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INDUSTRIAL AND ENGINEERING CHEMISTRY
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bubbles comes from the tip of the ebullition stick, B. I n order to determine the boiling point, experience has shown that a correction of a few degrees must be subtracted from the temperature of the oil bath a t this point. The boiling point is of the same order of accuracy as the boiling point taken by the Siwoloboff micromethod (1) for taking boiling points. An indication of the boiling point can be obtained only when the liquid is a pure compound, since the temperature recorded gives the boiling point of the liquid and not that of the distillate. When fractionation is necessary, each fraction is redistilled and the true boiling point thus approached. The liquid to be distilled is placed in the bulb, A , by means of a capillary pipet and the ebullition stick, B, is then replaced. B fits the capillary of the apparatus rather snugly and F~GURE 2. allows passage of the vapors of the liquid to the condenser, C, essentially only a t the boiling pointof the liquid. Herein lies the advantage of the apparatus described, since, without the capillary, vapors would pass from the liquid to the condenser a t a temperature considerably below the boiling point because of evaporation and rapid diffusion to the condenser through the larger opening. The conditions presented by distillation
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from the customary flasks used for macrodistillation are thus more nearly reached by use of the capillary constriction which is nearly filled by the ebullition stick, B. As many fraction8 as desired can be taken during each distillation. Each fraction, however, corresponds to only a flash distillation, as there is no fractionation from reflux. A complete distillation can be done very quickly and is practically quantitative, as only the condenser is below the temperature of the boiling point of the liquid and there is no loss due to a large condenser surface. Accordingly, the process may be repeated several times and thus arrive a t a fractionation in the truest sense of the word. Distillations under reduced pressure can be made equally well by evacuating the flask a t the opening, E, although one of the usual means of avoiding fluctuations of the pressure is necessary since the distillation must take place without bumping. Thus, the apparatus is adaptable for distillation of a wide range of substances at temperatures and pressures a t which any distillation can be accomplished on the customary macroscale. When the volume of the distillate is more than 0.2 cc., an apparatus of the type shown in Figure 2 has been found to be useful. This operates in principle like the previous apparatus, with the exception that provision has been made for the collection of the larger volume of distillate by modification of the condenser. A capillary tube is fitted to the top of the condenser at e with a small ground-glass joint and leads to a mercury leveling bulb, the purpose of which is t o change the pressure in the collection chamber, a, so that it can be made less than that in b. By proper adjustment, the droplet of distillate as it collects at c will be forced up through the capillary, d, and into a,where it can be obtained with a capillary pipet after the distillation is finished.
Literature Cited (1) Siwoloboff, Ber., 19, 795 (1886). RECEIVED February 20, 1936.
Borax as an Acidimetric Standard FRANK H. HURLEY, JR., Department of Chemistry, Rice Institute, Houston, Texas
T
H E use of borax, NazB407~10Hz0, as a primary standard in acidimetry was suggested many years ago (8). Experimental work tending to confirm its suitability for this purpose has appeared from time to time (1, 4, 6), but it has never been as generally used as its many advantages would seem to warrant. Kolthoff ( I ) , in comparing the relative merits of several substances as acidimetric standards, states that “the substance most to be recommended as a standard is borax.” Lindfors (3) recommends it highly for routine commercial work. The most probable reason for the failure of borax to come into common use is that it is a hydrated compound, and such compounds are not usually suitable for use as standards because of the difficulty of drying them under conditions such that none of the water of hydration is lost. The methods which have hitherto been proposed for drying borax are not entirely satisfactory. The process of simple air-drying of recrystallized borax recommended by Lindfors (S) is not very safe, because the solution which remains on the surface of the crystals is alkaline enough to absorb carbon dioxide from the air with the formation of sodium carbonate or bicarbonate as an impurity (2). The technic of drying borax over moist sodium bromide, which is recommended by Kolthoff (6),is open to two practical objections-namely,
that sodium bromide is relatively expensive, and that the drying process requires considerable time. Moreover, the suitability of this desiccant has recently been questioned by Menzel (5) on theoretical grounds. Menzel proposes in place of sodium bromide a solution saturated with respect to both sugar and salt (sucrose and sodium chloride), since this solution was found to have the proper vapor pressure for maintaining borax in the decahydrated state over a wide range of temperature. This last desiccant, while it is apparently free from all theoretical criticism, is nevertheless objectionable because of its slow action. Unless an easy, rapid, and exact method for drying borax is available, its usefulness as a standard substance is obviously limited. In the present paper a very easy method of drying with alcohol and ether is described, and the purity of borax dried in this way is tested by comparison with borax dried by Menzel’s method and also with pure sodium carbonate.
Purification and Drying of Borax Pure borax was prepared by the recrystallization of the P. material, using quantities of borax and water (50 ml. of water to every 15 grams of borax) such that no crystallization could occur above 55’ C., thus eliminating the possi-
c.
ANALYTICAL EDITION
MAY 15, 1936
bility of the formation of the pentahydrate of sodium tetrapentaborate (transition temperature, decahydrate hydrate, 61 " C.). The borax crystals were freed from mother liquor by suction, then washed twice with water.
*
The first method of drying consisted in washing the damp crystal8 with two portions of 95 per cent ethyl alcohol, followed by two portions of ethyl ether, each washingbeing followed by suction to remove the wash li uid. The portions of alcohol and ether used were about 5 ml. jor 10 grams of the borax crystals. This drying treatment has been applied with success in analytical work to magnesium ammonium phosphate hexahydrate. Following the treatment with alcohol and ether, the borax was spread out in a thin layer on a watch glass and allowed to stand at room temperature to permit the evaporation of the ether, The second method of drying consisted in spreading the damp crystals on a watch glass and placing it in a desiccator over a solution saturated with respect t o both sugar and salt, the desiccant recommended by Menzel.
Stability of Borax Before proceeding to analytical tests of the purity of borax prepared and dried by the methods described, it was desired to gain some information about the stability of the substance in air a t room temperature, and in the presence of saturated sugar and salt solution. One of the principal advantages in the use of borax is that it may be weighed directly in air, since it is not hygroscopic. It has long been known, however, that borax loses its water of crystallization very slowly when kept in air a t room temperature. I n order to determine the rate a t which this loss occurs, a 3-gram sample of borax, dried with alcohol and ether and allowed to stand overnight to permit the evaporation of the ether, was placed in a crucible and allowed to remain for a long period of time in contact with air a t room temperature (24" to 33" C.). From time to time the cruoible was weighed and the loss in weight noted. The loss of weight a t the end of several periods of exposure was as follows: 5 days, 0.027 per cent; 17 days, 0.050 per cent; 25 days,, 0.076 per cent; 52 days, 1.04 per cent. During the first 25 days the loss in weight was regularly about 0.1 mg. per day. Between the fifty-second and fifty-third days the loss amounted to 1.6 mg. It appears, therefore, that the rate of loss of water, though very small a t first, increases with time. It seems probable from these results that leaving borax for as long as a week or 10 days would be safe in ordinary work, except under very unusual conditions of temperature and hurnidhy . The stability of borax in the presence of saturated salt and sugar solution was also tested. Menzel (6) found that the maximum variation in weight of a sample of borax kept for 12 months in a vacuum desiccator over this solution amounted to only -0.02 per cent. To test the efficacy of this solution in the drying of borax, 3.5 grams of damp borax crystals were spread out on a watch glass and placed in a desiccator over this solution. After 3 days the crystals appeared to be quite dry. The loss in weight on the fourth day amounted to only 0.1 mg., and after 52 days to 0.9 mg. (0.026 per cent). Another 3.5-gram sample of borax which had been dried with alcohol and ether also showed a loss of 0.026 per cent in weight on standing over sugar-salt solution for 55 days. It appears from these measurements that the desiccant proposed by Menzel is satisfactory, and that borax may be safely stored over sugar-salt solution for an indefinite length of time. A further study of the stability of borax is now in progress.
Purity of Borax Dried with Alcohol and Ether To ascertain whether the alcohol-ether method of drying yields pure decahydrated borax, samples purified and dried by this method were compared with another sample dried by Menzel's method and also with pure sodium carbonate,
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by means of hydrochloric acid. The borax dried with alcohol and ether was allowed to stand overnight before it was used. That dried over saturated sugar and salt was allowed to remain in the desiccator for a week. The pure sodium carbonate used for the comparison was prepared from c. P. sodium bicarbonate by the method of Reinitzer (7), and was dried to constant weight by heating a t 270" to 300" C. Standard solutions (0.05 M ) of the borax samples and of sodium carbonate were prepared and 25-ml. portions of the solutions titrated with approximately 0.1 M hydrochloric acid. All the instruments used in the preparation of the standard solutions and in the titrations were calibrated to give an accuracy of 0.1 per cent or better. In the borax titrations methyl red was used as the indicator, and the end point was matched with a color standard prepared by adding 5 drops of methyl red to 70 ml. of a solution containing 1.0 gram of sodium chloride and 2.2 grams of boric acid in 500 ml. of water. This solution represents approximately the stoichiometrical point of the titration of 25 ml. of 0.05 M borax with 0.1 M hydrochloric acid, assuming 20 ml. of wash water to be used in the titration. This color standard must be boiled to remove carbon dioxide which may be dissolved in the water. If properly prepared it has a straw color, different from the pure yellow of methyl red in alkaline solution, but not yet pink. By the use of the color standard the borax titration may be performed without titration error (d), for the stoichiometrical point of the titration comes within the range of color change of methyl red. However, the use of a color standard in routine work with borax is practically superfluous, since the color change of methyl red occurs so rapidly and is so distinct that the first change from pure yeIlow may be taken as the end point without making any serious error. The sodium carbonate titrations were performed a t the boiling point, using methyl red. Under these conditions the titration error amounts to less than 0.01 ml. of 0.1 M acid, which is beyond the limit of error of the buret readings. Methyl red was chosen in preference to methyl orange for the carbonate titration because the indistinctness of the methyl orange end point renders the measurements uncertain. The molarity of the 0.1 M hydrochloric acid was calculated from titrations against three samples of borax, two of which were dried with alcohol and ether and one over sugar-salt solution, and from one sample of sodium carbonate. All four titrations gave the same molarity for the acid (0.1024 M ) . This concordance indicates that the alcohol-ether method of drying borax is entirely satisfactory. In view of the ease with which the method may be carried out, the only disadvantage which has hitherto prevented the extensive use of boraxnamely, the difficulty of obtaining the dry salt-appears to be now obviated.
Acknowledgment The writer wishes to thank Arthur F. Scott for his interest and cooperation in this work.
Literature Cited (1) Kolthoff, J. Am. Chem. Soc., 48, 1447 (1926). (2) Kolthoff and Furman, "Volumetric Analysis," Vol. 11, pp. 93-6, New York, John Wiley & Sons, 1929. (3) Lindfors, IND. ENQ. CHEM.,15, 1047 (1923). (4) Mellon and Morris, Ibid., 17, 145 (1925). (5) Menzel, 2.anorg. aZZgem. Chem., 224, 10-13 (1935). (6) Milobedski and Kaminska, Bull. 806. china., 41, 957 (1927). (7) Reinitser, 2. angew. Chem., 7 , 551 (1894). (8) Salser, 2.a n d . Chem., 32, 529 (1893). RECEIVED February 14, 1936.
