore in a brown polyethylene bottle. This reagent is filtered before using. ing coating of glassware. The high fiow rate apparently S o d i u m HJdroxide (0.184').Dissolve 7.2 grams sodium hydroxkeeps the precipitated barium sulfate homogenous ide in 1000 ml distilled water. throughout the system. The addition of alcohol to the Meth>Ithjmol Blue (3.3-bis-~V,:.'V-bis(carboxymethyl)amino sample stream produces a high flow rate and keeps the methylthymolsulfonephthalein pentasodium salt). Dissolve 0.1182 mixing coils clean. Both ethanol and methanol from glass gram of methylthymol blue in 500 ml of methanol. Add 25 ml of containers are satisfactory but some loss of sensitivity has the barium chloride stock, 4 ml of 1.ON hydrochloric acid and 71 ml of distilled water and dilute to 1000 ml with methanol. S o been associated with methanol stored in metal containers. wetting agents should be added to this solution. Methanol is less expensive, and methylthymol blue is Procedure. Initially, barium chloride and methylthymol blue more readily dissolved in it. are equimolar and equivalent to the highest sulfate concentration Tygon tubing is unsatisfactory for pumping any reagent expected. The procedure depends on the relative reactivity of containing methanol. Ideally, silicone rubber should be methylthymol blue and sulfate with barium. When no sulfate is used for pumping and passage of alcohols. Solvaflex tubpresent, all t h e barium is complexed with methylthymol blue producing a deep blue color. Barium sulfate is produced in the ing is satisfactory when silicone rubber is unavailable but presence of the sulfate ion and only excess barium is complexed with methylthymol blue producing a gradation of blue color. When methylthymol blue is completely uncomplexed, its color is 1330
A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
NO. 9.
( 2 ) "Technicon AutoAnalyzer Methodology." Technicon Corporation. Tarrytown, N . Y , Method No 226-72W, 1969.
A U G U S T 1974
tends to flatten in 6 or 8 hours of continuous operation. With the pump pressure plate removed, Solvaflex will return to shape overnight. The sensitivity of the determination is greatly affected by the quality of the methylthymol blue reagent. Weight of methylthymol blue and barium chloride is critical. Fresh methylthymol blue should be prepared daily. Certain cations, notably calcium, even in low concentrations, interfere with the methylthymol blue-barium reaction. An ion exchange column of Dowex 50W-X8 (Dow Chemical Company) has proven satisfactory for cation removal. The optimum length of the column should be determined experimentally and must be sufficient to remove the highest calcium concentrations experienced. Fourteen inches of 2-mm i.d. glass tubing is sufficient to remove all apparent interferences a t Coweeta where the calcium con-
centration never exceeds 7.0 mg/l. Since the cation exchange column tends to slow the response time, a longerthan-necessary column should be avdided. Received for review December 17, 1973. Accepted March 3, 1974. Research supported in part by the Eastern Deciduous Forest Biome, US-IBP, funded by the National Science Foundation under Interagency Agreement AG 199, 40-193-69, with the Atomic Energy Commission-Oak Ridge National Laboratory, and in part by the U. S. Forest Service, Southeastern Forest Experiment Station. Contribution No. 151 from the Eastern Deciduous Forest Biome, US-IBP. Mention of commercial products and sources does not constitute an endorsement of such products, to the exclusion of others equally acceptable, by the Forest Service or the Department of Agriculture.
Titrimetric Analysis of Carboxylic Acid-Anhydride Mixtures with Tetrabutylammonium Hydroxide C. A. Lucchesi, L. W. Kao, G. A. Young, and H. M. Chang D e p a r t m e n t of C h e m i s t r y , Northwestern Unrversity. Evanston. I / / . 60207
The majority of methods for the analysis of admixtures of an anhydride and its parent carboxylic acid are titrimetric in nature ( I , 2), and most of these involve the determination of small amounts of acid in the anhydride sample ( 3 ) .Essentially all of the methods reported for the determination of both the acid and the anhydride contents over the entire concentration range require two separate titrations with either two standardized titrants or one titrant in a two-step procedure. The oldest and probably still the most widely used procedure is that described by Smith and Bryant ( 4 ) . In this procedure, one portion of a sample is titrated for total acidity with sodium hydroxide after it is hydrolyzed with water in the presence of a pyridine catalyst. A second portion of the sample is titrated with sodium methoxide in methanol. The equivalent difference between the two titrations yields the anhydride content, and through calculation the acid content is found. Since the anhydride and acid concentrations are derived from the difference between two large numbers, the precision is poor-particularly, a t very low and very high acid concentrations. Other two-step methods also suffer from the same disadvantage and are even more excessively time-consuming. The purpose of the work reported here was to develop a simple and rapid single titration procedure applicable to a wide variety of anhydride samples over an extended concentration range. A number of investigators have demonstrated that anhydrides behave as monobasic acids when titrated with sodium methoxide in nonaqueous solvents ( 5 - 7 ) . Patchor(1) S. Siggia. "Quantitative Organic Analysis via Functional Groups," John Wiley. New York, 1967, p 187. (2) R D Tiwari and J P. Sharma, "The Determination of Carboxylic Functional Groups." Pergamon Press, Oxford, 1970. p 68. (3) E. J Greenhow and R. L. Parry Jones. Analyst (London) 97, 346 (1972). (4) D M . Smith and W. M . D. Bryant. J. Amer Chem. SOC.. 58, 2452 (1936) (5) J . S. Fritz and N. M . Lisicki, Anal. Chem.. 23, 589 (1951) (6) A. Berger, M. Sela, and E. Katchalski. Anal. Chem.. 25, 1554 (1953). ( 7 ) A. Patchornik and S. E. Rogozinski, Ana/. Chem.. 31, 985 (1959)
nik and Rogozinski ( 7 ) reported the use of Triton B (trimethylbenzylammonium hydroxide) in pyridine as a titrant for anhydrides and presented data for the titration of acetic acid-anhydride mixtures. Their procedure required heating the sample in a dioxane-pyridine-water solution before titration, and two moles of Triton B were consumed for each mole of anhydride. Tiwari and Sharma (2) state that the anhydride content of an anhydride-acid mixture may be obtained from a total acidity determination by a simple calculation, but no data are given. In this paper, a method for the simultaneous determination of an anhydride and its parent carboxylic acid in binary mixtures is presented. The method involves a simple, room temperature indicator titration of a sample in pyridine with tetrabutylammonium hydroxide (TBAH) in benzene-methanol as the titrant. The indicator is either Thymol Blue or Azo Violet. Under these conditions. the anhydrides tested behave as monobasic acids. Data are presented for acetic, benzoic, maleic, phthalic, and succinic acid-anhydride binary mixtures.
EXPERIMENTAL Reagent. All t h e acids and anhydrides used were the best available. Most were analytical reagent grade obtained from .J. T. Baker (Phillipsburg, N.J.), E a s t m a n Kodak (Rochester, N.Y.), Fisher (Fair Lawn. S . J . ) or Matheson, Coleman and Bell (Norwood, Ohio). T h e benzoic acid was NBS sample 140B. Except for acetic, all t h e acids were dried a t 110 "C for a t least 2 hours before use. T h e anhydrides were used as received or pulverized in a glove bag under dry nitrogen. The zone refined maleic and phthalic anhydrides were obtained from Aldrich (Milwaukee, Wis.). T h e pyridine ( E a s t m a n ) , acetone (Baker), and benzene (Fisher, spectranalyzed grade) were dried over 15% by weight of Linde (New York. N.Y.) molecular seives 4A before use. Absolute methanol (Baker) and 99% acrylonitrile (Aldrich) also were used as solvents, The TBAH titrant was made with silver oxide (Fisher) and tetrabutylammonium iodide ( E a s t m a n ) , and the T P A titrant was made with tripropylamine obtained from Eastman. T h e anhydrous barium perchlorate was from K and K Laboratories (Plain-
A N A L Y T I C A L C H E M I S T R Y , V O L . 46. NO. 9. A U G U S T 1974
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