ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979 1
A
B
C
u 5 0
10
MINUTES Figure 4. Chromatograms of levodopa and oxidization products: (A, B) levodopa, 1, oxidized at 0.8 V, stopped flow for 5 min; (C) levodopa oxidized with ferricyanide to dopachrome, 2. 0.2 mg/mL levodopa in 0.01 M H,P04; 50.0 NL injected; 1.0 mL/min
a t 280 nm yielded a peak (at a retention time of 4.6 min) that eluted just prior to the levodopa peak (Figure 4A). This peak was not evident a t 485 nm (Figure 4B). A chromatogram of a solution of levodopa oxidized to dopachrome with ferricyanide as above yielded a peak (Figure 4C) a t a different retention time than the electrochemically generated peak of Figure 4A.
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Apparently, adrenochrome is the most stable oxidation product of adrenaline in the time scale of the experiment. However, the o-quinone of levodopa does not appear to readily cyclize to form dopachrome a t the low pH of the eluent. This is consistent with the cyclization rate constants for oxidized catecholamines presented by Hawley et al. ( 2 ) . At low pHs, the rate constant for primary catecholamines is much lower than t h a t for adrenaline and isoproterenol, which are secondary amines. Using the present cell and stopped flow procedure, from 3 to 10% of the analyte undergoes electrochemical transformation, making the cell useful primarily for qualitative identification of oxidation products. Increasing the electrochemical yield to 100% might provide an important advantage; namely, the ability to alter the retention or to completely eliminate peaks of electroactive species. By carefully adjusting the potential of the cell, quantitative electrochemical derivatization of potentially interfering components of a chromatogram might be possible, thereby providing an additional variable for controlling selectivity. Such a coulometric cell is presently under design.
ACKNOWLEDGMENT The assistance of W. J. Blaedel and the use of the University of Wisconsin-Madison Chemistry Department machine shop facilities are highly appreciated.
LITERATURE CITED (1) Kissinger, Peter T. Anal. Chern. 1977, 49, 445A-456A. (2) Hawiey, M . D.; Tatawawadi, S.V.; Piekarski, S.; Adams, R. N. J. Am. Chem. SOC.1987, 89,447-50. (3) Sternson, A. W.; Mccreery, R.; Feinberg, B.; Adam. R. N. J . Electronanal. Chern. Interfacial Necfrochem. 1973, 46, 313-21. (4) Heacock, R. A. Chern. Rev. 1959, 59, 181-237. (5) Blaedel, W. J.; Schieffer, G. W. Anal. Chern. 1974, 4 6 , 1564-67. (6) Molnar, Irnre; Horvath, Csaba J . Chromatogf. 1978. 745, 371-81. (7) Manok, G. L.; Heacock, R. A. Can. J . Chem. 1964, 42, 484-85.
RECEIVED for review December 15, 1978. Accepted March 9, 1979.
Determination of Chromium(II1) in the Presence of Large Amounts of Chromium (VI) R. V. Whiteley, Jr. Sandia Laboratories, Albuquerque, New Mexico 87185
A black chromium plating process which was originally designed for preparing decorative coatings is being evaluated for solar applications because it produces a finish with high solar absorptance and low thermal emittance. I t has been shown that the thermal stability of these electroplate finishes is a function of the chromium(II1) content of the black chromium plating bath ( I ) . These plating baths are typically prepared from chromium trioxide and dilute acetic acid solutions. They are rich in chromium(V1) (ca. 150-200 g L-l) and may contain as much as 25 g L-I of iron(II1). The chromium(II1) is produced by reducing some of the chromium(V1) in situ. T h e previously used procedure for a chromium(II1) determination entails a determination first for chromium(V1) and then for total chromium content. The chromium(II1) content is found by difference. Several problems arise from this approach. Since the chromium(II1) content of these baths is usually less than one tenth the chromium(V1) content, the results are based upon subtracting one large value from another large value; the inherent inaccuracies are significant. Furthermore, this determination of total chromium requires the oxidation of the chromium(II1) to chromium(V1) with 0003-2700/79/0351-1575$01 .OO/O
ammonium persulfate which tends to be a rather slow process. An alternative procedure for chromium(II1) delermination which is more rapid and accurate has been developed. Willard and Young (2)have determined chromium(II1) by a potentiometric titration. The chromium(I11) is quickly oxidized to chromium(V1) with an excess of cerium(1V). The excess of cerium(1V) is determined by titrating the solution with a sodium nitrite solution and observing the equivalence point potentiometrically. This method appears to be quite accurate; however, the nitrite-ceric reaction is very slow. This results in a rather laborious titration because it requires several minutes to obtain a fairly stable potential reading after adding titrant. Also, the potential break a t the equivalence point is not particularly well-defined. This makes a plot of the titration curve necessary. Monnier and Zwahlen ( 3 ) have applied the principles of this procedure to a trace analysis for chromium(III), but they achieved better definition in their titration curves by using a platinum-tungsten electrode system. Still, the tungsten electrode does not respond rapidly, and because it is essentially s n “attackable” reference electrode, frequent cleaning is required. The problems associated with the slow nitrite ceric reaction were circumvented in the ‘E 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
Table I. Results of the Analysis of Simulated Black Chromium Plating Baths Cr(II1) gL-' experiment calculated found 1 2a 3 4 5 6b 7"
3.90 3.90 7.51 15.59 28.11 28.11 17.30
3.93 3.89 7.63 15.65 28.81 28.63 17.28
error, g L-'
av. dev., g L-I
0.03 0.01 0.12 0.06 0.70 0.52 0.02
0.02 0.01 0.01 0.01 0.01 0.02 0.02
A 10-mL buret was used for the sodium nitrite. Sample was diluted t o 0.1 its original concentration.
present work by using a constant current potentiometric titration.
EXPERIMENTAL Two sets of experiments were performed. The first set entails the analysis of simulated black chromium plating baths. A second set of experiments was run to provide a rigorous evaluation of the accuracy of this technique. Earlier work has been used to demonstrate the precision of the Willard-Young procedure ( 3 ) , but the accuracy has not been directly addressed. Also, these experiments provide a measure of the sensitivity that can be achieved with this procedure. A pair of platinum wire electrodes was polarized with a 10-pA current. Because this constant current forces electrolysis to occur at the electrodes, stable potential readings can be achieved within seconds of adding the titrant. The Ce4+/Ce3+couple is reversible under the conditions of this titration, so only a small (less than 100 mV) potential is observed during the first part of the titration. On the other hand, the N03-/N02-couple behaves irreversibly. This causes a large (greater than 500 mV) potential to appear after the equivalence point ( 4 ) . The equivalence point is characterized by a very sharp rise in the potential. A rise of several hundred millivolts was observed on adding only 5 yL at the equivalence point. A 0.1 F cerium(1V) solution was prepared from ceric ammonium sulfate (Fisher Scientific Co.) and was made 1.0 N in sufuric acid. It was standardized against National Bureau of Standards arseneous oxide in a manner similar to that described by Willard and Young (2). The 0.1 F sodium nitrite solution was prepared using freshly boiled, deionized water and reagent grade sodium nitrite (Fisher Scientific Co.). It was stored in a dark bottle and standardized against 10.00 mL of the cerium(1V) solution as described below except that no chromium was present. The simulated black chromium baths were prepared from a 1-L solution containing 365.4 g of chromium trioxide (Mallinckrodt 99.75% pure) and 100 mL of concentrated sulfuric acid. The chromium(II1) was generated by reducing part of the chromium trioxide with sodium oxalate. Each standard was prepared by adding the necessary amount of National Bureau of Standards sodium oxalate to a 25-mL portion of the chromium(V1) solution and warming for nearly an hour. After cooling, each partially reduced chromium(V1) solution was transferred to a 50-mL volumetric flask, and the solution was diluted to the mark with more of the original chromium(V1) solution. In this way, the total
chromium content of each solution was 190 g L-' while the chromium(II1) content varied. For experiment 7, however, only a slight excess of chromium(V1) was added to 6.6893 g of sodium oxalate, and the final dilution to 100 mL was made with deionized water. This produced a simple chromium(II1) solution with only a small amount of chromium(V1). A sample size of 1 to 5 mL was required. The sample was delivered from a volumetric pipet into a 100-mLbeaker containing a magnetic stirring bar and 10 mL of 5 F nitric acid. This was followed by the addition of an accurately known volume of the cerium(1V) solution: 20 or 25 mL was suitable. Then enough water was added to the beaker to bring the solution volume up to 40 mL, and solution was covered and heated to 50-60 "C for at least 5 min. The solution must be titrated while still hot, but the titration can be performed quickly enough that the solution does not cool appreciably before the end point is reached. The platinum wire electrodes were connected to an Orion model 801 digital pH/mV meter which also provided the 10-yA polarizing current. No procedure for cleaning the electrodes was required beyond a simple rinsing with deionized water between titrations. The sodium nitrite solution was delivered from a 5-mL buret with the tip kept below the surface of the stirring solution. The reading of the volume was facilitated by a buret reading lense. The titrant was added rapidly ( 1 mL 9-l) until the potential began to rise noticeably. As the potential exceeded 100 mV, the addition was slowed considerably. When it reached 125 mV, the addition was stopped for a few seconds and then resumed by adding the titrant in 5-10 yL increments. In the second set of experiments, the source of chromium was a 1-L solution containing 25.078 g of National Bureau of Standards potassium dichromate. The chromium(V1) to chromium(II1) ratio was regulated by delivering an appropriate volume of the dichromate solution to a 100-mL beaker and acidifying the aliquot with 2 mL of 1 F sulfuric acid (Ultrex grade from J. T. Baker Chemical Co.). The chromium(II1) was then generated by adding 0.5 to 5.0 mL of a 10.163 g L-' National Bureau of Standards sodium oxalate solution. The partially reduced solutions were warmed to 70 "C for 10 min before 10 mL of 5 F nitric acid and 10.00 mL of the cerium(1V) solution were added. The volume of each solution was brought up to 40 mL with deionized water. After heating to 50-60 "C for 5 min, the titration was performed as described above.
RESULTS AND DISCUSSION Table I contains the results of seven experiments performed on several simulated black chromium plating bath solutions. T h e chromium(II1) content ranged from one fourth t o twice that expected in such plating baths. Acetic acid and ferric nitrate were added to some of the standards to produce samples very much like authentic plating solutions; this had no detectable effect on the results. Experiment 7 was performed to show the general utility of this procedure for simple chromium(II1) determinations. Because each determination was done only in duplicate, the precision is expressed as a n average deviation. T h e results of the second set of experiments are found in Table 11. Experiment 8 is essentially a standardization of the sodium nitrite solution against 1 0 . 0 mL of the cerium(1V) solution, but it is in the presence of 15.00 m L of the di-
Table 11. Results of the Analysis of Potassium Dichromate Solutions with Varying Chromium(V1) to Chromium(II1) Ratios ex peri ment
mg Cr
total
mg Cr(II1)
mg Cr(VI)/ mg Cr(II1)
mg Cr(II1) found
error, crg
8 9 10 11 12 13 14 1
132.97 132.97 132.97 8.865 70.92 132.97 132.97 132.97
0.000 1.315 2.629 5.258 5.258 5.258 7.887 13.145
100.2 49.6 0.69 12.5 24.3 15.9 10.0
-0.016 1.366 2.698 5.299 5.372 5.234 7.939 13.144
16 51 69 41 114 24 52 1
Required 15.00 mL of the cerium(1V) solution.
rel, error, 0 ,y g
YG
55 66 151 161 169 44 73 27
3.9 2.6 0.8 2.2 0.5 0.5 0.001
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
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ertheless, the xror WGS always less than 0 . 8 ~ and was typically less than 0 'm. The anr,lytical method presented is shown to be rapid and accurate. The procedure is very well suited for black chromium :dating bath solutions and can be readily adapted for chromium(II1) determination a t the milligram level even in the presence of high chromium(V1) concentrations.
chromate solution. Because the result is well within ~ C of T the theoretical value, it is apparent that the nitrite-ceric titration is unaffected by chromium(V1) at these levels. This is important in that it indicates that one can determine the sodium nitrite titer without considering the possible chromium(V1) content of the analyte. Also, the accuracy of the technique shows no apparent dependence upon the chromium(V1) to chromium(II1) ratio. For example, the largest and the smallest errors (experiments 12 and 15, respectively) were observed a t virtually the same chromium(V1) to chromium(II1) ratios while the remaining analyses produced fairly constant errors which were near the average error of 46 kg. At these low chromium(II1) levels, precision tended to be somewhat of a problem. When the analyses were performed in triplicate (experiments 10-13), the standard deviation was usually quite large, but the precision was improved markedly by performing the determination with five analyses. Nev-
LITERATURE CITED (1) Soweil, R. R.; Pettit, R . 8.Plating and Surface Finishing 1978, 65, 42. (2) Willard, H. H.; Young, P. Trans. Necfrochem. SOC. 1935 67, 347. (3) Monnier D.; Zwahlen, P. Helv. Chim. Acta 1956, 39, 1859. (4) Lingane, J. J. "Electroanalytical Chemistry" 2nd ed.;Interscience: New
York, 1958. p 155.
RECEIVED for review January 29, 1979. Accepted March 22, 1979. This work was supported by the U.S. Department of Energy (DOE), under contract AT(29-1)-789.
Determination of Serum and Blood Densities Lorna T. Sniegoski' and John R. Moody National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
The apparatus for density determinations with which most people are familiar is the pycnometer, and a good review on its use is given by Bauer (1). Both the accuracy and precision may be very high; however, the sample size requirement is often excessive for use in a clinical laboratory. Reviews of methods of determining the specific gravity or density of physiological fluids have been published by Altman ( 2 ) and by Sunderman and Boerner ( 3 ) . Admittedly, a commercially available digital density meter based upon the mechanical oscillator principle would probably meet most of the requirements for small sample size. Recently, Elder has reviewed the capabilities of this instrument ( 4 ) . However, such instrumentation is expensive and difficult to justify in a laboratory where density measurements are not routinely performed. Thus a need still exists for a simple, inexpensive method for determining serum or other fluid densities. T h e methods employed for the determination of blood, plasma, or serum density have included gravimetric determination by a weighing tube ( 5 ) ,or by a pycnometer (61, a determination by the falling drop method (7), and a determination by the copper sulfate method (8). Recently, the previously mentioned mechanical oscillator has been applied to the measurement of the density of flowing blood in anesthetized animals (9) and has demonstrated a capability that would have been impossible by classical means. With the exception of the mechanical oscillator method, none of the methods for determining fluid density approach the precision or accuracy obtainable with a pycnometer. For a 10-mL pycnometer, the measurement can be made to an accuracy of within *1 part in 10000. However, the amount of sample required precludes the use of a pycnometer in most practical cases. Therefore, a reliable semi-micro method for determining blood serum densities is needed; the following procedure was developed t o meet this need.
EXPERIMENTAL Materials. Macrodeterminations were made using a 10-mL pycnometer (Type A, Ref. 1) equipped with a thermometer. Semi-micro determinations were made using an 0.5-mL glass Lang-Levy micropipet. A cradle was constructed of sheet aluminum (Figure 1)to hold the micropipet on the analytical balance. For convenience, a micrometer type of pipetting aid was used to This article not
subject to U.S. Copyright.
Table I. Densities of Blood and Serum Samples Density, g/mL
____--_
description of sample
by pycnometer, 23 " C
micropipet, 23 " C
blood serum,
1.0242
1.0235
whole blood, 1.0549 CD C whole blood, 1.0485 1287 whole blood, 1.0505 Pa-Pba porcine blood 1.0584 Sample contained small clots.
1.0543
by
WHO
a
1.0479 1.0502 1.0583
fill the micropipet. All measurements were made to *0.01 mg using a semi-micro analytical balance. Methods. For the macro determination, the clean, dry pycnometer is used in the usual manner. For blood or serum, it is necessary to rinse off the outside of the pycnometer to get reproducible results. For the semi-micro determination, the pipet and its holder are weighed in a similar fashion. The effects of static charge on the pipet are minimized by wiping the pipet with a damp (not wet) lint-free cloth prior to weighing. A stable weight can be obtained usually within 2 min. For calibration, the micropipet is filled with distilled water and weighed. After rinsing out the micropipet, a vacuum line may be used to draw a stream of air through the micropipet in order to dry it between measurements. The room temperature is noted. After calibrating with distilled water, the micropipet is filled with blood or serum and weighed. Care must be taken to avoid aspirating bubbles into the micropipet. The pipet is wiped carefully and weighed. If the sample is first allowed to equilibrate to room temperature for 1 h, and the handling of the micropipet is kept to a minimum, then the sample may be assumed to be at room temperature. Calculations are made in the same manner for both methods. A density of 0.99756 g/mL is used for water at 23 "C. The room temperature was constant over a range of f l "C during these measurements. Minor corrections for temperature are made by the volumetric expansion formula where Vz3is the volume at 23 "C, Vobsdis the observed volume,
Published 1979 by the
American Chemical Society
