Vol. 3, KO. 2
ANALYTICAL EDITION
152
In the next operation, the actual combustion, stopcock D of the gasometer is closed, stopcock E opened to the azotometer, the long burner TB is lighted, and the combustion tube heated to red heat. Then the Bunsen burner, BB, is lighted, the heating of the temporary charge is started a t the empty portion of the tube, and the flame gradually moved nearer and nearer to the long burner. The rate at which the bubbles enter the azotometer must not exceed 2 bubbles in 3 seconds, and this rate is carefully regulated by the proper movement of the burner. During the combustion the gasometer is refilled with carbon dioxide to a mark between 50 and 100 cc. (50 cc. have been found to be sufficient) When no more bubbles rise in the azotometer, the combustion is at an end and the gaseous combustion products must be conveyed into the azotometer by a current of carbon dioxide; stopcock D is carefully opened and adjusted to give a rate of one bubble per second for the 10 minutes. During this the temporary charge is again heated throughout with the Bunsen burner to insure complete combustion. The burner is moved in the same direction as before and then extinguished. The remainder of the gasometer charge can be passed through the system at a higher rate, about 3 bubbles per second, and as soon as the current of gas has ceased, because of the absence of pressure in the system, stopcock E is closed and the long burner extinguished. After 15 minutes, the readings are taken of the nitrogen volume in the azotometer, to within 0.001 cc., with the potassium hydroxide leveling bulb at the height of the meniscus, and of its temperature to within 0.5" C., with the bulb of the thermometer touching the wall of the measuring capillary, and of the barometric pressure to within 1 mm. In order to determine the air and absorption errors-i. e., that volume of gas collecting in the azotometer during an analysis, which originates from the air content of the carbon dioxide used and from air absorbed to the temporary chargea blank analysis is carried out combusting a nitrogen-free substance (a few milligrams of pure cane sugar) in exactly the same way as in the actual analysis. The sum of the air I
and absorption errors is deducted from the azotometer readings obtained in the actual analyses and should be determined as often as any changes in the Kipp apparatus or in the procedure of the analysis are made. Calculation
To the volume reading, the calibration correction of the azotometer (from the apparatus certificate) is first applied. From the resulting volume deduct: (a) the correction for air and absorption,errors (from the blank analysis) ; (b) 1.1 per cent-i. e., 0.5 per cent for adhesion of the potassium hydroxide to the wall, thus reducing the volume of the gas in the capillary of the azotometer (6); 0.3 per cent for the vapor pressure of the potassium hydroxide solution (50 per cent) ; 0.3 per cent (approx.) for the temperature reduction of the barometer reading (from 18" to 0" C.). The resulting nitrogen volume is reduced to normal conditions (760 mm. and 0" C.) and the percentage of nitrogen calculated. o-Toluamide sample, mg.. . . . . . . . . . . . . . . . . . . . . . . . . . . . Collected nitrogen, cc., (at 24.5" C. and 767 mm.) ............................ 0.321 Calibration correction, cc.. . . . . . . . . . . . 0.001 Air and absorption correction, cc.. . . . . . 0.008 1.1 per cent correction, cc.. . . . . . . . . . . . 0.003 Total corrections.. .......................... .O. 012 Net volume nitrogen to be reduced to normal conditions, cc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.309 Nitrogen found, yo.. . . . . . . . . . . . . . . . 10.33 Nitrogen calcd., yo.. . . . . . . . . . . . . . . . 10.37
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3,475
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Literature Cited (1) Diepolder, Chem.-Zlg., 43, 353 (1919). (2) Hein, 2. angew. Chem., 40, 864 (1927). (3) Lindner, Be?., 59, 2806 (1926). (4) Niederl, Trautz, and Saschek, Mikrochemie, Emich Festschrift, 227 (1930). (5) Pregl, "Quantitative Organic Microanalysis," Blakiston, 1924; Springer, 1930. (6) Trautz, iMikrochemze (1931).
Measurement of Consistency of Starch Solutions' J. C. Ripperton HAWAIIAGRICULTURAL EXPERIMENT STATION, HONOLULU, HAWAII
A study of the swelling of starches and its relation viscosity methods ordinarily HE consistency of t o viscosity has been made and a method of evaluating used, was devel.oped for starch s o l u t i o n s is starches by the swell method using a very simple factory control purposes. quite generally deterapparatus devised. mined by some type of visThis method is applicable chiefly to the tuber starches Preparation of Solution cometer or plastom'eter. with a large swell, such as those of potato, sago, arrowThese vary in principle and root, and to a lesser extent to cassava starch. It is well known that starch design, but they have 8s their exists as granules, rather than purpose the measurement of the internal resistance to flow of the viscous or plastic solution. as a homogeneous powder. When heated with water these Emulsoid colloids, such as starch, owe a large part of their vis- granules imbibe water and swell greatly. I n the natural cous properties to swelling on imbibition of water. Determina- starches, each granule exists as an inflated balloon consisting tion of this ability to swell was used by Harrison (1) as a meas- of a skin, called amylopectin, and a milky colloidal liquid ure of the consistency of the starch solution. Measurement of within, called amylose. If the solution is sufficiently dilute, the swelling of starch as an index of its strength has been used the swollen starch grains will settle on standing, leaving a occasionally but has never received general recognition. I n clear supernatant liquid. With increasing concentration, the course of an investigation to determine the relative proper- the granules fill more and more of the space until no settling ties of edible canna and potato starches, a study was made can occur. The starch solution begins to take on noticeof the swelling method and its relation to viscosity. The able viscosity when the swollen granules fill all the liquid method described, which has certain advantages over the space. It is common knowledge that when a starch solution is stirred or agttated vigorously it becomes less viscous and can 1 Receivrd November 7 , 1930
T
INDUSTRIAL A N D ENGINEERING CHEMISTRY
April 15, 1931
finally be reduced to a watery consistency. This has been shown to be owing to the bursting of the granules. Herein lies one of the chief difficulties in the development of methods for determining the consistency of starch solutions, especially with respect to the large-granuled tuber starches. The cereal starches have a smaller s ~ e l land a larger proportion of amylopectin and are not so subject to change. The greatly inflated granules of such starches as potato are very tender and easily broken by agitation and stirring. They will burst of their own accord if heated at boiling temperatures for any length of time. Any method
60
ri
u bo
vr 0
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v, >
kept in the water, the tip being near the center. To make the final adjustment of exactly 97" C., the flask is either heated for a few moments over a naked flame if too low, or removed from heat entirely until by constant swirling of the contents the exact temperature is reached. The stopper and thermometer are then quickly removed, the tip of the pipet inserted to near the center of the liquid, and the pipet quickly filled. After each removal more water is added so that the flask is kept nearly full a t all times. Cooking
As soon as the starch has been brought into solution, a loose cotton plug is placed in the mouth of the Erlenmeyer, and the flask placed in a water bath for 15 minutes at 80" C. A longer time causes a gradual increase in the swell, but the relative difference between starches remains about the same. The water in the bath is kept nearly to the top of the flasks which are held down with lead collars. The solutions are not stirred or shaken during any part of the process. With starches such as potato and edible canna, cooking may be dispensed with. Instead, the starch solution is simply allowed to stand at room temperature for a t least 15 minutes. Longer time of standing has no effect on the results.
100
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153
40
20
Measurement of Swell loo
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I
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I
IZO
140
IKI
in0
200
220
240
SWELL- CC . Figure 1-Relation of Vimcosity t o Swell
for the determination of the strength of a starch must take into account this breaking of the starch grains in order to duplicate results. Nivling (2) contends that the only viscosity measurement that has any significance in practical work is that reached after prolonged cooking has reduced it to a constant value. Most investigators seek to obtain the initial viscosity of the starch by adopting an arbitrary procedure whereby the amount of granules broken during the preparation of the solution is always the same. For example, Wolff (3) brings the starch into solution by heating over a steam bath and then boils the solution over a naked flame for exactly 21/2 minutes. The following method was developed to bring the starch into solution with the least probability of breaking the granules: A series of 125-cc. Erlenmeyer flasks, varying no more than 6 or 8 grams in weight are selected. The sample of air-dry starch is weighed into the Erlenmeyer and 10 cc. of distilled water added. Distilled water at exactly 97" C. is rapidly drawn up into a 100-cc. pipet whose tip has been filed off to about a 3-mm. opening. The starch is then brought into solution by allowing the hot water from the pipet to run rapidly into the starch suspension. The tip of the pipet is kept in the center of the flask and ,iust below the rising level of the liquid. This produces a rolling motion of the liquid, down in the center, out at the bottom, up on the outside, and in at the top. The entire 100 cc. of hot water are delivered into the flask in about 10 seconds. The final temperature of the solution is 80" * 0.5"C. The starch granules are thus kept perfectly dispersed during the time they are swollen. The granules do not reach their maximum swell for several minutes, so that during the addition of the hot water they are quite mobile, and perfect solution is produced without any stirring. It is obvious that radiation takes place rapidly during the filling of the pipet and delivery of the hot water into the solution. However, by following a uniform and rapid procedure, the final temperature of the starch solution in the flask will not vary more than 0.5" from 80" C. The distilled water is kept at 97" C. in a liter Florence flask over a small flame. A thermometer ifiserted through a rubber stopper is
The swell of the starch is measured in lipless cylinders of about 400 cc. capacity with a rim around the tops2 About 200 cc. of distilled water are measured into each cylinder. After cooking or standing for 15 minutes, the starch solution is poured into the cylinder. The .flask is rinsed with a few cubic centimeters of water which is also poured into the cylinder. The exact volume of water used is of no consequence. A rubber stopper is then placed tightly in the top and the cylinder is carefully inverted twice, but not shaken. It is then allowed to stand undisturbed overnight. The next IO0
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Olio
110
2bO
230
260
2bO
3!?0
350
3bO
CONCENTRATION X SWELL Figure 2-Relation of Viscosity to Concentration X Swell
morning the volume or swell of the starch granules is measured. This can be read off from graduations on the side of the cylinder or by bringing the edge of a strip of paper, held by a rubber band, to the exact level of the settled starch grains. The contents of the cylinder are poured out, water added to the exact level of the strip of paper, and the volume of the water thus used measured in a graduate. Relation of Swell to Viscosity
To determine the relation of the swell of a starch to its viscosity, determinations were made on different concentrations of the cooked samples in a Stromer viscometer at 80' C. The instrument was standardized with sucrose so that time seconds could be expressed as centipoises. The relation of 2
Similar to Catalog No. 4392, A. H. Thomas Company, 1921 edition.
ANALYTIC A L EDITION
154
viscosity, expressed in centipoises, to swell is shown in Figure 1 for two commercial samples* of potato starch. D-56 represents a starch of unusually high viscosity, and D-14 one of medium viscosity. It will be noted that a much greater swell of D-56 is required to produce a given viscosity than of D-14. This means that starches differ in the relation of swell to viscosity, hence the swell cannot be taken as a direct measure of the viscosity. If, however, the viscosity is plotted against the product of the concentration times the swell (Figure 2), the curves of D-14 and D-56 practically coincide. By this means a direct relationship between swell and viscosity is obtained. For example, if a 1.5-gram sample gave a swell of 150 CC., 1.5 X 150 = 225 (concentration X swell)
This, referred to Figure 2, gives a viscosity of 26 centipoises. It has been suggested by Wolff (3) that for practical work the viscosity values of different starches should be expressed in terms of a standard starch rather than in absolute viscosity units. This method seems decidedly advantageous for the reason that the viscosity value obtained for a given starch is relative. A slight difference of procedure in preparing the solution will give a different viscosity. Air-dry starch is very stable in its properties and makes a dependable standardpver a period of years. 100
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Vol. 3, No. 2 1.5 i 1.64 = 91 per cent
By using the same method it is possible to evaluate the unknown in terms of the standard from the values of the swell by referring the viscosity in centipoises obtained from Figure 2 to the standard curve in Figure 3. Evaluating a Starch without Use of Viscometer
Where a viscometer is not available, for practical work the swell may be used to determine the value of a starch in terms of the standard by the use of the curve in Figure 2 and the D-14 curve in Figure 3 without establishing the viscosity values for the particular starch which is to be used as a standard. For example, suppose 1.6 grams of the starch selected to be the standard (8)has a swell of 170 cc. 1.6 X 170 = 272
which is equivalent to 41 centipoises. 1.6 grams of the unknown starch (X)has a swell of 135 cc. 1.6 X 135 = 216
which is equivalent to 24 centipoises. Referred to the D-14 curve (Figure 3)) 41 cp. = 1.69 grams
and 24 cp. = 1.5 grams 1.5 + 1.69 = 88 per cent
the value of X in terms of 8. This method may also be used when the swell is determined without cooking, since the effect would be relatively the same for the standard as for the unkno.wn . It is thus possible, in a practical way, to evaluate starches by the swell method with very simple apparatus-namely, flasks, pipets, and cylinders. Importance of Cleanliness
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“.8
1.0
1.4
1.2
1.8
1.8
2.0
2.2
CONCENTRATION- G
Figure 3-Relation of Viscosity to Concentration
Viscosities of the standard starch are determined for a series of different concentrations. Plotting these values gives a curve similar to those in Figure 3. To evaluate an unknown starch, the viscosity of any concentration is determined. The ratio of this concentration to that of the standard required to give the same viscosity is the value of the unknown with respect to the standard. For example, taking D-14 as the standard starch, 1.6 grams of the unknown starch gave a viscosity of 24 centipoises. Twenty-four centipoises on the standard curve are equivalent to 1.5 grams. 1.5
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1.6 = 94 per cent
the value of the unknown starch (standard starch = 100). Determinations of the viscosity of the standard starch will often vary somewhat above or below that of the standard curve owing to small differences in procedure, impurities in the distilled water, etc. To correct for this, viscosity is determined in samples of the standard starch and the unknown starch a t the same time, and the correction applied as follows: 1.6 grams of the standard starch gave a viscosity of 36 centipoises. This is equivalent to 1.64 grams on the standard curve. The corrected value of the unknown starch in the previous example would then be as follows : 8 These were selected from a series of commercial starches obtained through courtesy of Joseph Morningstar & Co., New York.
I n any determinations of the consistency of starch solutions, special attention must be given to cleanliness of the apparatus, The minutest trace of salts getting into the solution from the glasswake, or differences in the purity of the distilled water, affect the results. AIthough these effects are partially compensated by testing the standard starch along with the unknown, special care should also be used. For example, the distilled water left in the hot-water flask from one day should be discarded and the flask filled with fresh distilled water. The Erlenmeyer flasks and cylinders should be carefully brushed clean, then rinsed repeatedly with distilled water, and stood upside down on pegs to drain. At least three determinations should be made and the average value taken. Literature Cited (1) Harrison, W., J. SOC. nyers Colourisfs, 27. No.4, 84-8 (1911). (2) Nivling, W. A,, “Significance of Starch Viscosity in the Manufacture of Paper and Textiles. A Comprehensive Survey of Starch Chemistry,” Vol. I, edited by R. P. Walton, 1928. (3) Wolff, O.,Chem.-Zlg., 108, 1001,1012 (1927).
Timber and Wood Pulp Combine in Sweden Ten independent Swedish timber and chemical wood pulp companies, representing a share capital of nearly $20,000,000 with a total annual output of 316,000 tons of wood pulp and about 135,000 standards of timber, have formed a joint sales and purchasing organization. The mills retain complete independence and the understanding does not affect the management in any way.
