The Rigidity of Starch Pastes BERNADINE BRIMHALL AND R. M. HIXON Iowa Agricultural Experiment Station, Ames, Iowa
M
AXWELL (IS) in 1868 proposed an equation relating
the rigidity of a body to its viscosity by means of the relaxation time. Twenty years later, Schwedoff (91) devised an apparatus for measuring the rigidity and relaxation time of sols. He stated that the viscosity of a liquid is not necessarily an index of its rigidity. A number of investigators (6, 17, 92) have since worked out formulations for structural viscosity on the assumption that it is due to the presence of elasticity. [Rigidity is the reciprocal of elasticity. For definitions, see (8).I Michaud (14, 16), by an ingenious method of his own, investigated the effect of acids, bases, salts, and some organic compounds on the rigidity of gelatin and agar gels. Philippoff (18) described a dynamic method for determining elasticity of cellulose solutions, while Neale (16) measured the elasticity of airdried starch films. Porst and Moskowitz (19) summarized Bingham’s concept of rigidity asrapplied to starch. Their attempts to measure the “yield shear value” by extrapolation of flow-shear curves gave indefinite results because of the gradual slope of the curves obtained. Farrow, Lowe, and Neale (7), using both capillary and Couette-type viscometers, observed flow in starch pastes at rates of shear below Bingham’s theoretical “yield value.” McDowell and Usher (12) advanced a simple mechanical explanation, supported by striking experimental evidence, for the phenomena of rigidity and anomalous viscosity in suspensions and gels: “If rigid particles suspended in a liquid in which they are insoluble are not prevented from coheringwhether by an electric charge or by an envelope of a soluble s u b s t a n c e t h e y will in time form aggregates, the presence of which will always cause the viscosity to be a function of the rate of shear; and which, if completely interlinked, will impart rigidity to the suspension as a whole.” They mention that variable viscosity is shown sometimes when rigidity is absent, and always when it is present. Hess and Rabinowitsch (9), after heating starch grains just above their gelatinization temperature, punctured the granule membrane with a microneedle and photographed a liquid oozing out at the point of puncture. They believe that swollen starch grains have a certain amount of inner structure which, like the membrane, possesses elasticity. Badenhuizen (2) described experimental evidence which would seem to contradict this concept of granule structure. Woodruff and MacMasters ($3) made measurements of relative viscosity and gel strength on starches from different varieties of corn and on starches from the same variety of corn treated in different ways. They found that viscosity differences were very small as compared with differences in gel strength and that the two properties frequently did not fluctuate in the same direction. They give this as further evidence that viscosity and gel strength seem to measure two different sets of properties in the starch. Most of the methods now in use for the determination of gel strength in starch pastes involve actual disruption of their structure-i. e., measure the degree to which they can be stretched before breaking. This value fluctuates with the rate a t which the force is applied, so that a high degree of accuracy is not obtained. The method developed by Schwedoff (21) measures the resistance offered by the paste to being stretched. The results are more accurate, since the gel is not deformed beyond its 358
elastic limit, and the values obtained are independent of apparatus constants. By this means, rigidity has been demonstrated in a variety of gels (1, 8, 10, 11, 15, 18, 20, 21), but in only two cases has mention been made of starch: McDowell and Usher (11) measured the rigidity of nonaqueous suspensions of raw starch, and Arcay and Etienne (1) included starch in the list of substances whose gels they tested for the presence of rigidity. Caesar (4, 6) has designed a consistometer for characterizing starch pastes by their relative resistance to violent mechanical agitation at high concentrations (10 to 30 per cent). Consistency, as measured by this method, is partly conditioned by rigidity, as well as by viscosity, plasticity, and thixotropy.
0”
I
b
FIGURE 1. DESIGNOB RIGIDOMETER The results obtained from the application to starch pastes
of the Schwedoff technique for measuring rigidity are reported in this paper.
ExperimentaI
DEMONSTRATION OF RIGIDITY IN STARCH PASTES.PreIiminary experiments were made using a MacMichael viscometer. The paste, after cooling, was placed in the viscometer cup and a definite twist given to the wire. With pastes of low concentration, the disk eventually swung back to its original position, but if the concentration was high
JULY 15, 1939
ANALYTICAL EDITION
enough, it came to rest before reaching the zero point, thus demonstrating the presence of elasticity. However, any disturbance of the paste--even the passage of the disk attached to the wire during measurementsresulted in a breaking up of the gel structure and subsequent lowering of elasticity values. Since this factor made it difficult to obtain accurate and reproducible results, the design of the MacMichael viscometer was modified into an apparatus resembling Schwedoff's (21) for the quantitative determination of rigidity. The design is illustrated in DESCRIPTION OF RIGIDOMETER. Figure 1. A MacMichael viscometer wire, W , encased in a metal shaft, hangs from the bottom of a dial, D, graduated in degrees. By means of a piece of rubber tubing, the shaft around the wire is firmly attached to a small glass cylinder, C, so that the latter may hang freely inside a larger cylinder, H . A mirror on the shaft reflects a beam of light from its source, L, onto a graduated scale in front of the apparatus. If water or other ideal liquid is placed in the larger cylinder, the angle through which the wire is turned by the dial will be identical with the angle through which the cylinder turns in the liquid (as read by the position of the light beam on the scale). If a paste showing rigidity is placed in the larger cylinder, the inner cylinder will be deflected (when a torque is applied to the wire) by an amount depending on the elasticity of the paste, providing the force applied through the wire is not great enough t o cause shear of the paste structure. Two modifications of this apparatus have been used. For most of the experimental work reported in this paper, the outer glass cylinder, H, and the calibrated dial, D, were mounted independently in a rigid brass frame inside a constant-temperature air bath. When the inner cylinder had been set in place and secured by means of screws through a brass collar, the paste was poured in through a hole in the side of the outer tube and the inner cylinder released by turning back the screws. A mirror was also attached to the dial to provide for extra precision in setting and reading. The second modification which is shown in Figure 1 was developed later t o allow for greater convenience and speed of handling. Hydrometer cylinders (250-cc. capacity) were used for the outer tubes. The paste was poured in through the top, the calibrated dial put in place, and a center piece, P, bearing the wire and inner cylinder lowered into it. The whole unit was then set in a constant-temperature water bath until equilibrium wys reached and finally moved over to the light source for measuring. The first method is the more precise, while the second method is simpler in operation. In the first-described apparatus, the paste may be poured in after it has been cooled and requires only about 3 hours to reach equilibrium. The unit is not moved from the time the paste is put into it. I n the second modification, the paste must be poured in while hot, so that it will be fluid enough for the inner cylinder
359
to center itself. A paste prepared in this manner and placed in a 25" C. water bath requires a t least 10 hours to reach equilibrium. No skin is formed on the surface of the paste, however, since the top piece prevents appreciable evaporation. STANDARDIZATION OF PROCEDURE. Experiments were first carried out to determine the effect of time and temperature on rigidity. If, as McDowell and Usher (12) imply, anomalous viscosity is correlated with rigidity, then a 5 per cent cornstarch paste which shows abnormal viscosity a t 90" C. should also show rigidity a t that temperature. Such was found to be the case. The rigidity increases with time of standing in an irregular manner during the first 3 hours a t 90". Measurements taken after 3 hours a t this temperature are subject to error because of evaporation of water from the paste. I n order to eliminate this difficulty, the pastes were first heated to 90" and then cooled to 25" before being allowed to set. The resulting data showed that rigidity increases rather rapidly with time during the first 3 hours and then becomes nearly constant. (This holds for the unmodified starches under the conditions of experiments reported. It is likely that the time required varies with the concentration and previous treatment of the starch.) Lampitt and Money (IO)have obtained similar results with pectin gels. During measurements of elasticity, it is possible, by imparting sufficient twist to the wire, to strain the paste beyond its elastic limit and shear it. At this point the gel ruptures and the inner cylinder drifts along in the direction of twist on the wire. With pastes of fairly low elasticity this point is evident as a well-defined break, but with pastes of high elasticity it cannot be accurately detected. Tendency to shear, expressed in reciprocal form as shear value, must be considered because of its important effect on rigidity values. (Shear value is the torsional moment of the wire, N , multiplied by the number of degrees through which the wire may be twisted before shearing the paste.) Shearing tends to break up gel structure, so that once a paste has been sheared it is useless to make further rigidity measurements on it. If a certain paste shears so easily that its rigidity is difficult to determine, the shear value may be increased by using a higher concentration of paste, or by preparing the paste a t a higher temperature, providing the granules have not been ruptured. The use of a lighter wire (one having a lower torsional moment) facilitates the measurement of pastes with low shear values. PROCEDURE. The studies on the effect of time, temperature, and shear led to the adoption of the following method for determinations of rigidity: The weighed starch sample, suspended in 200 cc. of distilled water, is heated at the desired temperature in a water bath until it has come to equilibrium. This requires approximately 60 minutes of heating at 70" C., 50 minutes at 80", 40 minutes at go", and 30 minutes at 99", as determined by the change in volume of centrifuged granules at regular time intervals. The paste is then cooled to 25" by shaking in a stream of running water and poured into the tubes of the rigidometer where, after standing undisturbed for 3 hours, it is measured by placing varying degrees of twist, 6, on the wire and noting the corresponding deflection, w , of the cylinder. These two values, 6 and w (converted to angular degrees), when plotted against one another, make a straight line. Then the slope of this line, 6 / w , may be substituted in the equation mentioned by Hatschek and Jane (8): N
E 3
4
5
6
7
Z STARCH
8
9
10
FIGURE 2. RIGIDITY-CONCENTRATION CURVES FOR CORNSTARCH PASTES PREPARED AT DIFFERENT TEMPERATURES
=
1
4nh (m -
1
6
a);
where E is the modulus of rigidity in dynes per square centimeter; N , the torsional moment of the wire used; h, the height of the paste on the inner cylinder; and ROand R I , the radii of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
360
inner and outer cylinders, respectively. (The wires were standardized by measuring the oscillation time of a suspended disk of known mass. Then N = 4 r 2 M R 2 / 2 T 2where , T is the period of oscillation and iM and R are the mass and radius of the disk used.) Wires with torsional moments of 0.0256, 0.0665, and 0.1460 erg were used interchangeably with entirely consistent results, showing that wire size may be suited to the strength of the paste being measured without introducing error in this re-
VOL. 11, NO. 7
very similar in shape, differing only in the temperature required to produce rigidity. Waxy maize has nearly the same temperature of maximum rigidity as tapioca, although a much higher concentration of the former is needed to produce the same amount of rigidity.
spect.
Characterization of Starches by Means of Rigidity
A comparison was made of three different starches showing extreme variation in physical properties: a commercial cornstarch, tapioca starch, and waxy maize starch, which gives a reddish brown color with iodine. Figure 2 shows rigidity-concentration curves for the commercial cornstarch. At 70", 80", and 90" C. the curves are nearly identical in shape, the rigidity changing very slowly up to a certain critical concentration, above which it increases enormously for each slight increase in the percentage of starch. If the lower arm of the curve a t 90" is extrapolated, it intersects the X-axis a t approximately the same concentration a t which structural viscosity sets in, 2.7 per cent. The same thing appears to be true of the curves a t 70" and 80", although they cannot be accurately extrapolated because the high tendency to shear makes rigidity values uncertain at lower concentrations. Above 90" a change in the condition of the granules begins to set in, and a t 99 ", instead of bending sharply and shooting upward almost vertically, the curve slopes upward only gradually. Finally, a paste heated in the autoclave at 120" shows comparatively little rigidity.
400
?
7P
85'
95"
105'
115'
TEMPERATURE roc.)
FIGURE4. EFFECTOF TEMPERATURE OF PREPARATION ON RIGIDITYOF THREESTARCHES
It is interesting to compare these curves with the consistency-temperature curves obtained by Caesar (4,6). Although very similar in shape, they are quite different quantitatively because of the difference in properties measured and methods employed. Differentiation of Cornstarches
% STARCH FIGURE 3. RIGIDITY-CONCENTRATION CURVESFOR TAPIOCA AND WAXYMAIZESTARCH PASTESPREPARED AT DIFFERENT TEMPERlTURES
With tapioca (Figure 3), it is evident that even a t 70" changes have begun to take place in this starch, and the slope of the rigidity-concentration curve decreases progressively with increasing temperature of preparation. Rigidity in waxy maize, even a t 70", is very low and a t 75" is no longer evident. Rigidity increases in strictly linear relationship to concentration after the initial bend in the curve has been passed, the bend being more or less sharp depending upon the degree to which the paste has been altered at that temperature. Another method of treating these data is illustrated in Figure 4, where rigidities a t a given concentration of paste are plotted against temperature. This type of curve shows the temperature a t which each starch exhibits its maximum rigidity. Tapioca and comnlercial cornstarch give curves
Having tried out rigidity measurements on different kinds of starch, an attempt was next made to apply them to different varieties of the same starch. Rigidity-concentration curves (Figure 5) were obtained on seven different cornstarches prepared in the small-scale milling plant in this laboratory and on two commercial cornstarches. In all cases the pastes were made up a t 99" C. and measured a t 25 '. Samples of these starches in 2.7 per cent concentration were then prepared at 99", cooled to 25", and viscosities determined by the capillary method. Table I is characteristic of the results obtained. Since viscosity in pastes of this concentration varies with pressure, the viscosity values are given a t three different pressures: a t 5 cm. of water, where the decrease in viscosity with increasing pressure is extremely rapid; a t 15 cm., where the bend in the viscosity-pressure curve is most pronounced ; and a t 30 em., where the curve has leveled off and viscosity remains practically constant with further increase in pressure. The lack of correlation between rigidity and viscosity values indicates that they measure different properties of the pastes.
Relation of Granule Membrane to Rigidity The downward trend of the rigidity-temperature curves after reaching their maximum (Figure 4) has been ascribed above to a change in the condition of the granules. In order to get a better insight as to the character of this change, microscopic technique was employed. A glass cell mounted in a hot-plate substage provided the means for observing the progressive swelling of granules in'water suspension inside the cell. It was noted with tapioca and waxy maize starch that after the temperature of maximum rigidity had been reached,
JULY 15, 1939
ANALYTICAL EDITION
the granules, rather than increasing further in size, began to become more wrinkled, the degree of wrinkling increasing with temperature. At the same time, the refractive index of the granules approached so nearly that of water that it was difficult to observe them. It appeared as though the membrane became so permeable that it offered no resistance to the free exchange of contents with the outside medium. In general, no distinct rupture or breaking open of the granule membrane could be observed. Since the cornstarch grains failed to exhibit any notable change under the microscope a t the temperature of maximum rigidity, they were modified by various methods designed to weaken the membrane. Table I1 shows that when the granule membrane is weakened, whether by heat, mechanical treatment, or chemical degradation, rigidity decreases.
TABLE I.
COMPARISON OF VISCOSITIES d N D RIGIDITIES O F CORNSTARCH PASTES (Prepared a t 99' C.; measured a t 25') Rigidity of Viscosity of 2 7% Paste Starch 5.5% Paste 5 om. 15 om. 30 cm. Dynes/sq. cm. Poises Popcorn 410 0.45 0.25 0.16 Mandan 325 0.87 0.39 0.25 Yellow Creole 0.40 320 0.20 0.16 Iogent 0.36 270 0.21 0.15 Starch I 220 0.58 0.37 0.25 Reid Yellow Dent 193 0.80 4.34 0.22 Country Gentleman 158 0.18 0.12 0.10 Starch I1 120 0.48 0.25 0.19 Waxy maize 0 1 05 0.66 0.51
TABLE11. EFFECTOF MODIFICATION ON STARCH PASTES Starch
Discussion
400
350
Commercial starch I1 '
None R u n through homogenizer eleven times None Electrolytic oxidation with 1/10 C1 equivalent 5.5 hours acid treatment b y Gore method Ground in ball mill for 144 hours Autoclaved for 30 minutes at 120' C.
Rigidity 240 20 125 20 0
12 15
Literature Cited Arcay, G. P., and Etienne, Compt. rend., 185, 701 (1927). Badenhuiaen, N. P., Protoplasma, 29, 246 (1937). Bingham, E . C., J . Rheol., 1, 511 (1930). Caesar, G. V., IND. ESG.CHEM.,24, 1432 (1932). Caesar, G. V., and Moore, E. E., Ibid.,27, 1447 (1935). Eisenschitz, R., and Rabinowitsoh, B., Ber., 64, 2522 (1931). Farrow, F. D., Lowe, G. M., and Neale, S. M., J . Textile Inst., 19, 18 (1928). (8) Hatschek, E., and Jane, R. S.,Kolloid-Z., 39, 300 (1926). (9) Hess, K., and Rabinowitsch, B., Ibid.. 64, 257 (1933). (10) Lampitt, L. H., and Money, R. W., J . SOC.Chem. Ind., 56, 290T (1937). (11) McDowell, C. M., and Usher, F. T., Proc. Roll. SOC.(London), 131A, 409 (1931). (12) Ibid., 131A, 564 (1931). (13) Maxwell, C., Phil. Mag., 35, 129 (1868). (14) Michaud, F., Compt. rend., 174, 1282 (1922). (15) Ibid., 175, 1196 (1922). (16) Neale, S. M., J . TertileInst., 15,443T (1924). (17) Philippoff, W., Kolloid-Z., 71, 1 (1935). (18) Philippoff, W., Physik. Z . , 35, 884 (1934). (19) Porst, E. G., and Moskowita, M., J. IND. ENG. CHEM., 14, 49 (1922). (20) Rohloff, C., and Shinjo, Physik. Z . , 8, 442 (1907). (21) Schwedoff, T., J . Phys., [2] 8, 341 (1889). (22) Saegvari, A., Z. physik. Chem., 108, 175 (1924). (23) Woodruff, S., and MacMasters, M. M., Ill. A@-. Expt. Sta., Bull. 445 (1938). (1) (2) (3) (4) (5) (6) (7)
250
z
-
I- 200.
s 0 1.50
O
RIGIDITYOF
Summary The apparatus described by Schwedoff for the quantitative measurement of rigidity in gelatin sols has been applied to starch pastes. The method gives reproducible results for a given set of conditions of the paste and is free from instrument constants. The application of rigidity measurements was demonstrated in the comparison of three starches showing extreme variation in physical properties and in the differentiation of nine samples of cornstarch. The assumption that rigidity is dependent upon the condition of the granule membrane is supported by the results of microscopic observation of the granules a t various stages of swelling and by rigidity measurements made on several modified starches.
300
E
THE
(5.5% paste; prepared a t 99": measured a t 25") Treatment
Commercial starch I
Blthough not often encountered in the literature, the lack of correlation between viscosity and gel strength of starch pastes is generally recognized in the industry. The relationship between breaking strength and elasticity is less definite. Rigidity measurements are made without shearing the pastes, presumably giving a method of characterizing the starch independent of its viscosity and plasticity. It might be said that the rigidity and breaking strength of a starch are related as are the elasticity of a steel beam and the force required to break it. In preliminary experiments there were found cases where these two properties differed decidedly, but more often they paralleled one another.
361
4.0
L 4
F
5.0
5.5
% STARCH
60
FIGURE5 . RIGIDITY-CONCENTRATION CURVESFOR CORNSTARCH PASTES PREPARED AT 99" C. A Po corn B: YeEow Creole a n d Iowa Mandan C . Iogent
D. Reid Yellow Dent E.
Country Gentleman F . Waxy maize I, II. Commercial starches
PRESENTED before t h e Division of Sugar Chemistry a n d Technology a t t h e 97th Meeting of the American Chemical Society, Baltimore, Md. Journal paper No. 5-598 of t h e Iowa Agricultural Experiment, Station, Project 426. Supported in part b y a grant from Corn Industries Research Foundation.
Colorimetric Determination of Fluorine with Ferron JOSEPH J. FAHEY Geological Survey, U. S. Department of the Interior, Washington, D. C.
HE difficulties encountered the quantitative determination of fluorine in rocks and minerals can be pointed out in no better way than by reference
Tin
The colorimetric determination of fluo-
Mix the sample (usually 0.5 gram) with 5.0 grams of sodium rine, using the ferron-iron reagent herein carbonate and fuse the mixture in a Covered Platinum crucible described, is applicable to a wide range of over a Bunsen flame, taking care materials, including rocks and minerals to keep the cover of the crucible After fusion having up to 10 per cent of fluorine and to the prolific literature on this attained further heating is needsubject. From the work of Bernatural waters with a minimum fluorine less and may cause a loss of fluozelius (1) in 1816 to that of rine. Cool and leach overnight content of 1 part per million. with 300 ml. of water in a platinum Stevens (9) in 1936-a period dish. In the morning bring just of 120 years-there have been to incipient boiling, covering the published more than twenty methods and modifications dish with a &ratch glass, and filter hot through a 7-cm. Whatthereof relating to the determination of fluorine and embracman No. 41 paper. Wash once with hot water and transfer the ing gravimetric, volumetric, colorimetric, and nephelometric ' residue back to the dish with a jet of water, Add water until the volume is approximately 100 ml., boil for about 1 minute, filter methods of analysis. As a review of the literature was made through the same paper and wash well with hot water, Reserve by Stevens (9), further efforts in that direction are unnecesresidue A for the silica determination. sary. However, Steiger's (7) method as modified by Merwin To the filtrate in a large platinum dish add a solution containing 1.0 gram of zinc oxide in 30 ml. of hydrochloric acid (1 to 3). (6) is the most widely used in the analysis of rocks and the Cover with a watch glass and heat to boiling. Allow the PrecipiHoffman-Lundell(6) lead chlorofluoride procedure is probably tate to settle and filter through a 12.5-cm. Whatman No. 41 employed more than any Other where high Percentages of paper. Wash the platinum dish and the residue on the paper fluorine, characteristic of some minerals, are to be determined. once with hot water. Transfer the residue back to the dish, boil with about 75 mi. of water, filter using the same paper, and The method herein described consists in matching in a comfinally wash several times with hot water. Reserve residue B parison solution the yellowish hue of green produced by the also for the silica determination. reaction of fluorine in the unknown Solution on the ferron-iron Carefully measure the volume of the solution and divide it into two equal parts. To one part add methyl orange indicator and reagent. hydrochloric acid (1 to 19) to the end,point from a buret. To the other part add the same volume of hydrochloric acid (1 to 19) The Ferron-Iron Reagent using no indicator, followed by a solution of 0.5 gram of zinc oxide Ferron (7-iodo-8-hydroxyquinoline-5sulfonicacid) was and 1 gram of ammonium carbonate in 2 ml. of concentrated ammonium hydroxide and 10 ml. of water. Cover the platinum described by yoe(11) in 1932 as a co~orimetricreagentfor dish with a large glass cover and boil gently until the odor of ferric iron. In Using this compound a6 a reagent for fluorine, ammonia is no longer noticed. This requires reducing to a a saturated water solution is combined with a water solution volume of about 75 ml. Add 50 ml. of hot water, stir +ell, allow the precipitate to settle, filter through a 9-cm. Whatman NO. 41 of ferric chloride and hydrochloric acid. The proportions of ferron, ferric chloride, and hydrochloric acid finally decided on a ~ ~ $ ~ ~ t t rvere the result of some nineteen attempts to Produce a water, stir well, and filter through the same paper. Wash once. tive and stable fluorine reagent. When the iron content was Residues C, A, and B contain the silica of the sample. Make up the filtrate to 250 ml. This volume will be adjusted during the too lo~v,the green color was too weak, and conversely, when course of the analysis, so that a 25-ml. aliquot portion will coniron than necessary was in the solution, the color was tain from 0.1 to 1.5 mg. of fluorine. When the fluorine content darker than desired. Hydrochloric acid was found to be the of the sample is less than 0.20 per cent, a 1-gram sample should best of the three acids tried in the reagent; sulfuric acid be used and the final volume after removing residue C reduced to 50 ml. causes a partial discharge of color and nitric acid a complete By fusion with sodium carbonate and subsequent leaching with decolorization. As the concentration of hydrochloric acid water only about per cent of the fluorine in phosphate rock is was increased beyond the desired point, the color became rendered soluble. paler, whereas a muddy green resulted from a deficiency of the acid. Procedure for Determination of Fluorine The iron as ferric chloride and the hydrochloric acid are contained in a water solution that is 2 N to hydrochloric acid A 25-m1. aliquot portion of the solution containing the fluorine of the sample as the sodium salt is pipetted into a 50-ml. beaker; and 0.1 N to ferric chloride. this will be referred t o as the unknown solution. Into another The composition of the ferron-iron reagent used is as folbeaker of the same size are measured 25 ml. of a solution having lows : the same pH and the same quantity of sodium chloride per milliliter as the solution containing the fluorine; this will be called the ~ i . comparison solution. To each beaker 2.00 ml. of the ferron-iron 9o Saturated water solution of ferron reagent are added. Unless the fluorine content of the sample is Ferric chloride and hydrochloric acid solution (described above) 10 100 Distilled 'water very low, a difference in color of the solution will be noticed in the two beakers without resorting to the colorimeter for comparison. indefiIt is believed that this reagent Will remain The colorimeter used was a Klett top reader of the plunger type, having glass cups 65 mm. deep, with black opaque sides and nitely. No change was noticed after the reagent had stood transparent bottoms. for 6 months in the light of the laboratory. A 0.02 N solution of sodium fluoride is now slowly added from a buret graduated to 0.05 ml. to the greener or comparison soluExtraction of Fluorine as Sodium Fluoride tion until the color almost matches that of the unknown. An equal quantity of distilled water is added to the unknown The preliminary extraction Of fluorine from a rock follows solution in order to maintain the same volume in each of the two with few changes that outlined b y Hoffman and Lundell (6). beakers.
~ ~ ~ t , a , n " , ~ h t ~ ~ ~ e w ~ ~ ~ t s.csttz ~ ~ w ~ ~ ~ ~~ ;~
362
