Polarographic Estimation of Starch and Its Application in Flotation T.
SHIOU C. SUN, DANIEL L. LOVE, and RICHARD
HOLZMANN
College o f Mineral Industries, The Pennsylvania State University, University Park, Pa.
b Polarography has been developed for the determination of the degradation products of starch in a supporting electrolyte of constant boiling hydrochloric acid. Few ions of the tested flotation liquors produce waves that interfere with the starch determination; those that do are easily removed or are present in amounts too small to cause any serious difficulty. The present method, with a working range of approximately 5 to 450 mg. of starch per liter, has been applied to three starches and is believed applicable to some other starches and carbohydrate materials. The time necessary to complete one analysis is less than 1 hour, and on a routine basis averages under 15 minutes.
S
and their derivatives have long been used in the flotation (5, 8, 21) and sedimentation (18, 22) of minerals. The starch contents of flotation liquors, however, are difficult to determine with the existing chemical methods (2, 3, 13, 20), because the liquors are frequently contaminated with organic and inorganic species. Under such a condition, ultraviolet spectrograms (24) become difficult to interpret, as so many materials absorb here. Colorimetric methods (9, 19, 85) suffer similarly, and polarimetric techniques (1, 6) are tedious. The purpose of the present investigation was confined to the development of a polarographic procedure for the rapid determination of starches in water and in flotation liquors. Love (14) observed that, under certain restrictive conditions, carbohydrates can be hydrolyzed with a strong acid to produce, among other hydrolysis products, the electroactive 5-hydroxymethylfurfural [5-(hydroxymethyl)-2furaldehyde], and thus to result in a characteristic polarographic wave. The yield of 5-hydroxymethylfurfural, in general, is a poor criterion of the degree of conversion of starches, because the aldehyde is subject to degradative cleavage as well as condensation under the conditions of formation. Severtheless, careful technique can overcome this difficulty. A great deal is knou-n about the polarography of j-hydroxymethylfurTBRCHES
1074
ANALYTICAL CHEMISTRY
fural ( I d , 14, 16, $3). Cantor and Peniston ( 4 ) reported that it has a half-wave potential of - 1.31 to -1.37 volts cs. S.C.E., a t the dropping mercury electrode, in a buffer of potassium hydroxide-potassium dihydrogen orthophosphate (pH 7.60) using lithium chloride as the supporting electrolyte. A linear range, 1 X to 2 x 10-3M, was noted in their plot of concentration us. diffusion current. Dean, Peniston, and Cantor (7) presented a paper on the polarographic determination of 5-hydroxymethylfurfural in dextrose and dextrose liquors, but this work iyas never published. APPARATUS A N D REAGENTS
-4k e d s & h'orthrup Electro-Chemograph Type E and an H-type cell with a saturated calomel electrode and a saturated potassium chloride agar bridge were used to obtain the polarographic data. The H-cell was jacketed in a constant temperature water bath a t 25.0' i. 0.1' C. Dissolved oxygen was removed satisfactorily by bubbling nitrogen through the solution for 10 minutes prior to each determination. Oxygen was removed from the nitrogen stream by bubbling the impure nitrogen through a vanadous sulfate solution (16). The characteristics of the capillary used were: drop time, 4.33 seconds per drop; m for the capillary, 1.79 mg. per second; and 1.88 mg.z'a sec.-112for m2/3tilB a t H = 71.6 cm. in distilled water a t -0.60 volt. All chemicals used were C.P. or analytical reagent grade, except that the starches, as shown in Table I, were the normal commercial products. Starch solutions were prepared immediately before use. Triple-distilled C.P. mercury was used in the dropping mercury electrode. Table 1.
Chemical Analysis of Starch Sa mp les"
Starch Sample Potato starch Cornstarch* Starch CCb
Starch '3% by Difference
Moisture, rlsh,
'3%
%
9.19 0.32 10.35 0.07 12.45 1.19 85.41 a Analyses performed by Xational Starch Products, Inc., Plainfield, N. J. b Starch contains about 0.35Y0 protein and 0.67, fatty acid. 90.49 88,63
PROCEDURE
Add a Iveighed amount of starch to an appropriate volume of concentrated hydrochloric acid in order to yield a starch concentration within the range of 5 to 450 mg. per liter. Boil the solution for a predetermined length of time and readjust the volume of the solution with constant boiling hydrochloric acid. Pass oxygen-free nitrogen through the solution for 10 minutes and make a polarogram over the range of -0.23 to -0.60 volt us. S.C.E. Convert the n-ave height t o microamperes and ascertain the concentration of starch in milligrams per liter by reading directly off calibration curves. This procedure for the purity determination of starches themselves can be readily adapted to the starch analysis of flotation liquors with the following modifications. First, the concentration of the interfering ions of copper and nickel, which may be present in the liquor to be analyzed, must be sufficiently reduced to eliminate their interfering waves. The technique employed was similar to the normal qualitative scheme, except that thioacetaniide (10, 11) instead of hydrogen sulfide was used as the precipitating agent. The precipitates were readily centrifuged. Secondly the flotation liquor must be evaporated to 1 t o 2 ml. nith great care to ensure that none of the starch chars. Lastly, the liquor concentrate is then taken up with an appropriate amount of concentrated hydrochloric acid so that a concentration range of not less than 5 mg. per liter is attained. RESULTS A N D DISCUSSION
Boiling Time. Exploratory data indicated t h a t the maximum wave was obtained by boiling a starch in concentrated hydrochloric acid for a suitable period of time determined empirically in the following manner: Add 20.0 mg. of the commercial starch to 100 ml. of concentrated hydrochloric acid in a 250-ml. beaker. Cover the beaker with a n-atch glass and place it on a preheated hot plate. Boil for a predetermined time, adjusting the watch glass for the last 5 minutes to cover only half the beaker in order t o achieve a constant boiling hydrochloric acid solution. Remove and cool to room tem-
perature, transferring the solution to a volumetric flask and adjusting to 100 ml. with constant boiling hydrochloric acid (20.24%). Run a polarogram from -0.23 to -0.60 volt us. S.C.E. Test data (Table 11) show that the diffusion current of starches in concentrated hydrochloric acid increases with an increase of boiling time; after the maximum is reached, further boiling reduces the diffusion current. Considerable time is needed for the complete hydrolysis of starches and subsequent degradation t o Lhydroxymethylfurfural, which is the electroactive species reducible at the dropping mercury electrode. The necessary time for such a transformation may be followed visually as the colorless starch solution turns first to deep yellow and then to deep brown, indicating the progress of the degradation. The lowering of the diffusion current by excess boiling may be blamed on the condensation of 5hydroxymethylfurfural with itself to form oxybis-(5-methylenefurfural) (I7 ) and,'or on the formation of huniins. The data show also that the optimum boiling times for the acid degradation of the potato starch, cornstarch, and starch CC are, respectively, 22, 20, and 15 minutes. This lack of agreenieiit is attributed to the considerable difference among starches in particle size, chemical composition, structure, and molecular packing. Although the data are hardly ahsolute values, they are reproducible with normal care. Concentration. Table I11 shows t h a t t h e values of concentration of t h e calculated pure starches, within t h e range of 5 t o 450 mg. per liter, fall on good straight lines when plotted against the corresponding diffusion current. The lower percentage of starch in any flotation liquor can be concentrated to fit this concentration range by evaporating a large portion of the liquor and adding only a small amount of concentrated hydrochloric acid into the evaporating liquor. The half-wave potential of the tested starches was found, a t the zero damping position of the galvanometer, to be -0.402 which is considered the volt us. S.C.E., true value. Normal polarographic accuracy-i.e., within 27&-is attained with this method. Interfering Ions. T h e effects of 1 t o 5 mg. of flotation reagents a n d ions on t h e polarogram of 5 mg. of cornstarch boiled for 20 minutes in 100 nil. of concentrated hydrochloric acid mere investigated. T h e results shoIved t h a t collectors, such as xanthates, soaps, and lauryl amine, had little effect on the i d of Cornstarch, though lauryl amine a t concentrations greater than 19 mg. per liter slightly affected the polarogram. Also, inorganic ions, including lithium, lead, zinc, barium, calcium, cobalt, silicate,
Table
II.
Effect of Boiling Time on Diffusion Current of Starches in Concentrated Hydrochloric Acid
Commercial Potato Starch, 200 Mg./Liter Boiling. id. 5.0
1Fi.n 15.0
17.0 i7.0
22.0 25.0 35.0 40.0
Table 111.
0.031 0 .517 0.717 0.753 0,785 0.729 0.253 0.186
Commercial Cornstarch, 200 Mg./Liter Boiling, id, 6.0 10.0 i15 5 . oO 20.0 21.5 25.0 30.0
0.000 0.451
0.496 0.497 0.496 0.425 0.376
Concentration vs. Diffusion Current,
Cornstarch Boiled 20 Potato Starch Boiled 22 Minutes in Concd. HC1 Minutes in Concd. HC1 id, Concn. of pure" idr Concn. of purea starch, mg./liter pa. starch, mg./liter pa. 0.019 0.021 5.5 5.3 0.034 0.045 11.1 8.9 0.066 0,103 22.2 23.8 0 145 0.194 44.3 45.7 97.0 0.420 88.6 0.281 0 497 0 786 177 S 183 0 1 018 354 5 320.2 1 361 1 296 4x3 2 457 5 1 922 a Starch corrected for analysis in Table I.
sulfate, and tungstate, which may possibly be present in the flotation liquors of minerals, had no adverse effect on the id of cornstarch. The only ions which interfered were copper and nickel, and these could be quantitatively removed by precipitation as the sulfides prior to the polarographic determination of the starch. Another series of tests showed that the process of evaporating aqueous starch solutions to 1 to 2 ml. resulted in no ill effect on the polarographic determination of the starch. Current. The nature of t h e diffusion current of 17.8 mg. of cornstarch boiled for 20 minutes in 100 nil. of concentrated hydrochloric acid n-as investigated by changing t h e height of the mercury column between 42.8 and 85.8 cm., and by studying the effect of these changes on the id of the starch solution. The current was found to be dependent on the height of the mercury column. The value of was constant (0.056 pu,/cni,1'2), indicating that the reduction process n-as diffusion controlled. I n another series of tests, the id of 17.2 mg. of cornstarch boiled for 20 minutes in 100 nil. of concentrated hydrochloric acid was measured at various temperatures, ranging from 20.1" to 44.8" C The teniperature coefficient, computed by means of the compound interest formula, was 1.8% for the starch solution, demonstrating again a diffusion-controlled process.
Starch CC, 200 hlg./Liter Boiling, id, 10.0
12.5 15.0 17.5 20.0 25.0 30.0
0.267 0.582 0.673 0.560 0.495 0.200 0.103
= -0.420 Volt vs. S.C.E. Starch CC Boiled 15 Minutes in Concd. HCl Concn. of pure" id, starch, mg./liter Ma. 0.031 6.8 21.4 0.089 0.170 42.7 85.4 0.339 170.8 0.686 1 002 256 2 1 399 311 6 1 721 422 1
Table IV. Adsorption of Pure" Cornstarch by 48/65 Mesh Mineral Particles
Concn. of Pure" Starch in Supernatant purp Liquor Cornstarch id, h!g./ .ldsorbed, Mineral pa, liter Rlg./G. Quartzb 0 026 7 9 0 73 0 29 Chalcosite" 0 13.1d 29 9 0 067 22 1 0 44 Barite' West Kentucky coalJ 0 041 12 8 0 63 a Starch corrected for analysis in Table I. Dodecylamine, potassium ethyl xanthate, sodium oleate, and kerosine used as collectors, respectively. d Copper ion quantitatively removed from liquor by thioacetamide precipitation. * , c p e , f
starch in flotation liquors. The data mere established by agitating, for 5 minutes, a slurry of 5.0 grams of 48/65 mesh, mineral particles, 5.0 mg. of cornstarch as received (4.43 mg. of pure material), 1.5 mg. of collector, and 100 ml. of distilled water, followed by starch determination of the supernatant liquor. I n another series of tests, the polarographic method was checked against the colorimetric method of T'iles and Silverman (35) in the determination of cornstarch in water, and results agreed reasonably well. ACKNOWLEDGMENT
APPLICATION
Experimental data (Table IT) illustrate the applicability of the polarographic method for determining corn-
The writers wish t o thank Charles Fuget for his assistance in this investigation, and National Starch Products, Inc., for its kind cooperation. VOL. 30, NO. 6, JUNE 1958
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Sorthwestern Cniversity, Kov. 16, 1945. (8) DeVaney, F. D., Ind. Eag. Chem. 39, 26 (1947). (9) DuBois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A., Smith, F., ANAL.CHEM.28, 350 (1956). (10) Flaschka. H.. Chemist Analvst 44. 2-7 (1955).‘ (11) Flaschka, H., Abdine, H., Zbid., 44, 30-1 (1955). (12) Grzhivo, V. S., Sazich. Chteniya 1962 1953, 78-90. (13) Kerr, R. W.,“Chemistry and Industry of Starch,” pp. 659-72, Academic Press, Pr’ew k’ork, 1950. (14) Love, D. L., Ph.D. thesis, Pennsylvania State University, University Park, Pa., 1955. (15) Mackinney, G., Temmer, O., J . Am. Chem. SOC.70, 3586 (1948). (16) Meites, L., Meites, T., SAL. CHEM. 20, 984 (1948). (17) Middendrop, J. A4,, Rec. trac. chim. 38, l(1919). (18) Mitchell, D. R., ”Coal Preparation,” pp. 609-47, Am. Inst. Mining,
LITERATURE CITED
(1) Bates, F. J., “Polarimetry, Sacchar-
(2)
(3)
(4)
(5)
(6) (7)
imetry and Sugars,” Government Printing Office, Washington, D. C., 1942. Brautlecht, C. .4., “Starch-Its Sources, Production and Cses,” pp. 330-53, Reinhold, Sew York, 1953. Browne, C. A., Zerban, F. R., “Physical and Chemical Methods of Sugar Analysis,” 3rd ed., pp. 1124-33, Wiley, New York, 1941. Cantor, S. M., Peniston, Q . P., J . A m . Chem. SOC.62, 2113 (1940). Chang, C. S., Cooke, S. R B., Huch, R. O., Trans. Am. Inst. JIznzng M e t . Engrs. 196, Mznzng Eng. 5, KO.12, 1282 (1953). Clendenning, K. il , Can. J . Research C20, 403 (1942); B23, 113, 239 (1945); F26, 185 (1948). Dean, G. R., Peniston, Q . P., Cantor, S. M., Technical Conference,
.
I
Met., Petrol. Engrs., S e w l-ork, 1954. __._
(19) RIorris, D. L., Science 107, 254 ( 1948). (20) Radley! J. S., “Starch and Its Derivatives,” Vol. 2, 3rd e d , pp. 35686, Chapman & Hall, London, 1953. (21) Schulz, S . F., Cooke, S. R. B., Ind. Eng. Chem. 45, 2767 (1953). (22) Sun, 8.C., “Effect of Organic Flocculants on Coal Sedimentation,” Chicago Meeting, -4m. Inst. Nining, Met., Petrol. Engre., February 1955. (23) Turner, J. H., Rebers, P. A , , Barrick, P. L., Cotton, R. H., A s . 4 ~ CHEX. . 26, 898 (1954). (24) Van Dyk, J. W., Caldwell, 11. L., Ibid.,28, 318 (1956). (25) Viles, F. J., Jr., Silverman, L., Ibid., 21,950 (1949). RECEIVEDfor review August 16, 1957. Accepted February 3, 1958. College of Mineral Industries Contribution So. 57-54.
Condensed Direct Current Arc Excitation for Spectrochemical Analysis of Plant Materials I
H. E. BRAUN Department of Chemistry, Ontario Agricultural College, Guelph, Ontario, Canada
b A method is described for the simultaneous quantitative determination of calcium, magnesium, phosphorus, iron, manganese, copper, and boron in plant tissue. An acidic aqueous solution of the plant ash is impregnated into powdered graphite held in a cratered electrode. The electrode is dried and then excited by means of a condensed direct current arc. A comparison of spectrographic and chemical analyses of various samples within several plant species indicates that the accuracy of the method is adequate for routine quantitative analysis of plant tissue and the reproducibility of the method is excellent.
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analyses of plant materials for the major elements calcium, magnesium, and phosphorus are generally obtained by direct excitation of plant ash or plant ash solutions, as chemical concentration is seldom required. Because i t is adaptable t o routine analysis, the copper electrode procedure of Fred, Nachtrieb, and Tomkins (3) was investigated in this laboratory. Sample solution residues were mounted on flat opposed copper electro es and excited by means of a higR voltage alternating current spark. These investigations revealed that large discrepancies between chemical and spectrographic analyses for calcium, magnesium, and phosphorus PECTROGRAPHIC
1076
ANALYTICAL CHEMISTRY
were prevalent, presumably as the result of the high salt concentrations encountered in most plant materials as noted by Feldman ( 2 ) . A related disadvantage of the copper spark technique is the need for preparing characteristic sets of standard concentration curves for each particular species of plant material under investigation because of the dependence of emission characteristics on the concentrational levels of the constituent major ash elements. Consequently, one established set of working curves cannot be applied to the analysis of a variety of plant species, as in agricultural research. I n the search for a method which would equal the convenience of the copper electrode procedure, investigations with a direct current condensed arc type of excitation revealed the possibilities of overcoming the above disadvantages. This paper describes the technique developed to permit simultaneous quantitative spectrochemical determinations of calcium, magnesium, phosphorus, iron, boron, manganese, and copper from one established set of standard concentration curves by means of condensed direct current arc excitation of plant ash solutions. CHEMICAL PROCEDURE
Purification of Reagents.
trated
C.P.
Concenhydrochloric acid ( 1 2 N ) is
diluted n i t h a n equal part of n-ater and t h e mixture is distilled through a n all-glass distillation apparatus. The strength of the purified acid is very close to 6 S . This is the only reagent required by the method which needs to bepurified: Internal Standard Solution. Cobalt serves satisfactorily as t h e internal standard. Under t h e conditions of this method, the trace amounts of cobalt present in plant material are not detectable. A standard cobalt solution IS prepared by dissolving 4.939 grams of reagent grade cobaltous nitrate [Co(N03)2.6HzO]in 100.0ml. of water to yield a concentration of 10.0 mg. of cobalt per ml. To 80 nil. of redistilled 6 S hydrochloric acid, 4.0 ml. of the standard cobalt solution are added, and the total volume is adjusted to 100.0 ml. with water. Preparation of Standards. Individual standard stock solutions of t h e elements to be determined are prepared from analytical reagent salts and pure metals. B y using appropriate amounts of these solutions, a composite stock solution is prepared so that 2.0 nil. of final solution contain a concentration of: Mg. Potassium Calcium Magnesium Phosphorus
25
15 3 3
Y
Iron
Boron Manganese Copper
150 50
25 20
This composite solution contains the
