V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 Rinse the cell and electrode several timefi with the solution to be analyzed. After purging for 5 minutes with nitrogen, electrolyze the solution over the potential range of 0 to -1.5 volts us. the saturated calomel electrode, maintaining the nitrogen atmosphere above the solution. If the electrocapillary curve for the base solution is not known, note t, the drop time, a t potentials of -0.2, -0.5, and -1.5 us. S.C.E. Run a Pimilar curve on the base solution and, if necessary, correct the sample curve for the latter curve. Using the intercept method, determine the diffusion current for each of the three waves. Calculate the amount of each ketone present from its diffusion current constant, C = id/Im2W6, where C is the ketone concentration in millimoles per liter, i d the measured diffusion current in microamperes, Z the diffusion current constant for the ketone (Table 11),m the mass of mercury in milligrams per second flowing from the capillary, and t the drop time in seconds measured at -0.2, -0.5, or - 1.5 volts. The weights or percentages of the three ketones present may then be calculated from the millimolar concentrations by conversion to grams and correcting for the dilutions involved. .4CKNON LEDG\IE\-T
The authors n ish to thank the -1tomic Energy Commission which supported the xvork described. LITER4TURE CITED
(1) Buchman, E. R., and Sargent. H , J . Am. Chnn. Soc., 67, 401 (1945). (2) de Clermont, P., and Chautard. P., Bull. soc. chim Paris, 43, 614 (1885).
1913 I.. editor), rol. 1, p. 264, Oxford Univ. Press, New York. 1946. (4) Elving, P. J., Record Chem. P ~ O Q (Kresge-Hooker T. Sci. Lab.), 14,99 (1953). ( 5 ) Elring, P. J., Rosenthal, I., a n d Kramcr, 11.K., J . Am. Chem. Soc., 73, 1717 (1951). (6) Elring, P. J., and Rutner. E., ISD. ENG.C H E M . . - 1 s . 4 ~ .E n . . 18, 176 (1946). (7) Komyathy, J. C., llalloy, F., and Elving. P. .J.. . ~ L L C . HFx, 24, 431 (1952). ( 8 ) Levene, P. A., Org. Synchesea, 10, 12 (1930). (9) Linnemann, E., Ann. Chim., Justus Liebigs. 134, 170 (1863). (10) Aleites, L., "Polarographic Techniques," pp. 137-9. Interscience, Xew York. 1955. (11) Orlemann, E., and Kolthoff, I. AI., J . Am. C k e m . 9 o c . . 64, 1070 (1942). (12) Pasternak, R., and Halban, €I. yon, Hela. Chirn. A d a . 29, 190 (1946). (13) Scholl, R., and Matthaiopoulos, G., Ber. deut. chem. Ges.. 29,1557 (1896). (14) Saper, J., Bu2l. soc. chim., 51, 653 (1932). (15) Van .Itts, R. E..Zook, H. D., and Elving, P. J . . J . A i m , C h c m . SOC.,76, 1185 (1954). (16) Winkel, A., and Proske, G.. Ber. deut. chem. Ges , 69, 693 (1935). ( l i ) Ibid., p. 1917. (:?) "Ilirtioiiary of Organic Compounds," (Heilhron,
RECEIVED for review April 23, 1955. .Iccepted Septemher 18. 1935. .Ihstracted from a thesis presented t o T h e Pennsylvania State College in .lugtist 1952 h y Robert E. Van A t t a i n partial fulfillment of requirements for Ph.D. degree. Detailed tables of d a t a covering t h e polarographic hehavior of the three haloacetones are available from the senior author.
Polarographic Nitrate Determination RANDALL E. HAMM and C. DEAN WITHROW Department o f Chemistry, University o f Utah, Salt Lake City, Utah
A new- polarographic wave has been found for the induced reduction of nitrate ion when present in a chromium(II1)-glycine complex solution. The current measured from this waw is proportional to the nitrate ion concentration. Procedures have been developed for determining nitrate in the concentration range 1.0 X 10-6 to 2.5 X mole per liter, with as good or better precision than the older polarographic methods for nitrate. The effects of gelatin, pH, and interfering ions on this reduction wave have been studied. The nitrite ion gives a similar reduction ware. As a result of the study of nitrate and nitrite reduction a rough calculation has been made of the number of electrons involved in the reduction processes.
A
S C M B E R of polarographic methods (1, 4-73 for the determination of nitrate by means of reduction induced in the presence of polyvalent cations have been described. The original observation of Tokuoka ( 8 ) and Tokuoka and Ruzicka (9) showed that the reduction potential of nitrate TTas shifted to more positive potentials when certain polyvalent cations tvere present. Lanthanum (9),uranyl ( 5 ) : molybdate ( 1 , 4),zirconyl ( 6 ) , and cerium ( 7 , 9 )ions h a r e been studied. Collat and Lingane ( 2 ) have recently niade a more thorough study of the reduction products in the case of lanthanum, cerium, and uranyl induced reductions of nitrate and have reported that nitrate can be reduced directly from acid solutions. I n the coime of a study of the complex ions of chromium(II1) with glycine it n-as observed (3) t h a t a chromium(II1)-glycine complex, prepared by aging a 1 to 3 mixture, gave an induced nitrate reduction 1%-arewhich had some unique properties that should make it especially valuable for the determination of ni-
trate. This paper is a description of the polarographic nictliotl which has been developed as the result of this new wave. Further work is in progress in this laboratory in an attempt to esta!)lish the nature of the processes that are responFilIle for the unusual shape of the wave; ho\i-ever, the present n.ork has estahlishetl a reproducible and highly sensitive method for the tletPrniination of nitrate or nitrite. EXPERIMENTAL
Equipment. h Sargent, Model XXI, polnrograph n-as used throughout this investigation. A Beckman, Xlodel G. pH meter, calibrated with 0.05.V potassium acid phthalate solution, was used for making all pH measurements. Analyses were performed in an H-type cell which had a saturated calomel reference electrode in one side, the experimental solution in the other side, and a saturated potassium chloride-3% agar bridge in the cross member. This cell was suspended in constant temperature bath which Tvas maintained at 25.0" =k 0.1 C. for making all measurements. Dissolved osygen Tvas removed from the esperimentsl solut,ion hy bubbling purified kiydrogen through the cell for about 5 minutes before the recording of a polarogram. An external potentiometer circuit v a s used for determination of the exact potential applied to the cell, wherc measurements of potential %-eretaken. The dropping mercury electrode wed was n piece of marine barometer tubing with the characteristics, 1112 3!iii; = 1.G3 nig.2'3 sec. -l'z. Reagents. All of the reugents used except the following were reagent grade chemicals. Chromium perchlorate, hydrated Cr(C10,)7, 0.Frederick Smith Chemical Co. Glycine, Eastman Kodak (white label). Gelatin, Baker and Adamson. A stock solution which was 0.10036 in potassium nitrate xms prepared from a dried and weighed sample of analytical reagent grade potassium nitrate. More dilute solutions Tvere prepared from this bv normal volumetric t,echniques. A stock solution of sodium nitrite which was approximately 0.10M in nitrite was
ANALYTICAL CHEMISTRY
1914 prepared from reagent grade sodium nitrate (assay 98.8% NaN02). This solution was analyzed for nitrite by oxidizing with excess standard permanganate in acid solution and back-titration of the excess permanganate with standard ferrous solution. More dilute nitrite solutions were prepared from this solution by normal volumetric techniques. Preparation of Complex. The chromium(II1)-glycine complex used as a reagent in the nitrate determination was prepared by mixing 50 ml. of 0 . l M perchloric acid solution, 50 ml. of 0.144 chromium(II1) perchlorate solution, and 50 ml. of 0.3M glycine solution in a 500-ml. Erlenmeyer flask. The flask was placed in a boiling water bath and heated for 2 hours. During the heating period the violet hexaaquochromium solution gradually became wine-red in color. After cooling, the wine-red solution was neutralized with 0.5M sodium hydroxide solution to a pH of about 6 . The resulting solution was diluted to 500 ml. This solution as normally prepared was clear, but occasionally a small amount of chromic hydroxide precipitate would form. If a precipitate formed it was ignored, since it was found t o have no effect on the results obtained. A sample of complex prepared in this manner was found to be still completely effective after a period of 6 months.
Figure 1. Polamgrams with each curve starting at -1.2 volts A. B. C. D.
0.5 XjlO-'Mpomplex, no nitrate Solution A with 4 X 10-4 molea per liter of nitrate 2.5 X lO-aM complex no nitrate Solution C with 1.6 2 10-8 moles per liter of nitrate
Procedure. Nitrate or nitrite was determined by adding an appropriate quantity ( 5 to 25 ml.) of the complex, prepared as above, 50 ml. of saturated potassium perchlorate solution, 0.4 ml. of 1% gelatin solution, and a pipetted sample of the nitrate or nitrite solution t o a 100-ml. volumetric flask. The volume was adjusted t o the mark, the solution was mixed, and the H-cell waa rinsed thoroughly with this solution before filling. After placing the cell in the constant temperature bath and removing dissolved oxygen by bubbling hydrogen through the solution for about 5 minutes, a polarogram was recorded by scanning over the potential range -1.2 to -2.0 volts us. saturated calomel electrode. The solution should have a H between 3.4 and 7.5 for best results. The nitrate wave heigEt was taken as the vertical distance from the peak of the maximum t o the blank curve, or t o the level where the current dropped. The latter was easy to determine even when no blank was run. For the reagents used in this investigation, it was found necessary to run and subtract a blank determination for low nitrate concentrations, because small amounts of nitrate were present in the reagents.
Table I. (In 1.0
x
Variation of Nitrate Diffusion Current with pH 10-aM KNOt, 2 . 5 X 10-aM complex, 5 X l O - * M KClOd, and 0.004% gelatin) PH Microamperes 3.30 3.38 3.60 3.88 4.25 4.67 5.50 6.70 7.11
1.46 ,.9s 8.11
21.6 29.5 29.8 29.6 29.4 29.2 29.3 29.4 29.0 30.4 31.4 32.5
RESULTS AND DISCUSSION
The polarographic waves that resulted from solutions containing the chromium(II1)-glycine complex described are shown in Figure 1. There are two sets of curves shown; A and C mere from solutions which contained no nitrate, whereas B and D had nitrate added, 4.0 X 10-6 and 1.6 X 10-3 mole per liter, respectively. The complex concentration was 0.5 X molar for A and B and 2.5 X 10-3molar for C and D. The unique part of these waves is the n a y in which the current drops suddenly from the high value to a reduction current which is identical to the current given by a solution containing no nitrate. With the nitrate present the reduction wave obtained appears to be a maximum in the reduction wave from the chromium( 111)glycine complex alone; however, it differs from maxima in general in that its height is proportional to the nitrate ion concentration. A tentative explanation for this reduction wave may be proposed by assuming that the complex is reduced a t the dropping mercury electrode to some intermediate unstable state, and that in some way an electron is transferred to the nitrate. Once the nitrate has accepted this electron, further reduction proceeds rapidly by a series of steps until the final reduction product or products are formed. At some more negative potential this process becomes competitive with the reduction of the complex to a stable state and the nitrate can no longer get the first electron t o start its reduction. Kolthoff, Harris, and Matsuyama (6) found that the potential a t which a sudden current increase took place for nitrate reduction induced by lanthanum was a function of the direction of scanning the polarograms. In the present case, on the contrary, it was found that the potential a t which the sudden current decrease occurred was essentially independent of the direction of scan. With a rate of voltage scan as low as possible, the difference in the potential at the sudden current change, for forward and backward scan, was found to be only 0.004 volt. This difference was more likely to be a lag in the recorder than a difference in the system. In all other ways the curves obtained appeared to be independent of the direction of voltage scan. Effect of Gelatin. The gelatin was found to have a suppressing effect on both the blank and the maximum; however, without a small amount of added gelatin the maximum was difficult to measure because of irregularities. A small amount (0.0040/,) of gelatin was found to smooth out the nave and still cause very little supressing action. As much as 0.01% gelatin reduced the height of the complex wave (blank) from 5.4 to 4.4 pa. and reduced the height of the nitrate n-ave from 29.8 to 22.0 pa. For all of the determinations reported 0.004% gelatin was used. Effect of pH. In order to study the necessity of carefully controlling the p H in making nitrate determinations, a series of samples was analyzed polarographically over a range of p H while all other variables were held constant. These runs are reported in Table I. These results shoF that for any pH in the range 3.4 to 7.5, within experimental error, essentially constant results can be
V O L U M E 2 7 , N O , 12, D E C E M B E R 1 9 5 5 obtained for a given amount of nitrate, and careful buffering of the solutions for a nitrate determination is not necessary. Nitrate Determination. The results of measuring the height of the maximum polarographic current at the point of change as a function of nitrate ion concentration are shown in Table I1 for two different values of the complex concentration. A t complex concentrations in the range betvieen these values similar results were obtained.
Table 11.
Nitrate Diffusion Current
(In 0.05.11 KC104 and 0.004% Gelatin Solution) complex, Sitrate. Diffusion k = id/C, Rlmoles/Liter Mmoles/Liter Current, @a. ua./Mmoles/Liter 2.5 0.100 3.04 30.4 0.200 5.92 29.5 0.400 11.9 29.8 0,800 24.2 30.2 1.40 42.7 29.5 1.80 52 4 29.5 2 40 67.9 28.3 3.00 82.1 27.4 0.020 0.61 30.6 0.5 0 040 1.25 31.4 0,060 1 81 30.1 0 080 2 33 29.4 0 160 4 83 30.2 0.?60 7 72 29.7 0 R6O 10.8 30.0 0.360 13.6 29.5 0 561 16 1 29.3 0.600 18 i 27 4
Kolthoff, Harris, and Matsujania ( 6 ) observed that it was necessary to have a certain minimum uranyl ion concentration in order to get a diffusion current proportional to the nitrate ion concentration. The results in Table I1 show when the nitrate to complex concentration ratio became greater than about one, the proportionality of wave height to concentration did not hold. At the other end of the scale it was found that when the nitrate to complex concentration ratio became less than 1/25 it became difficult to measure the curves with accuracy. For this reason the amount of complex used was decreased for l o x values of nitrate ion concentration. Nitrite Determination. The nitrite ion gave similar polarographic waves a hen reduced in the presence of the chromium(111)-glycine complex Table I11 lists the data obtained using this method for nitrite ion.
1915 Interfering Ions. Previous work has indicated that the sulfate ion interferes with the lanthanum induced reduction of nitrate, and that 20 times or more excess of sulfate oven nitrate interferes with the uranyl induced reduction of nitrate ( 5 ) . The data given in Table I V show the list of substances and concentrations which have been checked for interference in the course of this investigation. Chloride, sulfate, oxalate, and acetate may be present in concentrations many times that of the nitrate being determined, however, there was evidence that the highest chloride and sulfate concentrations investigated gave slightly low results because the current did not drop completely to the blank. This effect may have been caused by the high ionic strength of the solution.
Table 11-.
Interferences in Solutions Containing 1.00 X M Nitrate
Salt Added KC1
KPSOI
IGHPOI
Mmoles/Liter 5.05 29.9 (8.0 159 19.3 28.0 55.2 107 0.20 1.54 6.92
11.0 20.0 0.50 1.62
Nitrate Found, Mmoles/Liter 1.01 1.02
1,oo
0.97
1.00 1.01 1.00 0 96
(0.97) ( 0 79)
None 1.02 0.99 (0.95) ( 0 61)
Phosphate and borate which reacted with the chromium-glycine complex to form more stable chromium complexes prevent thip method from being effective. The values found for phosphate and borate are given in parentheses because the actual value. found would depend upon the time between mixing and actually making the measurements, as the reactions between these ions and the chromium-glycine complex is a slow one. The presence of metals which are reduced a t the dropping mercury electrode a t potentials more positive than the potential a t which nitrate reduction takes place would interfere if they were present in large amounts. Small amounts of these metals would only increase the currents by a small amount and would not seriously interfere.
Table 111. Nitrite Diffusion Current ( I n 0.05M KC104, 2.5 X I O - a M complex, and 0.04% gelatin) h-itrite, Diffusion k = id/C sa./Mmoles/Liter Mmoles/Liter Current, +a. 0.101 2.10 20.8 4.20 20.8 0.202 20.3 8.20 0.404 21.5 17.4 0.808 20 2 32.7 1.62
It may be assumed that the end products should be identical for the nitrate and the nitrite reductions. Since a single capillary was used under identical conditions, the ratio of the diffusion current constants should give the ratio of the number of electrons involved in the two cases. This ratio for nitrite to nitrate was 0.698. The ratio would be 0.75 if 6 and 8 electrons were involved, or 0.667 if 4 and 6 electrons were involved. This rough method of calculation indicates that in this induced nitrate reduction there is general agreement with the results of Collat and Lingane ( 8 ) for the uranyl and lanthanum induced reductions, where the actual end product was a mixture of substances, but in part was reduction to ammonia.
ACKNOWLEDGMENT
This investigation was supported in part by a research grant from the National Science Foundation. LITERATURE CITED
(1) Chow, D., and Robinson, R., ANAL.CHEX,25, 1493 (1953). (2) Collat, J., and Lingane, J. J., J . Am. Chem. SOC.,76, 4214 (1954). (3) Davis, R. E., Ph.D. thesis, University of Utah, 1954. (4) Johnson, M., and Robinson, R., ANAL.CHEM., 24,366 (1952). (5) Kolthoff, I. M., Harris, W. E., and hlatsuyama, G., J. A m . Chem. SOC.,66, 1782 (1944). (6) Rand, RI., and Heukelekian, H., ANAL.CHEM., 25, 878 (1953). (7) Scott, E., and Bambach, K., IND.ENG.CHEM.,A 4 ~E D ~ , 14, ~ . 136 (1942). (8) Tokuoka, M., Collection Czechoslov. Chem. Communs., 4, 444 (1932); Mem. Fac. Sci. Agr., Taihoku Imp. Univ., 9, No. 6 , 197 (1936). (9) Tokuoka, hi., and Ruzicka, J., Collection Czechoslov. Chem. Commum., 6, 339 (1934) RECEIVED Eo1 review July 11, 1955. Arcepted September 12, 1955.
