Determination of Surf ace-Active Substances in Refined Sugar Classijcation by the Polarographic Method IVAN VAVRUCH Research Institute of the Czechosloz.uk Sugar Industry, Prague, Csechoslouakia The polarographic method, owing to its great sensitivit), makes it possible to determine the surface-active substances in refined sugar. These substances represent organic nonsugars in the sample under investigation. It has been ascertained by means of about 850 analyses that the content of surface-active substances, expressed numerically in special units, is a very important constant of the sample and consequently it forms a new and suitable basis for the classification ofrefined sugars.
T
HE analysis of refined sugar has been the subject of a large number of studies and publications which have led to exact and convenient methods. Considerably less attention has been paid to the determination of surface-active substances in refmed sugars, though this subject has proved of great importance, especially in studies of the surface tension of sugar solutions, and in the sugar-consuming industries. Because of the very low content of surface-active substances in refined sugars, only highly specific and sensitive methods may be used for their determination. Heyrovskfs polarogr!phic method ( I ) , used for the first time in sugar analysis by Sandera and Zinimermann ( 3 ) .proved most suitable for this purpose.
Figure 1.
Table I.
Conditions for Polarographic Analysis 0.05 or 6.5 t 0 . 0 1
26.0
0.002N KiSOi 0.08 * 0.001 3.30 t 0 . 0 5 20.0 * 0.3
EXPERKMENTA L
A solution of 26.0 grams of the sugar in 100 ml. of 0.002 N potassium sulfate solution is prepared with distilled water of highest purity, and is analyzed with free access of air, from 0 to -1.0 volt. The potassium sulfate electrolyte forms a very pronounced oxygen maximum, which is readily suppressed by the surface-active substances in the sample, lowered by the increased viscosity of the solution, and therefore varies inversely with the content of surface-active substances in the sugar. By analyzing under uniform conditions, it is possible to determine the content of the surface-active substances through comparison of the heights of the maxima, h. Comparable precision of these determinations is not attainable by any other method (6). The value of h is measured from the highest point of the curve to the upper part of the teeth of the oxygen diffusion current, and expressed in microamperes (see Figure 1). The height of the polarographic maximum, h, often depends very much on the conditions of the analysis. I n order to be able to compare the results directly, it is necessary to carry out the analysis under uniform conditions. The conditions which gave the best results are summarized in Table I. The drop time, t , has to be determined in the solution undei investigation, without the applioation of external e.m.f. The lower concentration (6.5 grams) is used if the maximum in a 26gram solution is lower than about 5 mm. The polarographic method is extraordinarily sensitive. Special attention must therefore be paid to the occasional presence of minute traces of surface-active substances, introduced during the preparation of the solution for analysis and therefore not present in the sample under investigation, which may affect the results. I t is necessary to use only purest analytical reagents for the preparation of the electrolyte, to keep the equipment scrupulously clean, and to work only with mercury of highest purity and with a clean surface. I t is advisable to analyae a special standard sugar of exactly known height of oxygen maximum for the fist curve of a series of samples, to make sure that the results are perfectly reliable. If all the described conditions are maintained with care (the change in the value of h due to variations in the atmospheric
Current-Voltage Curves
Under certain conditions, polarographic current-voltage curves with so-called maxima (Figure 1) are encountered, which are most readily suppressed by the surface-active substances. This occurs to an extent such that it is usually possible to detect them molarity. I t ha6 been found in concentrations as low as that the formation of polarographic maxima and their suppression by the surface-active substances are essentially an adsorption phenomenon ( 1 ) . The original method has been improved and extended by the author, and the polarographically determined content of the surface-active substances in relined sugar-Le., the “polarographic purity”-has been made the basis for its classification. About 850 analyses made with individual samples during 7 years have proved that this polarographic purity is a very important property of refined sugars. I t is an excellent characteristic of these sugars, especially in connection with the ash content of any individual sample.
930
V O L U M E 2 2 , NO. 7, J U L Y 1 9 5 0
931 __._I
pressure may usually be neglected), the variation in h does not exceed *2 mm. For the clasiification of refined sugars by the polarographic method, the content of surface-active substances must be known as exactly as possible. Consequently, it is necessary to have a polarographic zero standard sugar which is practically free of surface-active substances. In the first series of experiments a refined sugar of the highest technical purity was used. I t had a higher polarographic purity than any one of the 850 analyzed samples, but in order to have a still more reliable basis for the numerical classification, a sample of purest sucrose was prepared. This sample did not contain any surface-active substances, within the limits of the sensitivity of the polarographic method, because special care had been taken to ensure their complete removal. This zero standard sugar was prepared by making use of the adsorptive properties of specially purified active carbon (carboraffin), followed by recrystallization from ethyl alcohol and ether (4). I t seemed most appropriate for the numerical expression of the surface-active substances in refined sugars to work out a uniform scale of suppressive effects of some surface-active substances under uniform conditions (Table I). Such a scale may be worked out readily by adding increasing quantities of such a contaminant substance to the solution of the polarographic zero standard sugar. The dependence of h on the concentration of the suppressive agent in the solution can best be demonstrated graphically, and the equations of the curves can be calculated, or often deduced from Langmuir’s sdsorption isotherm (4). By measuring the h of the solution under investigation, and by comparing it with the standard scale, it is possible to determine easily the amount of surface-active substances in the refined or other sugars, in terms of the standard scale. A standard beet sugar molasses has been used as the suppressive agent. The results for the sugar in solution were expressed either as milligrams per cent of molasses (referring to the standard technical refined sugar), or as milligrams per cent of corrected molasses (referring to the polarographic zero standard sugar) ( 5 ) . Because of the somewhat different effects exhibited by different beet molasses, two “model” scales have been proposed. These make it possible to express the results, if desired, in special units which are practically independent of the different suppressive effects of molasses. The results may thus be related easily to a new basis and compared with those obtained with the standard molasses. The first model scale is based on methyl orange (Merckj, the results being expressed in mg. ’30 X 100 of methyl orange on the zero standard sugar in solution (conditions as in Table I). The second model scale is completely independent of any suppressive agent. The amount of surface-active substances in the zero standard sugar is here set equal to 0, and the amount of these substances in a sample completely suppressing the maximum (conditions as in Table I ) is set equal to 100. The relation between h and the amount of the surface-active substances in the solution is amumed to be linear, to simplify the evaluation of the results. The polarographic test (P.T.) of the sugar may then be calculated by means of the expression :
P.T. = ( h , - h ) X 100 ha where h, is the height of the maximum of the polarographic =ro standard sugar ( h , = 14.23 microamperes). The molasses scale is very comprehensive, and is preferred by the author. The model scales should be used only for comparing accidental discrepancies in the results. The numerical values, in terms of the standard corrected molasses scale together with some values in’terms of the model scales for comparison, are shown in Table I1 (conditions as in Table I). In order to simplify the procedure for expressing the results in
Table 11.
Scales for Polarographir i n d j seb
(Using 26 grams of sugar i n 100 ml of 0 002 S K2504) Corrected Rlrthyl Orange, Molasses, % i I g . y& x 100 I’,‘r.
w4.
some of the above-mentioned units, it is advisable to make an exact graph of the values summarized in Table 11. ‘I’o express h in microamperes, it is necessary to know the full sensitivity of the galvanometer, because the height of the niaxiniuni on the polarogram can be directly measured usually only in units of length, and not in microamperes. The folhv ing exariiplr niay clarify the method of calculation: The full serisitivitj of the author’b gaivarioriieter was 2.62 X A sample of refined sugar had under standard conditions (see Table I), using l / 5 0 of the full sensitivity, the height of the oxygen maximum, h, X 50 X 45 = 5.67 equal to 45 mm. This equals 2.52 X microamperes. Comparing this result with Table 11, a-e find the following content of surface-active substances in the sugar under investigation: 100 mg. % ’ corrected molasses, 0.65 mg. % methyl orange, or P.T. equal to 60. Using the above equation, we have: 10-9 ampere per 1 mni. on the polarogram.
PI’ . .
(11.23 - 6 tii) x 100 14.23
fj(j
in good agreement with Table 11. Table 111 presents the averages of about 250 anal) ses of refined sugars from the years 1940 to 1312. “Ash” means coriductometrically determined content of inorganic salts ( 2 ) . The average ratio, f,between the polarographic purity and the ash content-Le., the approximate ratio between the “molasses content” and the inorganic salts-is in normal samples, as expected, between 8 and 12 (average 9.83). The calculation o f f seems to be very suitable for judging the normal course and the unusual interferences during the production of the sample under investigation. The lower the polarographic purity of the sugar-Le., the higher the milligrams per cent of corrected molasses-the worse is the quality of the sample, and vice versa. The differences in the average results for the various classes of sugar in Table I11 are comparatively great. They are in good agreement with the quality of the raw materials used for the production of the various types of sugar, and demonstrate the practical value of the above-described classification. As little as 1.3 grams of sugar may be sufficient for the polarographic analysis. Each analysis, with the development of the polarogram, takes about 15 minutes, and the results are precise to about +4%.
932
ANALYTICAL CHEMISTRY
A characteristic polarogram of four different refined sugars, a t a concentration of 26 grams in 100 ml., is shown in Figure 1 (conditions as in Table I), depicting the following results: Curve h. Pa.
1 2 3 4
7.82 3.43 7.15 0 00
Corrected Molasses, Mg. % 49 153 64 >235
of the helpful interest and assistance in preparing this paper for publication. LITERATURE CITED
Ash, Mg.
%
5 15 6 28
f 9.8 10.2 10.6 >8.4
Quality
I
111
I1 IV
ACKNOWLEDGMENT
The writer wishes to express to F. W. Zerban his appreciation
(1) Heyrovskg, J., "Polarographie," Vienna, Julius Springer, 1941. (2) gandera, K.,and Zimmermann, B., Listy Cukrovar., 46, 343 (1928). (3) Zbid., 47, 377 (1929). (4) Vavruch, I., Collection Czechoslov. Chem. Cmrnun., 14, in press. (5) Vavruch, I., Listy Cukrozlar., 60,119,229 (1942); 65,155 (1949); Centr. Zuckerind., 50, 263 (1942). R E C E I VNovember ~D 19,1949.
Determination of Cerium Photometric Method with Radiometric Correction ARTHUR J. FREEDMAN' AND DAVID N. HUME Massachusetts Institute of Technology, Cambridge, Mass.
A rapid method has been developed for the estimation of small amounts of cerium in mixtures of rare earths and other elements, based upon a separation of part of the cerium in pure form, followed by a spectrophotometric determination of the cerium and a measurement of the efficiency of the recovery process with a radioactive tracer. The procedure
T
H E direct determination of a constituent in complex mixtures is often not possible because other substances which are present interfere in the measurement. At the same time, the quantitative separation of the element to be analyzed may be prohibitively difficult. Long procedures and many repeated steps are sometimes necessary to ensure complete recovery of a small amount of an element from macroquantities of other materials. However, it is frequently possible to separate part of thrb substance in very pure form in only a few steps. If some independent method is available for determining the efficiency of the separation procedure, a short-cut method may be developed which will greatly reduce the time necessary for a complete analysis. Recauqe measurements of radioactivity afford a simple indication of the efficiency of a recovery-process, the availability of radioactive isotopes of many elements offers opportunities for the development of such procedures. This technique has been used by other workers. Hevesy and Hobbie @), for example, applied it in the determination of microgram amounts of lead in rocks, and it is used in modified form in the isotope dilution method of analysis (7). However, except in unusual circumstances, the methods available up until the present have not permitted the use of radioactive tracers in precise analytical work. It is the purpose of this paper to illustrate the potentialities of the general procedure as an analytical tool of high precision when good techniques for measuring the radioactivities are employed. The present paper describes the determination of small amounts of cerium in mixtures containing rare earths and other elements with which cerium is often associated, The activity used is Ce14', which decays according to the following scheme:
Ce,44 275 days 8- = 0.348 mev.
~
prlp, 17.5 minutes B- = 3.07 mev.
Nd144
A known number of counts per minute per milliliter of active cerium are added to the cerium-containing sample, and the cerium 1
Present address, Loa Alamos Scientific Laboratory, Los Alamos, N. Mex.
takes about 4 hours, and under the condi! m s used is accurate to about 1.8%. A method of counting radioactive liquid samples with an end-window Geiger tube to a reproducibility of better than 0.5% is reported. Applicability of the combined photometric-radiometric technique as a general method for determination of trace amounts is discussed.
is separated by precipitation as ceric iodate, ceric hydroxide, and perceric hydroxide, the number and order of precipitations depending upon the other constituents of the solution. The separated cerium is then determined spectrophotometrically by measuring the color of ceric ion in sulfuric acid solution, The completeness of recovery of the cerium is found by measuring the number of counts per minute per milliliter in the final solution. The radioactive isotope was available in sufficiently high specific activity so that the amount needed to give a convenient activity in the solution did not affect the spectrophotometric determination. The over-all accuracy with which an snalyRis of this type can he carried out depends, of course, on the errors involved in the determination of the separated element and in the measurement of the activities. Up until the present, the limiting factor has generally been the activity determinations, because most of the techniques in common use for this purpose are accurate to only 2 to 5%. It waa felt, therefore, that a necessary preliminary t o the analytical work was the development of a precise and reproducible method of counting radioactive samples similar to the ones found in this study. ACTIVITY DETERMINATIONS
Geiger Tubes. As part of the investigation of the factors influencing the determination of the radioactivity of a sample, a study was made of the characteristics, reproducibility, and stability of several commercial Geiger tubes of various designs. In order to be useful in precise analytical work, a tube must count reproducibly to within the limits of error desired for the experiment, and it should be stable during the period of the investigation. Wide variations were found among the tubes tested, and many tubes were completely unsuited for use in the present work. Some of the tubes could not reproduce the activity of a given sample to better than 5 to 10%. Most of these tubes showed a marked voltage hysteresis effect-that is, the counting rate for a given sample depended upon whether the
