that excessive anodic polarization did not occur during generation, the solution was stirred as fast as possible by manual rotation of the electrode, and particular attention paid to monitoring its potential. As the end point, corresponding to complete oxidation of the available Y b(II), was approached, current densities at the tungsten anode were reduced from an initial value of ca. 2 mA cm-* to ca. 0.2 mA cm-* and the length of time required for potential stabilization after each generation was observed to increase. However, a smooth S-shaped curve (Figure l), consistent with normal potentiometric titration behavior, was obtained for the dependence of potential on total charge passed. The lower portion of this corresponded to the oxidation of Yb(I1) to Yb(II1) and the upper portion to the oxidation of tungsten metal. Good agreement was noted between the position of the point of inflection in this curve and the theoretical end point for the first process. Any indications of the liberation of chlorine, as manifest in a constant potential of the order of $0.3 volt with respect to the platinum reference, were entirely absent. By re-plotting potentials corresponding to points in the upper portion of the curve os. log (q - qinJ where q is the total charge passed, and qinfis the charge passed up to the point of inflection, a Nernst plot for the oxidation potential of tungsten was obtained (Figure 2). By carrying out the
same procedure with an electrode of platinum foil in place of the tungsten electrode, the position of the end point of the Yb(I1) oxidation could be checked, and a Nernst plot for the oxidation potential of platinum obtained (Figure 3). Nernst plots for the anodic oxidation of ytterbium could also be obtained from the first portion of each curve and were in agreement with data obtained by reduction of Yb(II1) (Figure 4). It is of interest that both titration curves have the predicted unsymmetrical shape for processes with different numbers of electrons. Although only a limited number of points appear in Figure 2, the linearity is excellent and the slope corresponds to a twoelectron process at 450 "C. The formal potential for the W(1I)-W(0) couple [-OS85 V U S . 1.0m Pt(I1)-Pt(O)] is also such that Eu(II1) should oxidize tungsten metal. Further work is in progress to establish the mechanism(s) of the processes taking place on tungsten in the solvent. The demonstration of the validity of the reference potential (Figure 3) is encouraging. RECEIVED for review February 3, 1969. Accepted June 27, 1969. This work was supported by the Ministry of Technology. We also thank I.C.I. for a grant to purchase rare earth compounds.
Determination of Microgram Amounts of Nitrogen Using Distillation, Flow Spectrophotometry, and Data Acquisition by Computer J. W. Frazer, G . D. Jones, Robert Lim, and M. C. Waggoner Lawrence Radiation Laboratory, Unicersity of California, Licermore, CaliJ 94550
L. B. Rogers Chemistry Department, Purdue UniGersity, Lafayette, Ind. 47907
A QUANTITATIVE METHOD was needed to determine microgram amounts of ammonia in solutions that contained high concentrations of lithium ion. Scheurer and Smith (1) have described a method in which small amounts of ammonia react with hypochlorous acid and sodium phenate to give a blue color. We found, however, that high concentrations of lithium interfered with a direct determination. Consequently, distillation was selected as the method for prior separation. Decision was made to explore the possibility of improving the sensitivity of this method by measuring the ammonia continuously as the distillate was collected, then integrating the colorimetric signal by means of a computer. If the approach proved feasible, it was believed that the results would give valuable insight for later, more refined use of a digital computer for closed-loop control of experimental variables. Britt ( 2 ) adapted the Scheurer and Smith method for the Technicon AutoAnalyzer for continuous measurement of ammonia in water, and reported greater sensitivity and precision. Unfortunately, part of the reported gain in sensitivity resulted from the use of a 50-mm sample cell which re(1) P. G . Scheurer and F. Smith, ANAL.CHEM., 27, 1616 (1955). (2) R. D. Britt, Jr., ibid., 34, 1728 (1962).
quired a sample of too large a volume for the present application. The use of a slightly modified procedure for the AutoAnalyzer provided an easy way to carry out a test of the possible advantages of making continuous measurements. EXPERIMENTAL
Reagents. SODIUM PHENATE.16.7 g of phenol and 9.4 ml of 5 0 x NaOH were diluted to 300 ml with water. HYPOCHLOROUS ACID. Water was saturated with chlorine gas and diluted 1 :3. The diluted reagent was kept in an ice bath to minimize loss of chlorine by volatility. Both reagents were made fresh daily. Standardizations will have to be made with each new batch of reagents. AMMONIUM CHLORIDE, which had been dried at 110 "C was used to prepare a standard solution that contained 2.00 pg of nitrogen per ml. Deionized (DI) water was used throughout. Equipment. A Technicon AutoAnalyzer was used to mix the distillate with reagents prior to spectrophotometric measurement in a 10-mm absorption cell. The flow rates of the sample, hypochlorous acid, and sodium phenate were 4.50, 0.8, and 0.8 ml/min, respectively. The distillate was mixed with hypochlorous acid in a 30-sec coil, sodium p,he*l?+ .e u'as introduced, and the mixture was passed through an addiI
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Table I. Nitrogen Determination by Batch Method DeterminaNitrogen Recovery, Sample tions found, pg Std dev 75 50 ml of 50% NaOH 9 3.3 1 .o 9 . 4 pg N 3 8.9 0.6 94.7 1 8 . 8 pgN 7 16.1 2.0 85.6
tional 210-sec coil in the AutoAnalyzer system to permit more nearly complete color development. A double-beam sepctrophotometer (operated on a voltage regulator) set at 620 mp was used. The output was conditioned by a logarithmic amplifier circuit, including a Philbrick Model PPLA-P transconductor, which converted photometric transmittance to absorbance. The logarithmic element was temperature-compensated and had an error of less than f1 of actual input over the range from to 10-8 A. After the photometric signal had been converted to absorbance, it was fed in parallel to a recording potentiometer and to a computer interface. The interface included a Vidar model-260 voltage-to-frequency (V/F) converter with a 1-MHz maximum output frequency. In turn, the V/F output was fed to a 12-bit binary counter, which was controlled by a PDP-8/S Computer (Digital Equipment Corp., Maynard, Mass.). The counter has an overflow flag that signals the computer when the maximum capacity of the counter has been exceeded. Since the computer can keep track of the overflow, the total count capacity of the system could be extended as much as desired. Distillation was carried out in an all-glass 250-1111 flask equipped with a thermometer well and an inlet tube, the tips of which went to the bottom of the flask. The sample and the wash water were added to the flask through the inlet. During the heating period, air was passed through that inlet into the solution. To start the distillation, a 500-ml heating mantle was raised around the bottom of the flask and was operated at 140 V by a Variac transformer. That particular combination of heating mantle and operating voltage was found to give a satisfactory distillation rate over the entire concentration range of nitrogen samples. In practice, a distillation flask and its thermometer well lasted for approximately one week before pin holes developed because of attack by the concentrated sodium hydroxide. Distillation Procedure. A 50-1111 volume of 50% sodium hydroxide was transferred to the empty flask and DI water was added to make a total volume of 110 ml. The ammonia was then boiled out of the flask using the procedure described below. A succession of samples or standards were next added, until the build-up of salt necessitated replacing the sodium hydroxide Before adding a sample or standard, a few ml of DI water were added to the inlet tube to displace most of the sodium hydroxide from its submerged tip. After an appropriate volume of the sample (or standard) had been added to the cold solution, DI water was used to rinse the inlet tube and to restore the volume to 110 ml. The heater was moved into place and turned on. Air was fed through the inlet to produce 2 to 3 bubbles per second in the cold solution. The condenser was adjusted so that its outlet was just below the surface of 1 ml of deionized water in the cone-shaped tip of a centrifuge tube used as the receiver. Prior to collection of distillate (about 4 minutes after turning on the heater), deionized water was fed into the AutoAnalyzer system through a T-valve in the sample line. When the first drop of distillate was collected, the T-valve was switched to the sample line, and a stop-watch was started to indicate when to start the computer. The receiver was lowered so 1486
ANALYTICAL CHEMISTRY
that the tip of the condenser was 1-2 cm above the surface of the liquid and against the side of the tube. Distillation was continued until the temperature reached 130 “C,following which the heater was quickly replaced by an ice-water bath to bring the temperature below 40 “C prior to adding another sample or standard. Meanwhile, the T-valve in the sample line was switched to deionized water, so that the collection flask and the samples line could be flushed out in preparation for the next sample. A relatively long time (about 285 seconds) was required for the sample to be pumped from the collection flask, through the AutoAnalyzer, to the spectrophotometric cell. Data acquisition by the computer was started at any predetermined time after distillation had started-as long as there was at least 50 seconds in which to establish the base line before the ammonia peak began to emerge. For our program, the computer was started 100 seconds after the start of sampling from the collection flask. The computer program idled for 100 seconds, took a base line for 50 seconds, integrated for 500 seconds, then took a second base line for 50 seconds. The average of the two base line counts was subtracted from the integrated counts to obtain the net counts of the run. RESULTS AND DISCUSSION Preliminary Studies with Batch Method. At the beginning of the study, an ASTM type steam generator (3) was used for distillation. The color of the distillate was developed according to the method of Scheurer and Smith ( I ) . Using their method, sodium hydroxide blanks and standard recoveries were tabulated. The precision of this method was not sufficient for our purpose (Table I). Studies with Continuous Measurement Method. By carrying out a determination and following the absorbance as a function of time, it was found that most of the ammonia was evolved in the first 30-50 seconds of distillation, Virtually all the ammonia had been pumped out of the collection flask in the first 200 seconds. Nevertheless, even after 500 seconds, the absorbance usually had not quite reached the original base line value. For samples containing 10 pg or less of ammonia, 400 seconds of integration time for the absorbance appeared to be just as satisfactory as 500 seconds. However, above 10 pg, 500 seconds gave significantly larger values than did 400 seconds, but not significantly different than 600 seconds. Therefore, an integration period of 500 seconds was selected. The reproducibility of the measuring system alone was determined by pumping reagents through the system, using deionized water as a sample. Upon subtracting base lines from the integrals of the reagent blanks, a totalxange of 12 counts was obtained. Since one count corresponded roughly to 0.01 pg of nitrogen and the range was assumed to represent four standard deviations, the estimated standard deviation that developed from the system was ~ t 0 . 0 3pg of nitrogen. The uncertainty was attributed to the spectrophotometric measurement. The transmittance of the blank varied f 0 . 5 z over relatively short periods of time; and occasionally varied as much as 1 A similar series of measurements, in which a stable voltage source was substituted for the spectrophotometer, usually produced a net count of zero or, at most, a single count for the integral. The early studies demonstrated the need for prior distillation of the NaOH charge. However, the number of succes-
z.
(3) “Recommended Specifications for Microchemical Apparatus,” ANAL.CHEM., 23, 527 (1951).
sive samples that could be run was limited by the build-up of water in the still. Furthermore, the increase in volume for successive samples appeared to affect the reproducibility and to retard the evolution of ammonia. Therefore, the use of a direct heater and a supplemental “carrier” air stream was adopted. The effect of the concentration of sodium hydroxide in the solution also was studied. When the amount of 50z sodium hydroxide was progressively decreased from 50 to 25 ml, then to 10 ml, the reading for 8 pg of nitrogen decreased from an average of 760, to 752, to 717 counts, respectively. Although 50 ml of the 50z sodium hydroxide did not give significantly more recovery than 25 ml, the former was selected in the hope that it would lead to somewhat more reproducible results through a faster (if not more nearly complete) evolution of ammonia. Another important factor was the carry-over (or loss) when successive samples differed by a factor of 10 or more in nitrogen level. For example, when an 8-pg nitrogen sample, having a count of 844, was followed by DI water, a count of 69 was obtained (compared with a normal value of 36). Less convincing evidence pointed to the probable occurrence of a slightly lower value for a large amount of nitrogen when it followed a blank, or when a sample contained less than a microgram of nitrogen. Another variable, which did have a noticeable effect on the peak height and shape, but less effect on the integral, was the variation of volume of water in the collection flask. As expected, higher and more narrow absorbance peaks resulted when the volume was kept to a minimum. In addition, the absorbance decreased to the base line faster with minimum volume. The response of the system was not linear over a wide range of nitrogen. This was probably because of the fact that color development was not complete when the solution passed through the spectrophotometer flow cell. If the colored solution was allowed to stand in the cell, the absorbance increased with time. This reaction was probably less complete with higher concentrations of ammonia. Results with Continuous Measurement Method. The nonlinearity was shown in a series of runs, the values (not corrected for nitrogen in DI water) for 0.2, 0.4, 1.0, 2.0, and 6.0 pg of nitrogen were 47, 57, 142, 272, and 717 counts, respectively. In another series of nitrogen runs (6, 10, 14, and 20 pg) the respective values were 763, 1099, 1352, and 1852 counts. Typical results at different levels of nitrogen are given in Table 11. Concentrated samples of lithium chloride, which contained 0.5 g of lithium per 10 ml and trace amounts of ammonia, proved to be difficult to handle because a precipitate, presumably lithium hydroxide, appeared toward the end of the distillation, when the concentration of sodium hydroxide was high. Once formed, the precipitate dissolved slowly and, possibly incompletely, upon addition of a second aliquot of sample. Typical results obtained on running two aliquots of a lithium chloride solution in succession using the same charge of sodium hydroxide were 156 counts for the first aliquot and 99 counts for the second aliquot. After the first pair of aliquots was run, the flask was cleaned and fresh sodium
Table 11. Counts Obtained for Different Levels of Nitrogen Nitrogen DeterStd Re1 taken, pg minations Counts dev, counts std dev 8.0 4.0 0.4 DI water
7 6 6 5
750 410 37 31
12.5 12.0 8.5 7
1.7 2.9
9.8 22.6
Table 111. Nitrogen Determination in a OS-gram Sample of Lithium Metal Sample Counts Nitrogen, pg 1 2 3 4
Av Std. dev Re1 std dev,
664 643 642 657
5.53 5.40 5.35 5.47
653 9.7
5.43 0.08 1.5
hydroxide was used. The results for another pair of aliquots were 149 and 79 counts, respectively. Because of the difference between the first and second aliquots of each pair, there was doubt as to whether or not the recovery for the first aliquot of each pair was quantitative. Therefore, a spike of standard ammonium chloride solution was added to the flask before the first portion of a lithium aliquot. It was noted that, even when using a spike as small as 0.2 pg of nitrogen, recovery was complete. To obtain best results for lithium samples, it was necessary to use two charges of sodium hydroxide. After the first charge of sodium hydroxide had been boiled out, a lithium sample was run to get an estimate of its nitrogen content before selecting the bracketing standards. Then, a new charge of sodium hydroxide was boiled out and dupiicate runs were made of the high and low standards. Finally, a second portion of the lithium sample was run to complete the series. Using this approach, a lithium metal sample was analyzed four times. The results are given in Table 111. The procedure described above lowered the amount of nitrogen that.could be determined by a factor of at least 10. The standard deviations of the single measurement method was about 1-2 pg nitrogen, whereas the standard deviation of the described method was about 0.1 pg nitrogen. ACKNOWLEDGMENT We are indebted to K. C. Anderson for making some of the runs and to Purdue University for granting academic leave to L. B. Rogers. RECEIVED for review January 30, 1969. Accepted July 3, 1969. Work partially supported by the U. S . Atomic Energy Commission under Contract AT(11-1)-1222 with Purdue Research Foundation. This study was made at the Lawrence Radiation Laboratory under the auspices of the U. S. Atomic Energy Commission.
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