from analysis of three different licorice samples by both methods. This wide divergence in results indicates that in the acid precipitation procedure extraneous material is being precipitated as glycyrrhizic acid. This was borne out in th,st the precipitate was dark brown in color, had poor water solubility, and analysis of the precipitate by the spectrophotometric method indicated a maximum glycyrrhizic acid content of 49-50%. I n addition, glycyrrhizic acid was found in varying amounts in the starches and gums precipitate, as well as in the mother liquor after acid precipitation. The fallacy of using the acid precipitation method is further demonstrated by comparing results on samples 2 and 3
in Table 11. Results by acid precipitation are not significantly different; however, there is considerable difference uhen using the spectrophotometric method. The conclusions may be drawn that the acid precipitation method is empirical and yields high results, which are not really indicative of the glycyrrhizic acid content. The precipitation method is reproducible in replicate analyses of the same sample, but the results between different licorice extracts would be subject to question] and could lead to faulty evaluation of the licorice. ACKNOWLEDGMENT
The author expresses appreciation to N. W. VanHoy for technical assistance,
and to A. J. Sensabaugh, Jr., for preparation of the figure. LITERATURE CITED
(1) Brieskorn, C. H., Mahran, G. H., Arch. Pharm. 293, 1075 (1960). (2) Houseman, P. A,, J . Assoc. Ofic. Agr. Chemists 6 , 191 (1922). ( 3 ) Nieman, C., “Advances in Food Research]” E. M. Mark and G. F. Stewart, eds., Vol. 7, pp. 339-81, Academic Press, New York, 1957. ( 4 ) Onrust, H., Jansen, A. P., Wostmann, B. S. J., Rec. Trav. Chim. 74, 1515 (1955). (5) Wiest, F., “Diss.” Buchdruckerei W. Zurchers Erbens, Zug (1949).
ROBERT H. CUNDIFF R. J. Reynolds Tobacco Co. Winston-Salem, N . C.
Volta mmetric Determination of Permanganate at the SIR: Determination of manganese by the voltammetric reduction of permanganate offers several advantages. The reduction can be performed in acidic solution with a five-electron change and therefore a relatively large current and sensitivity. The high positive potential of permanganate reduction prevents the reduction of most other ions. The ineasurements are independent of c o b r of sample or interferences. The voltammetric reduction of permanganate at the mercury electrode has been investigated (6). The positive potential range of the diffusion plateau is severely limited because of oxidation of the mercury. Reduction of permanganate at the rotating platinum electrode in 1N HzS04yields apparently normal current-voltage curves (3, 6). The platinum electrode surface is oxidized a t these potentials. Analytical application is limited because of currents produced by reduction of dissolved oxygen and by reduction of excess periodate used as oxidizing agent. The use of a rotating gold electrode offers several advantages. Gold surfaces are oxidized at potentials more positive than platinum (2); the overvoltage for the reduction of dissolved oxygen and for the reduction of periodate is greater a t 1% gold than at a platinum electrode. These considerations suggest the possibility for a sensitive, selective, and convenient manganese determin,ation method. EXPERIMENTAL
Apparatus and Reagents. A conventional rotating electrode assembly
was employed. T h e polarographic cell consisted of a 150-ml. beaker. A mercurous sulfate external reference electrode was used t o prevent a n y possible reduction of permanganate solution b y chloride ions. T h e salt bridge contained 1N perchloric acid. A recording Patwin Model G-1 Electropolarizer was used to make all measurements. The synchronous rotator oprated a t 600 r.p.m. Several electrode designs were used. The electrode giving best response was constructed as shown in Figure 1. This electrode design eliminates possibility of electrolyte leakage to contact leads. All reagents used were reagent grade. Procedure. T h e sample was prepared in perchloric or nitric acid such t h a t t h e concentration of acid at t h e time of measurement was between 1 a n d 3 N . For organic samples, a wet ashing with a perchloric-nitric acid mixture was used. T h e solution containing manganous ions was heated with the addition of potassium periodate and boiled for 5 t o 10 minutes t o ensure complete oxidation of manganous t o permanganate. Excess periodate was added t o prevent t h e reduction of t h e permanganate by water which otherwise takes place at moderate rates for such dilute solutions. T h e solution was diluted to volume and the concentrations determined. Measurements were made by adding 20.0 ml. of 1 to 3N perchloric or nitric acid inert electrolyte solution (previously boiled with periodate to remove reducing impurities) to the voltammetric cell. The rotating gold electrode was pretreated by oxidizing a t +2.5 volts us. S.C.E. for 15 seconds followed by reduction of 0.0 volt for 5 seconds. The potential was then shifted
Gold Electrode
directly to +0.80 volt and the residual current was permitted to decay. After about one minute the residual current had reached approximately 0.1 pa. and was changing only slowly. The sample and the standard addition were added in various volumes of 1 to 10 ml. Measurements were taken as indicated in Figure 2. The results were computed in the usual manner. The solution to be used for the standard addition is prepared by reducing a standard permanganate stock solution with sodium sulfite, boiling to remove excess sulfite, and oxidizing to permanganate by the same procedure used for the samples. RESULTS A N D DISCUSSION
Current-voltage curves of permanganate a t the rotating gold electrode were obtained. I t was ascertained that a t 0.80 volt us. S.C.E a current is obtained which is proportional to concentration. At more positive potentials the rising portion of the polarogram was encountered. Use of more negative potentials was not attempted because of possible loss of specificity and because of changing nature of the electrode surface. Current us. time measurements a t this potential indicate that the electrode surface is not stable, but is oxidized by the permanganate. This agrees with the findings of other investigators (8, 4 ) . i2ccordingly, the nature of the electrode surface, and therefore the measured current, changes slowly with time. This necessitates the use of a standard addition technique when making analytical measurements. The type of plot obtained is shown in Figure 2. VOL. 36, NO. 9, AUGUST 1964
1873
The initial residual current is obtained in solution containing only electrolyte. The electrode was pretreated at highly oxidizing potentials and the residual current was permitted to decay to approximately 0.1 pa. before sample was added. Interestingly, if the residual current due to the reduction of oxidized gold surface was permitted to decay to values less than 0.1 pa., the permanganate currents obtained were several-fold smaller. This indicates that the oxidized gold electrode surface provided either very much more surface area than a gold metal surface, or that the reduction of permanganate is reduced more reversibly a t a gold oxide than a t gold metal surface. -4nson (1) found similar behavior for the reduction of iodate and periodate at the platinum electrode. The method provides high sensitivity because of convection in the mas5 transfer process and a five-electron change in the electrode reaction. The selectivity of the method is intrin.ic:illy favorable because of the highly poiitive potential a t which measurementq are made. Treatment of the solution before measurement was relatively simple and convenient. Ordinarily, filtrations were not necessary and the procedure was carried out in one vessel except for the final dilution to volume. Deaeration of the solution was not necessary because dissolved oxygen ia not reduced a t the relatively positive potential employed. Although all of the work reported was done with a recording polarograph, it would seem that measurements with a manual instrument could be made with reasonable convenience. Solutions of known concentrations of permanganate were prepared. The samples from these were added in various volumes. The results are tabulated on Table I. Measurements
1.0
/-
v)
w a w
a
50 a V
f
-
0.5
steel
i2
-
0
t
L
"
'
l
'
l
'
l
50 TIME(seconds)
Figure 2.
l
l
f
t
l
100
Standard addition analysis plot
il, current due to sample i p , current due to sample plus standard addition
wax
Figure 1 . Rotating gold electrode construction
of less than 0.1 pg. of manganese are accomplished with reasonable accuracy and precision. Positive error in the case of the most dilute solutions may be due to instability of the permanganate after being added to the cell solution.
Substances which were found not to interfere are: Ca, Ba, Mg, Al, Cd, Zn, Mo, W, V, S i , Co, Cu, and Fe. Chromium would be expected to interfere since chromate yields a small reduction current a t 0.8 volt v s . S.C.E. Chromium interference can be circumvented by saturating the perchloric or nitric acid inert electrolyte solution with barium nitrate. The chromium, when oxidized, precipitates as barium chromate and is not reduced a t the electrode. Barium permanganate is quite soluble. Mercury and silver interfere. ACKNOWLEDGMENT
Joanne M. Gilbert and Heinz E. Stapelfeldt collected much of the data. Table I.
LITERATURE CITED
Data for Manganese Analysis
(1) Anson, F. C., J . Am. Chem. SOC.81,
Twenty milliliters of electrolyte, 0.400 pg. std. (in 1 ml.) added Standard solns. Added, pg. 4.00 (in 10 ml.) 0,400 (10 ml.) 0.200 ( 5 ml.) 0.080 ( 2 ml.) 0.400 (1 ml.). S.B.S. KO. 86bb Oxidized soln. 1 m1.e
Oxidized soln. 1 ml.d a
1554, (1959).
(2) Baumann, F., Shain, I., ANAL.CHEW
Av. found, pg. 4.02 0,402 0.200
XO.
detns. 4 10
Std. dev., pg. 0.007 0.007 0.007 0.002
0.082
0,418
5 6 2
0.41
8
0.02
0.41
6
0.01
yc Mn
0 014
0.014
In presence of equal concentration of chromium. Alloys: 0.013% Mn, 88.67, Al, 7.877, Cu, 1.53c7, Fe, 1.50% Zn.
* Aluminum
0.46-pg. std. added. d 4.62 p g . std. added.
c
1874
ANALYTICAL CHEMISTRY
29, 303, (1957). (3) Huber, C., Shain, I., unpublished results, University of Wisconsin, 1956. (4) Lee, J. K., Adams, R . K.,Rricker, C. E., Anal. Chim. i l c @ 17, 321, (1957). (5) Sernst, W.,hlerrlam, E. S., 2. Physik. Chew. 5 2 , 345, (1905). ( 6 ) Songina, 0. A., Rozhdestvenskaya, Z . , Zh. Anal. Khim. 11, 717, (1956). CALVIN 0. HUBER Rockford College Rockford, Ill. and Vniversity of Wisconsin-Milwaukee Mdv;3ibikPe, 1 Present address. THISW O R K supported in part by a Research Corp. grant.
