498
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 11 illustrates the results of fractionating, as an unknown, a drop of a mixture of equal volumes of ethyl acetate and butyl acetate. The work shows the very large number of fractions that can be collected from a single drop. Each fraction weighed not more than 0.43 mg. This determination was stopped as soon as a constant boiling point of the butyl acetate portion was assured. I n Figure 12, the boiling points of two of the three components present are judged correctly. Presence of a t least three substances is clearly evident. The results show that a satisfactory determination can be made on a component present to the extent of 33 per cent only. The minimum concentration for which correct determinations can be made by
Vol. 13, No. 7
this method lies between the 20 per cent level observed in Figures 8 and 9 and the 33 per cent content in Figure 12. Literature Cited (1) Benedetti-Pichler, A. A., and Spikes, W. F., “Introduction to
Microtechnique”, Douglaston, N. Y., Microchemical Service, 1935. (2) Emich-Schneider, “Microchemical Laboratory Manual”, p. 34, New York, John Wiley & Sons, 1934. (3) Gettler, A. O., Niederl, J. B., and Benedetti-Pichler, A. A., Miki-ochemie, 11, 174 (1932). (4) Lanyar, F., and Zechner, L., Monutsh., 43, 405 (1922). (5) Morton, A. A,, and Mahoney, J. F., IND. ENG.CHEM.,Anal. Ed., 13, 498 (1941). CONTRIBUTION No. 244 from the Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology.
Copper Blocks and Optical System For Determining Boiling Points (Emich Method) and Melting Points M
AVERY A. MORTON AND JOHN F. MAHONEY Massachusetts Institute of Technology, Cambridge, Mass.
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BSERVATIONS of boiling points in capillary tubes by the Emich method (1) involve use of a liquid I I I I heating bath. The possiA B C bilities in cleanliness and high temperatures make the FIGURE 1. CONSTRUCTIOK OF COPPERBLOCKFOR CSEWITH BOILING POINT CAPILLARIES ~. A , top view. B , side view parallel t o light beam. C,.side.view at right angles $0 (facing) light beam. Visual construction of a metal block observation can be made through slits 8 if a strong light is placed on t h e far side. T,thermometer hole. H, very desirable. Figure 1 holes for boiling point capillaries in mica cover plate. M , mica cover plate. C Pyrex glass plates 3.5 mm thick for passage of light through block. R , resistance wire (40 feet of No. 30). ‘ B p , binding posts. Blook is shows a form which has 3 S cm. across and 10.2 cm. high. proved very useful. By means of an optical system (Figure 2) the inverted image of the boiling point capillary A and drop can be observed on a paper screen with a minimum of eyestrain. The determination of the boiling point in this apparatus need not vary from the usual practice in a liquid bath. The block is heated by the resistance wire until the droplet ap/ pears above the surface of the metal. Heating is then discontinued, the block is cooled by the air jet, and the determination is repeated until the values can be duplicated. When the apparatus is used for a series of boiling points as in the fractionation (9) of a drop of liquid, the repeated observation on a single fraction consumes too much time in a process which is itself time-consuming. I n such a case the L4 block is heated to within about 5’ of the boiling point before insertion of the boiling point capillary. A small amount SC P B C L of vapor sometimes condenses in the capillary above the surface and hinders the upward movement of the droplet through FIGURE 2. OPTICALSYSTEM FOR BOILING POINTDETERthe length of the tube. This difficulty can often be avoided MINATION by breaking the capillary until it protrudes just above the Sc, screen. P , projection lens B t o p view of cop er block for boiling point determination. A , air b l k t dkected against si& of block. T,8 , H, surface, so that the vapor escapes. Such treatment occaa n d G, as in Figure 1. C, condenser lens, 8.5 inches focal length. L,100 sionally results in a failure to get any value for one or two of watt lamp. -
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499
ANALYTICAL EDITION
July 15, 1941
is difficult to attain in individual cases. T h i s a p p a r a t u s was used in most of the observations recorded by the authors (2).
MELTING POINTBLOCK. The authors constructed a copper block from 1.5 inches (38 mm. square) of copper for use with the same optical system (Figure 3). T h e r e l a t i v e l y large opening in the side needed for the use of the optical system is protected by two Pyrex glass plates fastened closely t o the side of the block by bolts as illustrated. A , top view looking down upon mica cover glass showing thermometer with cross slit and holes for capillary melting point tube. B , side view facing beam of light. C , side view parallel t o light beam. N , 1.5-mm. hole for melting point tube. L,5-mm. hole for light. G, PyFex glass plates fastened t o copper block by cross bolts, S . R , resistance wire (40 feet of No. 30). B p , bindmg posts. Blocks are 3.8 cm. across and 10.2 cm. high.
Literature Cited (1) Emich-Schneider,
the fractions, but the loss is not serious where so many determinations are made and a curve is plotted, ~~~h separate determination in a series can usually be made more accucase, for the ‘perator rately than is possible in an soon acquires a perfection in every detail of operation that
“AVi-
crochemical Laboratory Manual”, New York, John Wiley & Sons, 1934. (2) Morton, A. & and Mahoney, J. F., IND.E m . C H E M . , - h a l . Ed., 13,494 (1941). CONTRIBUTION from the Research Laboratory of Organic Chemistry, hlassachusetts Institute of Technology, NO. 245.
A Photometric Method for the Determination of Magnesium W. SHERIIlAN GILLAMI IIZichigan State College, East Lansing, Iklich.
I
N A recent investigation involving the analysis of several soil extracts and fertilizers, it was necessary to determine fairly small quantities of magnesium. As a rapid and reliable method was desired, i t was concluded that a colorimetric method, if sufficiently sensitive, might afford the most suitable procedure. Several organic compounds which give color reactions with magnesium were studied, but none yielded as sensitive a test for this ion as did titan yellow. In an alkaline solution magnesium ions react with this compound to produce a red or pink color, which can be readily matched with standards or read in a photoelectric colorimeter. The use of titan yellow for the determination of magnesium was first suggested by Kolthoff (d), who stated that it was capable of detecting 0.2 part of magnesium per million of solution. He suggested using it in the analysis of tap water but reported only one determination. I n this investigation the method has been adapted t o soil and fertilizer analysis and to a photoelectric colorimeter. 1
Present address, Purdue University, Lafayette, Ind.
Preliminary Investigation Some difficulty was originally encountered in retaining the pink coior deveioped after the addition of sodium hydroxide. I n many instances the color faded rapidly and consistent readings could not be obtained; a small amount of hydroxylamine hydrochloride effectively stopped the fading. I n order to determine the stability of the color developed, various solutions of known concentration were read a t intervals in the photometer over a period of several hours. The instrument was set on zero per cent light absorption and if necessary, adjustments were made to bring the galvanometer to its null point after each reading. The results obtained were very consistent-for example, one solution containing 1 p. p. m. of magnesium fead 31, and when read a t 20-minute intervals for the next 3 hours gave 31.0, 31.0, 31.0, 30.8, 30.8, 30.8, 30.5, and 30.5. Solutions containing 4 p. p. m. of magnesium were found to be stable for 1.5 hours, whereas solutions containing 3 p. p. m. of magnesium mere stable for 4 hours, and the more dilute solutions were stable for over 12 hours. Ammonium ions up to a maximum concentration of 500 to
