. [ O- 0
L
W 4
2
6
4
8
10
I
1/11 ( KOtlYS-l)
Figure 3. Dependence of differential pulsed stripping peak current on the reciprocal of the added uncompensated resistance
Solution: 8 ppb Pb in 1.70 acetic acid/1.25F potassium acetate. Instrumental conditions: same as for Figure 1
Similar experiments of O'Dom (8) using the hanging mercury drop electrode indicate that a similar effect of uncompensated resistance obtains a t this electrode, but that an added uncompensated resistance of -50 kl2 is (8) G O'Dom. Princeton Applied Research Corporation, Princeton N J , private communication, 1973
necessary for the effect to show clearly. That a higher resistance is necessary to demonstrate the effect is clearly due to the smaller area of the drop as compared with the thin film; the smaller area manifests itself in a smaller interfacial capacitance and a smaller stripping current, both of which require a larger uncompensated resistance to exhibit the effect seen a t lower resistance values with the thin-film electrode. A complete theoretical and experimental study of pulse anodic stripping voltammetry is under way in our laboratory. Until results of this complete parametric study are available, we suggest that pulsed stripping voltammetry and other voltammetric techniques be used with caution in high resistance solutions. The response depends strongly on the resistance of the test solution which may not be under the analyst's control unless he adds additional supporting electrolyte to swamp the effects of the variable amount of electrolyte naturally present in his sample. Thomas R. Copeland Joseph H. Christie Rodney K. Skogerboe Robert A. Osteryoung Department of Chemistry Colorado State University Fort Collins, Colorado 80521
Received for review November 30, 1972. Accepted February 8, 1973. This work was supported. in part, by the National Science Foundation under Grants GP-31491X and GI-34813X.
I AIDS FOR ANALYTICAL CHEMISTS Comparative Study of Different Methods of Packaging Liquid Reagents G . J. Curtis, J. E. Rein,l and S. S. Yamamura
Analytical Chemistry Branch, Allied Chemical Corporation, P. 0. Box 2204, ldaho Chemical Programs-Operations Office, ldaho falls, ldaho 83401 Liquid reagents of many types are an integral part of the operation of any laboratory performing chemical studies. The reagents, stored in a variety of containers, are used for periods ranging from a day to several years and it is not uncommon to have one question the integrity of a given reagent that has been stored for any length of time. In addition to precipitation or chemical change, evaporation and/or transpiration is the major cause for observed change of reagent concentration with time. To determine the magnitude of evaporation-transpiration change with time, a one-year study of the containment of different liquids in various glass and plastic containers was conducted. Included in the study were 9 common aqueous solutions and 5 organic liquids and 8 different container-closure combinations. In all, more than two thousand observations were made among the 135 specimens involved in the study. The significance of those observations is presented in this paper. I J E Rein I S presently employed at Los Alarnos New Mexico
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
EXPERIMENTAL The investigation involved measurement of the weight of various bottles containing different liquids on periodic intervals (weekly for the first month, then monthly thereafter) for one year. The bottles were not opened until the weighings were concluded. The container-closure-liquid combinations covered in the study and the details of certain special experiments are summarized in Table I. The weighings were made on a single-pan 200-gram capacity balance; hence, the quantity of liquid originally packaged was maintained at about 100 ml with the plastic bottles and about 50 ml with the heavier glass bottles. There was one exception t o this. In the case of the polyethylene bottle, a 50-ml water sample also was included to determine whether the amount of vapor space in a container affects the loss. Selected container-reagent combinations were stored in a vapor atmosphere similar or closely similar to that within the container. Cerium(II1) at about a 0.075M level was included in all nitric acid solution and water tests to permit correlation of any observed evaporation losses with corresponding concentration gains.. The cerium(II1) was determined by titration with EDTA to a visual xylenol orange end point. A straightforward base titration to a phenolphthalein end point also was performed on the 1M nitric acid specimens.
Table I. Overall Per Cent
Loss (or Gain) over the 1-Year Perioda Plastic containers
Liquid
Polyethylene, screwcapb
Polypropylene, screwcap. waxedC
Glass containers High density poly propylene, screwcap,waxed'
Teflon-FEP screw-cap waxedC
Inverted stopper greased
Inverted stopper
Polyethylene, screwcap*
Standard glass stopper
A. Storage in Air between Weighings
Water, 100 ml Water, 50 ml 1 2 M HNO3 1 M "03 1 2 M HCI 1 M HCI 1 2 M HF 1M HF 1 2 M NaOH 1 M NaOH 9 5 % EtOH Ether Hexane
MIB K ~ Kerosene Blank
0.35 0.37 0.03 0.23 1.34 0.23 0.19 0.40
(0.05) 0.23 1.80 100.0 100.0 13.2 33.5
0.26
0.32
0.01 0.47 1.02 0.18 0.21 0.39 (0.15) 0.33 0.81
(0.01) 1.05 1.53 0.44 0.23 0.32 (0.09) 0.96 3.27
0.07 (0.01) 0.09 0.24 0.07 0.07 0.26 (0.02) 0.12 0.02
57.3
49.9
22.4
0.00
0.00
0.14 0.03
0.01 0.1 1
2.13 1.21
1.71
1.72
3.93
0.17 1 .oo 17.9 3.43 0.70 0.02
0.01
0.60
0.09 100.0
3.72 100.0 11.5 2.72 0.52
2.50 0.51 0.10
0.07
0.00
B. Storage in Desiccator above Similar Solvent between Weighings
Water, 100 ml
(0.16) 0.20 (1.55) 1 2 M "03 ( .29) (0.05) 1 M "03 95% EtOH (0.01) 0.08 0.48 Hexane 3.24 0.44 1.22 %02 plastic bottles and 2-02 glass bottles were used. For a given liquid in a given container. the absolute loss (or gain) is essentially independent of the amount of liquid in the container. The per cent change is calculated on the basis of 100 grams of liquid. A value in parentheses denotes a weight gain. bCommercially-suppiied black screw-caps with conical polyethylene liner. CThe caps supplied with the polypropylene and Teflon-FEP (1,1,2-trifluoro-2-methylethylene polymer) bottles did not give an air-tight seal, so a heavy wax coating was applied. dMIBK is methyl isobutyl ketone.
RESULTS AND DISCUSSION Table I lists the observed per cent loss or gain for the various container-liquid-storage condition combinations over the 1-year period. The data in this table are calculated on the basis of 100 grams of liquid. Except for the anticipated high evaporation losses for the volatile organic solvents, the weight change, hence, the solution concentration change for most of the container-liquid combinations was remarkably small and linear with time (Figure 1). Generally, the observed change was less than 1% for the aqueous solutions. It should be kept in mind, however, that the containers were not opened during the 1-year study. The cases where significant evaporation losses occurred are: (a) storage of 12M HC1 in glass containers and in all plastic containers except Teflon (Du Pont)-FEP, ( b ) storage of aqueous as well as organic liquids in common glass reagent bottles with conventional ground glass stoppers, and (c) storage of volatile organic solvents in all types of containers. The commonly used glass reagent bottles with ground glass stopper showed 2.1, 1.2, and 3.9% losses of water, 12M H N 0 3 and 12M HC1, respectively. The plastic containers, particularly Teflon-FEP, were more satisfactory; however, nitric acid discolored the containers and 12M HC1 produced a rough texture on the outside surfaces of the bottles. With the glass containers, inverted ground glass stoppers and polyethylene-lined screw caps were significantly better than the conventional ground glass stoppers for both aqueous solutions and organic solvents. There were several unexpected observations. These are described in Figure 1. In polyethylene, 12M H N 0 3 gained slightly in weight a t the outset, then lost weight to show a slight net loss over the 1-year period. Twelve molar NaOH
IM
NaOH
-0.2
!L 2
s +-
\
n
WATER
4
\
\
-0.41
\
I
2
g-0.6
+-
z
w
0
a
-0.8
w a -1.0
-1.2
E T H E R , I O O X L O S S IN L E S S THAN 2 MONTHS
2
4
TIME
6
IN
8
\ IO
12
MONTHS
Figure 1 . Evaporation c u r v e s for various liquids
stor'ed
in poly-
e t h y l e n e bottles w i t h s c r e w c a p s .
showed a slight linear weight gain over the 1-year period, presumably due to absorption of carbon dioxide; however, 1M NaOH showed a linear weight loss over the same period. The smaller weight loss compared to water is again attributable to carbon dioxide absorption. Greasing the glass stoppers seemed to reduce evaporation losses of liqA N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
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uids of low volatility but with highly volatile ether, greasing the stopper greatly enhanced evaporation, presumably by preventing the stopper from reseating properly after rising due to internal pressure. Storage of samples in a desiccator containing the same solvent greatly minimized evaporation and/or transpiration losses, especially in the case of the volatile organic solvents. Some gained weight. For example, in polyethylene, 1211 H N 0 3 showed a 1.6% gain. Time was allowed for evaporation of solvent condensed on the exterior of the container, but part of the gain could still be attributed to condensate. The weight gain on the 12M "03 specimens undoubtedly includes gain due to absorption of moisture. It was noted earlier that the 12M H N 0 3 specimens in open air also gained weight at the outset. Because of the small changes on most of the containerwater and container-"03 combinations, the acid and cerium(II1) titration data did not add materially to the interpretation of the weight data. In general, when a signifi-
cant weight loss was observed as in the case of water and 12M H N 0 3 in glass bottles with standard glass stoppers, a corresponding increase in the concentration of cerium(II1) or HNOJ was observed. The results of this study provide some valuable guidelines for the storage of liquid reagents. Evaporation and transpiration should be anticipated and when necessary, provisions should be made to minimize their effects. When small quantities of liquids are involved, the relative loss and corresponding concentration change can be highly significant. Containment in sealed glass ampoules has been found to be the most satisfactory packaging procedure at this laboratory. When this is not feasible, packaging in high density polyethylene or polypropylene or glass bottles fitted with tight sealing caps is usually satisfactory for periods up to a t least 1year. Received for review July 28, 1972. Accepted November 29, 1972.
Low-Cost integrator with Digital Character Stephen R. Pareles' Analytical Services, Aarlab, lnc.. Jersey City, N.J. 07307
The advantages of digital electronic integrators in the analytical laboratory are well established. Their most frequent application is quantifying gas chromatographic effluents. Unfortunately, their cost continues to be prohibitive for most small laboratories. Less versatile, but relatively inexpensive short-term analog integrators using one or more operational amplifiers have been described in detail for several years ( 2 - 4 ) . A typical circuit is shown in Figure 1. A chopper-stabilized operational amplifier integrator capable of accurate and precise long-term integration was described by Harrar and Behrin ( 5 ) . Recently, low-drift chopper-stabilized operational amplifiers have become available in hybrid form for less than $100. Most recent still are FET-input, low-drift "chopperless" operational amplifiers available for less than $50. Manufacturers are Burr-Brown, Analog Devices, Teledyne-Philbrick, Bell and Howell, Analogic, Datel, and others. A long-term integrator of excellent precision and accuracy built from an FET-input operational amplifier was recently described in detail by Kendall(6). Kendall used a Burr-Brown Model 3420K operational amplifier with a 1000 ohm input resistor and a 1 mF (200V polystyrene) capacitor in the feedback loop. The sensitivity of the integrator so built was thus
L"Figure 1. Typical operational amplifier integrator Null provision varies with manufacturer
R E S E T RELAY
Eo(V)/Eln(V-sec)= 1/RC = 1000 Linearity and precision of better than 1% were achieved I
I
lPresent address, D e p a r t m e n t of F o o d Science, Rutgers-The State University, N e w Brunswick, N.J., 08903. (1) C. N. Reiiiey, J. Chem. Educ., 39, (11) A 8 5 3 ; (12) A 9 3 3 (1962). (2) H. V. Maimstadt and C. G. Enke. "Electronics for Scientists," W. A. Benjamin, New York, N. Y., 1963, p 356. (3) J. R. Barnes and H. L. Pardue, Anal. Chem., 38, 156 (1966). (4) "Operational Amplifiers, Design and Applications," Burr-Brown Publications, Tucson, Ariz.. 1972. ( 5 ) J . E. Harrarand E. Behrin,Ana/. Chem., 39,1230 (1967). (6) D.R. Kendall, Anal. Chem., 44, 1109 (1972).
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A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. u, M A Y 1973
ANALOG OUTPUT " % over last reset''
DIGITAL OUTPUT
"wmbcr of resets"
Figure 2. Functional diagram of automatically resetting operational amplifier integrator
