JULY 15, 1939
ANALYTICAL EDITION
TABLEVI. EFFECTOF IONSOTHERTHANBORONON DETERMINATION OF BORON No.
@
Soil Extraeta Added M1.
60.0 grams of
Standard Boron Solutions Boron ~1.p.p.m.
Boron Found P.p.m.
Total Boron Present P.P.~.
Decatur clay extracted for 24 hours with 400
OD.
Error
%
of water.
Discussion The proposed method is most satisfactory when used with a photoelectric colorimeter in routine determinations. A calibration curve is readily formed from the readings obtained with a series of standards. From this curve, a table may be made to facilitate calculations of concentrations of boron in the samples. Obviously, the amount of curcumin or turmeric per unit volume must be kept identical with that of the blank; when this is done, dilutions of deeply colored solutions may be employed. Where a photoelectric colorimeter is not available, series of standards or balancing methods may be used but these methods require more time and are less accurate. The fact that the colors are reduced on standing after approximately 2 to 3 hours renders the series of standards method laborious unless artificial standards are used. The substitution of 50 per cent ethyl alcohol, by volume, for the 95 per cent ethyl alcohol used for extracting the colored
409
residue gave satisfactory results when the solutions were read within an hour; on standing the aqueous extracts faded more rapidly than the alcohol extracts. The substitution of 1 per cent turmeric extract for the 0.10 per cent curcumin is advisable, since the former is inexpensive and does not “crawl” in the evaporating dishes as much as the curcumin. These extracts deteriorate on standing in light and it is recommended that the extracts of turmeric be prepared daily. All reagent flasks should be boron-free, since either acids or bases might extract boron from glass containing this element; Kavalier Bohemian glass is satisfactory. Calcium hydroxide was used to prevent the volatilization of boron and the 0.10 N suspensions served the purpose of obtaining a more intimate contact of the reacting substances a t the drying point, the point a t which the color is developed.
Summary
A microgram procedure for a colorimetric microdetermination of boron involving the reaction between boric acid in the presence of oxalic acid and curcumin is outlined. It is accurate for the extremely low amounts of boron generally found in soil extracts and in plants. Literature Cited (1) Bertrand, G., and Agulhon, H., Bull. SOC. chim., 7, 90, 125 (1910). (2) Cassal, C. E., and Gerrans, H., Chem. News, 87, 27 (1903). (3) Chapin, W. H., J . Am. Chem. SOC.,30, 1691 (1908). (4) Holmes, W. C., J . Assoc. Oficiul Aor. Chem., 10, 522 (1927). (5) Gooch, F. A., Am. Chem. J., 9, 23 (1887). (6) Scott, W. W., and Webb, S. K., IND.ENQ.CHEM.,Anal. Ed., 4, 180 (1932).
Microviscometer JOHN R. BOWMAN, Mellon Institute of Industrial Research, Pittsburgh, Penna.
A microviscometer is described having absolute accuracy better than 4 per cent, and precision within 0.1 per cent in the range from 2 to 10,000 centistokes. The method is simple and rapid, and requires only one drop (about 0.03 gram) of sample.
continuous below the meniscus. The method is capable of good precision but is slow, because experimentally determined corrections must be made for surface tension and drainage. Levin’s method (1) is rapid, but not capable of great accuracy. A short capillary dipping into a minute reservoir is again employed, but the transit of the meniscus rising with surface tension is timed. Many inherent errors exist in this method which cannot be easily corrected.
N CONNECTION with some exhaustive fractionation of oils a t this laboratory, a new viscometer has been developed. The instrument was designed primarily for examining extremely small samples, less than 0.1 gram, but is so simple, rapid, and accurate that it is believed to be as satisfactory in general viscometry as any of the popular macro types. The apparatus is entirely self-contained, and stands about 60 cm. (24 inches) high from a base 25 X 30 cm. (10 x 12 inches). While not intended for use as an absolute method, accuracy better than 4 per cent is obtainable in this sense; the precision is much closer, within 0.1 per cent. Even in the hands of an unskilled operator, the complete cycle of a determination, including sampling, charging, timing, and cleaning, requires less than 10 minutes. The microviscometer discussed here is of the capillary type, but differs from the numerous modifications of the Ostwald pipet in that it has no bulb. Two such instruments have been described. That of Lidstone (2) depends in principle on the fall, under gravity, of a liquid column previously drawn up into the capillary from a small reservoir, the column being
Principle
1
The present method depends on the rate of fall, under gravity, of a short segment of liquid contained in a longer capillary. The tube is vertical, straight, and of uniform bore; a length of 25 cm. has been found convenient. It bears three etched marks, two near the bottom and one near the top, and is shown with its vapor jacket in Figure 1. In making a determination, a minute drop of the liquid is placed on the open lower end of the tube, and, with the aid of suction, a column is drawn up t o the first mark, C. The lower end is then wiped clean with filter paper or a lintless cloth, and suction again applied until the upper meniscus of the column segment is drawn above the top mark, A. Finally, the upper end of the capillary is opened t o the air, and the transit of the upper meniscus between marks A and B is measured with a timing device.
Theoretical Discussion The classical law of Poiseuille for the flow, u, through a tube of radius r and length 1, under a pressure p is as follows:
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 11, NO. 7
bore are used. Calculation of the appropriate correction is simple. where 11 is the viscosity of the fluid. Applied to the present problem, this reduces to the simple form. k = - sr2 t
8L
where k is the kinematic viscosity, g is the acceleration of gravity, and L and t are the length and time of transit,respectively, of the meniscus between the two marks, A and B. Note that the length of the liquid column does not appear. Poiseuille’s law, however, is not strictly accurate for capillaries of finite length; corrections for end effects are required, and several well-known a p proximation formulas for these have been proposed. Experimentally, the magnitude of the error was determined by measuring the rate of fall of columns of different length, using the same liquid and tube. For the useful range of tube bores, this was found to be constant, or very nearly so, when the length of the segment was 100 or more times the diameter of the capillary. However, even though this error is very small, it is easily eliminated by fixing the length of the segment by the use of the third mark. Surface tension introduces no error if the two menisci are identical. Primarily, this requires FIGURE 1 that they be of the same radius. This condition is fulfilled when the films of liquid on the walls of the capillary are of equal thickness before and behind the column. This state, in turn, is easily attained by drawing the column up a t about the same rate a t which it will later fall in making the test. The instruments must be equipped with vacuum-control bleed valves adjustable to give a head just twice that exerted by the column, so that the column is drawn up a t the same rate as it will fall under gravity. Since the error is a secondary one, and oils never differ greatly in density, a fixed setting for the suction head has been found entirely satisfactory for such liquids.
Calibration Considering the basic principles discussed above, it is possible to use this type of viscometer as an absolute instrument. The following is a typical absolute calibration check run, using a highly refined petroleum lubricant of accurately known viscosity: Bore of capillary, diameter, p Distance, A - B, om. Time, meniscus fall, A - B, sec. Kinematic viscosity, centistokes: Known Observed Error, % ’
33.97
15.0 128 2
12.01 12 28 2.2
The tube bore was determined by weighing mercury threads of measured length in it. Several such calibrations have been checked, using bores from 25 to 5 0 0 , ~and ~ oils from 2 to 10,000 centistokes’ viscosity. The precision of this type of viscometer is far better than its absolute accuracy, however, and is apparently limited by the human element in timing a t the low end of the range. Check runs can nearly always be made to agree within 0.1 per cent for reasonable flow times ( > 100 seconds). This being true, far better results can be obtained using the instrument as a relative one. A typical calibration curve is given in Figure 2. The primary standards are refined petroleum oils kindly supplied by M. R. Fenske, of Pennsylvania State College, and the secondary ones similar oils run against them in a FitzSimons viscometer.
0 - P R I M A R Y STANDARDS ‘I X-SECONDARY
200
400
600
800
1000
S E C O N O S FLOW T I M E
FIGURE2. TYPICAL CALIBRATION CURVE Flattening of the advancing (lower) meniscus, together with a corresponding increase in the curvatuve of the retreating one, is another possible source of error. Qualitative observation, however, under high magnification failed to show the effect. The tubes used are so small that the menisci appear to remain very nearly hemispherical a t all times. The viscosity of the air contained in the empty portions of the capillary introduces an error when the method is applied to liquids of very low viscosity, or when tubes of very small
FIGURE3. FRONTOF MICROVISCOMETER
JULY 15, 1939
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ANALYTICAL EDITIOK
Description of the Instrument Several viscometers of the present type have been built a t this laboratory, and t h e form shown in Figures 3 and 4 has been found most satisfactory.
complete instruments have been in use a t this laboratory for general routine work for nearly 3 years without difficulty. Temporary improvised viscometers of this type have been made from broken thermometer stems. Two stoppers and a large glass tube form the jacket, used as a water bath. An ordinary -10" to 110" C. thermometer capillary was found to give very nearly Saybolt seconds as flow time for the more viscous oils when the A-B distance was 8.9 cm. An all-glass, vapor-bath type, however, is recommended if many determinations are to be made.
Procedure T h e operation of the viscometer is so simple t h a t detailed description is scarcely necessary.
FIGURE 4. BACKOF MICROVISCONETER
Two capillaries, of different bore, for different viscosity ranges, are jacketed and mounted a t the front of the frame, near the sides. Between them is a small "showcase" lamp, protected by a metal plate shade, which also serves to truss the frame and support a calibration chart (not shown). Directly behind the lamp is a small boiler supplying vapor for temperature control, and a reflux condenser. The boiler heater is operated in parallel with the lamp from a small switch in the base. Two vacuum controls are placed a t the back corners of the frame. One, using s-tetrabromoethane, regulates the pressure at the top of the condenser, and hence the capillary temperature. Temperatures are read on an A. S. T. M. Saybolt thermometer hanging with its bulb above the liquid in the boiler. For Pittsburgh's elevation, methylene chloride, acetone, and diisobutylene are satisfactyy for temperatures of 37.78", 54.44",and 98.89' C. (loo", 130 , and 210" F.), respectively. The other bubbler, filled with kerosene, controls the suction used to draw the sample into the capillary. For recise work, it should be adjusted according to the density of tfle unknown, but for petroleum products a mean fixed setting has been found satisfactory. Both adjustments are readily made with a valveless rubber atomizer bulb communicating with the reservoirs. The apparatus is assembled with glass seals throughout. The model described is easily portable and entirely self-con-tained. Connections are required to a 110-volt electric line (power demand about 100 watts), vacuum, cold water, and drain. Although the glass part is complex, it is so compact and well protected by the frame that the unit is very rugged. Two such
First the boiler is started. After steady operation has been obtained, as indicated in the condenser, the temperature is adjusted with the right-hand vacuum control. Response is complete within a few seconds, and seldom more than two or three trials are necessary t o set the temperature to within 0.01" C. (0.02' F.) of that desired. Occasional small readjustments are sometimes needed to follow the changes in barometric pressure through the day. The left-hand control should be set so that its head of kerosene is equal to twice the head of the sample column, but a fixed setting may be used for most oils. This adjustment is not critical. The liquid is best applied to the capillary with a small wire loop, similar to those used in blowpipe bead tests. After the drop is in place, suction is applied to the tube by o ening the appropriate three-way stopcock a t top front. As t l e sample rises, a filter paper or lintless cloth is held in readiness to wipe the end clean. This may be done in several ways. With a little practice, an operator can learn to break the column without interrupting the suction just as the meniscus passes the first mark. A slower method, perhaps better, especially for light oils, consists in drawing the column up slightly beyond mark C, turning the stopcock to open the upper end of the tube to the air, and simultaneously pressing the wiper squarely and firmly against the lower end of the tube. The meniscus will then fall slowly, as the liquid is absorbed in the wiper, and may easily be caught exactly a t the mark by turning the stopcock back to the vacuum side, which will cause the column to break sharply a t the lower end of the capillary. The liquid segment is now allowed to rise above A , the upper end of the tube is opened to the air by means of the stopcock, and the time required for the upper meniscus to pass from that mark to B is recorded. No waiting is necessary, since the liquid comes to temperature almost instantly. Alternatively, to obtain a shorter flow time for heavy samples using the same pipet, the liquid is raised so that its lower meniscus is just above mark B, and its time of fall from that point to C is observed. In this way a two-capillary unit may have four ranges. Choice of proper range is important; flow times should be kept between 100 and 1000 seconds, the first limit for accuracy, the second for convenience. Check runs may be made, after a test has been completed, using the same sample column. For cleaning the capillary, 2 or 3 drops of volatile solvent are poured into it by removing the tiny ground stopper a t the top of the apparatus, and wiping the solvent off a t the lower end as it comes through. If the solvent used boils just a few degrees above the working temperature, the tube may be dried in a few seconds by suction. An occasional thorough cleaning (once a month during continuous operation) with dichromate-sulfuric acid solution is recommended.
Acknowledgments The writer is indebted to S. Frederick Kapff for some of the work of calibration of the instruments, and for many of the details of the procedure, and t o William E. Barr, of the Gulf Research and Development Company, for his skillful and patient glass working.
Literature Cited (1) Levin, H., IKD.ENG.CHEM.,Anal. Ed., 9,147 (1937). (2) Lidstone, F.M., J.SOC.Chem. Ind.,54,189T (1935). PRESENTED before the Division of Petroleum Chemistry a t the 97th Meeting of t h e American Chemical Society, Baltimore, Md.
