Semi-Micromethod of Analysis for Nitrogen ANTHONY R. RONZIO, University of Colorado, Boulder, Colo.
The sample, 12 to 20 mg., weighed to 0.01 mg. on an analytical balance (5 to 10 minutes), should yield about 1.0 to 2 cc. of nitrogen, and is prepared for analysis as described by Pregl. After the combustion tube is connected to the nitrometer and carbon dioxide generator, the stopcocks are opened and the tube is scrubbed free of air. While the air is being removed, the burner, c, is lighted and the xylene brought to boiling as soon as possible. The flame is then lowered so that onlv gentle boiling occurs during the entire a n a ry s i s . When the tube is free from air, the nitrometer is filled with potassium hydroxide and the c o m b u s t i o n performed in the usual manner. The rate of burning may be such that as many as three to four bubbles of gas per second rise in the nitrometer, but a much faster rate for a few seconds does not endanger the result. The time required for burning the sample is usually about 30 to 45 minutes. The complete determination from the time of weighing of the sample until the results a r e o b t a i n e d usually requires an hour.
FIGURE 1. DIAGRAMOF APPARATUS
T
With a freshly filled tube high results are obtained for the first 7 or 8 analyses, because traces of oxygen are generated by end of the combustion tube catalytically reduces the carbon the iodine pentoxide. This may be redioxide to carbon monoxide, and (2) this and the intermediate moved from the gas by washing with a little products of combustion are often incompletely oxidized unless pyrogallol-potassium hydroxide solution the burning produces not more than two bubbles per 3 secthrough funnel e (Figure 2). The only onds. other difficulty encountered in a series of The semi-micromethod described here avoids both difficulover two hundred analyses was the rare ties by placing at the end of the tube a heated section conFIGURE 3. case where methane, a difficultly comMERCURY taining iodine pentoxide, made by heating iodic acid in an oil VALVE bustible gas, was generated as one of the bath for an hour a t 170" C . At this temperature a minute products of decomposition of the burnamount of iodine pentoxide is decomposed, so small, however, ing c o m p o u n d and was not that it may be neglected. 1 completely b u r n e d (1). In A section of the crushed oxide this case there was no de13om. long, a, is placed in a comcrease in volume after washbustion tube 56 cm. long and ing the gas first with pyro1 cm. in d i a m e t e r (Figure 1). The section is kept dehydrated gallol-potassium h y d r o x i d e by enclosing it in a copper s o l u t i o n , a n d then with jacket, b, c o n t a i n i n g boiling ammoniacal cuprous chloride xylene (b. p. about 120' C. at solution. Boulder, Colo., about 130' C . a t 760 mm.). Since this temWith careful work analyses perature even in high altitudes do not vary more than *0.05 is well above the melting point per cent. of iodic acid (110' C.), the iodine A modified nitrometer dep e n t o x i d e is kept sufficiently d e h y d r a t e d and active. The vised for use with this apparaxylene is heated by burner c. tus (Figure 2) consists of a n Following this are a section of absorption tube connected by cupric oxide 5 cm. long and a means of a three-way stopcock section of reduced copper 5 cm. long. This is followed by a sect o a g a s b u r e t surrounded tion of cupric oxide 15 cm. long, by a water jacket. The stopkept permanently in place by a cock, a, is a capillary threesmall roll of copper gauze. The way stopcock of bore not exPyrex condenser, d, should have at least four bulbs, each of about c e e d i n g 1.5mm. T h e g a s 150-cc. capacity to avoid danger b u r e t , b, is g r a d u a t e d in from fire, and is sealed t o the 0.05 cc. and can be read accucopper jacket with cementite, e. rately to 0.01 cc. The asbestos screens, f, prevent heating of the nitrometer. The When t h e c o m b u s t i o n is combustion tube is connected finished the gas collected in to the nitrometer by means of a the a b s o r p t i o n buret, d, is rubber stopper, g, which is covtransferred into the buret, 6, ered with a short piece of glass tubing to prevent diffusion of and the volume read in the air into the apparatus. The usual manner. c o m b u s t i o n stand and set-up The mercuric iodide which are similar t o t h a t of Pregl forms in the nitrometer inlet FIGURE 2. MODIFIEDNITROMETER (3).
HE Dumas method for nitrogen (3) suffers from two important drawbacks: (1) the copper spiral placed at the
n
.
122
MARCH 15, 1936
ANALYTICAL EDITION
should be removed after about 40 combustions. It is advisable to treat all rubber tubing by Pregl's method to prevent diffusion of gases. Figure 3 shows a diagram of a mercury valve which replaces the one used by Hein in his Kipp generator (2). It is simpler and the materials for making it are more easily available.
Summary The advantages of the semi-micromethod for nitrogen are: an ordinary analytical balance is used; the sample may usually be burned in the same length of time as that required for the microsample; using the modified nitrometer, the volume of nitrogen may be read in 2 minutes; a sudden increase
123
in the evolution of gas above the normal rate for a few seconds has no effect on the result; and special training is not necessary for accurate work. A simple and easily made valve for the Kipp generator is illustrated.
Literature Cited (1) Dennis and Nichols, "Gas dnalysis," revised ed., p. 159, New York, Macmillan Co., 1929. (2) Hein, Z.angew. ~ h m .40, , 864 (1927). (3) P w l , "Quantitative Organic Microanalysis," 2nd ed., PP. 81-4, 90, 94-5, Philadelphia, P.Blakiston's Son & Co., 1930. R~~~~~~~
J~~~ 4, 1935.
Studying the Hardness of Butter Fat Apparatus and Routine Procedure WILLIS D. GALLUP, Oklahoma Agricultural Experiment Station, Stillwater, Okla.
IN
A STUDY of factors which influence the physical properties of butter fat there was need for an inexpensive piece of apparatus which would measure accurately the relative hardness of different samples of butter fat. Available pieces of apparatus designed to measure the hardness of materials of similar consistency were not found applicable for various reasons, chief among which were lack of precision and inconvenience of operation within a small constant-temperature chamber. Perkins (2) in discussing several types of apparatus used in studies of f a t hardness points out the inadequacies of each and describes a device which determines hardness by the depth of penetration into the fat of standardsize needles falling through a given distance. Apparatus based on this principle require relatively large samples of fat and the application of different weights, depending on the hardness of the fat and the diameter of the needle employed. Coulter and Hill (1) studied butter hardness with a modified apparatus designed to measure the body of cheese. It was desired to determine the hardness of butter fat a t ordinary temperatures with an apparatus which would operate within a constant-temperature water bath and give dependable results over a wide range of values. After building and testing several pieces of apparatus to determine hardness by the depth of penetration of needles of different sizes, by the weight required to crush or force plungers through a given amount of fat, and by a determination of the time required for plungers to sink a given distance into fat, the device here described was found most satisfactory. With this device, hardness is expressed as grams of mercury required to force LL plunger 5 mm. in diameter through a disk of butter fat 6 mm. in thickness a t 20" C. The fat disks are made in a two-piece mold which is thoroughly chilled by placing it on the metal divisions of a tray of ice cubes. The base of the mold is a brass plate commonly used in histological technic for paraffin imbedding. A lead ring 4 cm. in diameter and exactly 6 mm. high forms the side of the mold. The melted fat, after thorough mixing at 50" C., is slowly poured into the mold and allowed to cool. A small label bearing the number of the sample is inserted at the edge of the sample for the purpose of identification. When thoroughly hardened, the top of the sample is leveled off even with the top edge of the mold with a straight-edged knife. The whole thing is then placed in ice water and after removing the base of the mold the fat disk is forced out of the ring with very slight pressure. A thin sheet of paper moistened and pressed firmly against the base of the mold t o remove air bubbles may be used t o prevent the sample of fat from sticking to the base as it hardens. Usually 2 or 3
disks are prepared from each sample of fat and kept in ice water overnight or longer. Before the disks are ready for hardness determinations they must be kept for an arbitrarily chosen length of time at the temperature at which the determination is to be made. Preliminary investigations revealed that the disks soften rapidly during the first 3 or 4 hours when taken from the icewater bath and placed in the 20" C. bath. After 6 hours very little change in hardness takes place, Since 20" C. has been chosen as the optimal temperature for making these determinations, the disks are kept in a constant-temperature bath held a t 20" 0.2" C. for 8 hours previous to performing the test. Above 20" C., soft samples of butter fat are difficult to handle; a t lower temperatures hard samples tend to crack during the determinations. The hardness determinations are made with the apparatus sketched herewith. The fat disk, A , 6 mm. thick, is placed over the 9-mm. hole of the cork, B, which is cemented to the bottom of the water bath, C. The glass rod, D,5 mm. in diameter working in the glass sleeve, E, is brought to rest on the disk. The plunger is 19 mm. in length and together with the mercury receptacle, G, and its support, H , weighs 50.0 grams. Mercury from the 25-cc. buret, F, is now run into the 50-cc. beaker, G, at a constant rate of 10 cc. per minute. The tip of the buret is approximately 1 cm. above the bottom of the beaker. The flow of mercury is stopped when its weight becomes sufficient t o foGe t h e plunger through the disk of fat. When this occurs the rubber support, H , comes tto rest with a sudden thump on the top of the sleeve, E. The volume of mercury is d e t e r m i n e d from the buret readings and its weight caloulated. The entire apparatus can be made of ordinary laboratory equipment. There are no mechanical parts to require special attention or repair. Constant temperatures are easily maintained with two water baths, one a t the temperature of a mixture of ice and water and the other close
BP
