MARCH 15, 1938 (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
(15) (16) (17)
161
ANALYTICAL EDITION
Bredig, Franck, and Fiildner, Z. Elektrochem., 38, 158 (1932). Britton, J . Chem. Soc., 130, 614 (1927). Cameron and Hurst. J . Am. Chem. Soc., 26, 885 (1904). Davey, Gen. Elec. Rev., 29, 127 (1926). Davies, Chem. News, 64, 287 (1891). Dieckmann and Houdremont, Z . anorg. allgem. Chem., 120, 129 (1922). Drakunov, Udobrenie i Urozhai, 2, 409 (1930). Glass and Jones, Quart. J. Phurm. Pharmacol., 5, 442 (1932). Hendricks, Hill, Jacob, and Jefferson, IND. ENQ.CHEM.,23, 1413 (1931). Hinds, quoted from Larson (BO). Holt, La Mer, and Chown, J. Bid. Chem., 64, 509 (1925). de Jong and Kruyt, Kolloid-Z., 50, 39 (1930). Julian, Am. J . Sci. and Arts, series 11, 40, 367 (1865).
(18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
Klement and Trdmel, 2. physiol. Chem., 213, 263 (1932). Korber and Tramel, 2. Elektrochem., 38, 578 (1932). Laraon, 1x0. ENQ.CHEM., Anal. Ed., 7, 401 (1935). Naray-Szabo, Z. Krist. Jfineral., 75, 387 (1930). Rindell, Compt. rend., 134, 112 (1902). Romberry, Hastings, and Morse, J. Biol. Chem., 90, 395 (1931). Schleede, Schmidt, and Kindt, 2 Elektrochem., 38, 633 (1932). Taylor and Sheard, Proc. Soc. Ezptl. Bid. Med., 26, 257 (1928). Tromel, Phosphorsaure, 2, 116 (1932). Trbmel and Moller, 2. anorg. allgem. Chem., 206, 227 (1932). Washburn and Shear, J . B i d . Chcm., 99, 21 (1932). Wendt and Clark, J. Am. Chem. Soc., 45, 881 (1923).
RECEIVED M a y 13, 1937. The work was supported by a g r a n t from the Rockefeller Foundation.
Analysis of Caustic Liquors for Traces of Impurities 0. S. DUFFENDACK
AND
R. A. WOLFE, University of Michigan, A n n Arbor, Mich.
T
HE technic for the spectrochemical analysis of caustic liquors for small traces of aluminum, calcium, chromium,
copper, iron, lead, magnesium, mankanese, silicon, and strontium has been worked out a t the Physics Laboratory of the University of Michigan for The Mathieson Alkali Works, Inc., under the auspices of the Department of Engineering Research. It became important in the alkali industry to be able to determine correctly the amounts of certain elements present in caustic liquors, especially the caustic materials supplied to the rayon industry. Some of the elements cannot be determined very precisely by chemical methods in the range of abundance in which they occur in caustic liquors. The problem was undertaken because the development of spectroscopic methods had reached the point where it seemed possible to attain the necessary precision in this manner. I n addition, saving of time is an important item and is enhanced because the spectroscopic analyses for all the elements can be carried out in one operation. The problem was not a simple one. The material to be analyzed was caustic liquor, a solution of sodium hydroxide, .01 009 008 ,007 ,006 ,005 .004
,003
d z N
.\.'
,0002
.0001
which could be controlled as to strength; but, no matter what the concentration of the solution, the ratio of the impurities to the sodium hydroxide was extremely small. Since sodium is ionized and excited very easily, it was a problem to find a type of source in which the impurities would be sufficiently excited in the presence of the overwhelming abundance of sodium to permit accurate measurements on their lines. The source had to possess all the characteristics of constancy and sensitivity required for precise analytical work. The method employed has been previously described (8). It is a method of internal control (8) in which the analysis is made from measurements on the relative intensities of spectral lines of the test elements and of a control element which is present in or is introduced into the specimen in a definite and constant amount. The relative intensity of such s pair of lines is therefore a function of the abundance of the test element, and this function must be determined for each element. By measuring the relative intensities of a selected pair of lines excited in a suitable spectroscopic source for a series of prepared solutions in which the amount of the test element is varied over the range desired, the required function can be discovered. The function is usually expressed as a relationship between the logarithm of the relative intensities of a selected line of the test element and of the control element and the logarithm of the percentage abundance of the test element. The graph of this function is used as the working basis for the determination of that element. Those lines are used ordinarily which give a linear function when plotted as described; a typical working curve is shown in Figure 1. The relative intensities of the spectral lines are measured by well-proved methods of spectral photometry (4). The technic has been developed to such a degree of reliability that repeat measurements on a single plate agree within * 1 per cent and on different plates usually within *3 per cent. The principal problem, therefore, in the development of a technic for spectrochemical analysis lies in the discovery or development of a suitable spectroscopic source of sufficient constancy.
Development of Source /
I
LOG AI
3090 M o 31 58
FIGURE1. TYPICAL WORKIWQ CURVE
After trying several sources suggested by previous experience, the high-voltage alternating current arc (1) ww found the most suitable. This consists of an arc between two electrodes of carbon of the highest purity, upon each of which a drop of the test solution has been dried.
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
162
r----I
1
I
I
I I
I
I
I I I
I I
I
I
I I
I
I
I
I
I
1
I
1 1 I
I
I
I
I
I I
1 I
I
~
FIGURE 2. ARC STAND
II
VOL. 10. NO. 3
carbons covered with a thin film of solid material. The entire end must be covered, leaving no bare spots where the carbon is exposed, so that the arc plays at all times upon the test material and not upon the carbon. A Pyrex glass rod must be used for caustic liquors, as the sodium hydroxide dissolves soft glass sufficiently to affect the analyses for lead, silicon, and calcium. The electrodes thus prepared are placed in position in the spring clamps of the arc stand, but are not allowed to touch, so that the solid material is not disturbed. With the required voltage applied t o the arc gap, the electrodes are brought close enough together for the arc to strike. Bfter 30 seconds of operation, the circuit is opened and the gap is set at 0.5 mm. by bringing the electrodes into contact and then reversing the screw one full turn. The arc is now ready for record exposures. The arc is excited by an ordinary 5-kilowatt 60-cycle pole transformer, wired to produce 2200 volts at the secondary terminals. Sufficient resistance is introduced in series with the arc in the secondary circuit to limit the arc current to 2.5 amperes. For this purpose a bank of ordinary cone-type electric heater units has been found convenient. A sufficient number of these are used in combination so that they are not strongly heated by the arc current and the resistance remains constant. This is important because it is necessary to maintain a constant current during the exposure. As the resistors are in the high-voltage secondary circuit, it is wise t o mount them in a protecting cage as is shown in Figure 3, in which the entire apparatus is pictured. The effect of a variation in the arc current on the relative intensities of the spectral lines is shown graphically in Figure 4; the importance of maintaining a constant current is a t once apparent. I n pract'iae, a current fluctuation of more than =t 0.05 ampere will introduce errors greater than is generally desirable; therefore the primary current supply should be steady. The service lines should have sufficient capacity so that normal variations of the load on the line will not cause sharp fluctuations of the current. The arc gap must always be set a t the same value when exposures are made, berause the conditions of excitation of the spectra and the relative intensities of the spectral lines vary with the arc length, as shown by Figure 5 . Because the log ratios of intensities of a number of lines with respect t o those of the internal control have a maximum for an arc gap of 0.5 mm., this width was chosen as the standard.
The arc stand is shown in Figure 2. A , A are spring clips for holding the carbon electrodes; B , B are Bakelite insulators; and C is a screw of 0.5-mm. pitch which may be turned by an insulated handle, D. The lower electrode is adjusted in position Determination of Working Curves by sliding it on the uprights, and the upper electrode is set by means of the screw. As the arc is operated in a 2200-volt circuit Working curves for the analyses were determined from of considerable power, the operator must be guarded against danmeasurements on prepared solutions in which each test elezerous shock. For this reason, the aw qtand is enclosed in a proTectmg cage wit4 an automatic switch on the door, so that the circuit is broken when the do01 is opened. Theonly part of the arc stand protruding through the cage is the insulated handle €or adjusting the position of the upper electrode. The electrodes of the arc are made from the highest grade of spectrosco ic carbons. They are carefdy cut at right angles to their length, so that the ends are perfectly flat and smooth, and the edges are rounded off slightly with a clean file in ordei to,keepthe arc fromstrikingthe sharp edges of the electrodes. After the carbons have been prepared, a drop of the sample to be analyzed is placed on one end of each carbon. A glass rod is dipped into thesolution, prop erly adjusted as t o concentration andcontaining the internal standard, and the ends of the carbons are touch& with the droplet on the end of the rod. Sufficient liquid is thus transferred to cover the end of the carbon, the exact amount not being important. The solution is dried by any convenient FIGURE 3. SPECTROSCOPIC ARRANGEMEST method, leaving the ends of the
MARCH 15, 1938
ANALYTICAL EDITION
ment in turn was varied in percentage abundance. A single internal standard, molybdenum, introduced into the test sample as sodium molybdate, sufficed for all the elements. One cubic centimeter of sodium molybdate solution containing 2 per cent of molybdenum was added to 100 cc. of 25 per cent sodium hydroxide solution. The molykldenun; furnished spectral lines near enough to those of the test elements to serve as the internal control for all, and the lines selected were of such intensity that analyses could be made for all the test elements from a single exposure. The only exception was calcium, for which a separate exposure had to be made
163
A method for estimating the amount of a given element present in the stock solution had to be discovered. When the log ratio of the intensities of the selected lines was plotted against the log percentage abundance of the test element for a series of prepared solutions in which the amount of the test element varied systematically, the graph, instead of being a straight line as is to be expected (Figure 1). was curved in the range of the lowest concentrations of the test element.
GAP
WIDTH
IN
MM.
FIGURE 5
The departure from a straight line was in the direction expected if the materials from which the solutions were prepared contained a trace of the test element as a n impurity. A specimen graph is given in Figure 6. The fact that the graph becomes straight for larger amounts of the test element indicates that the amount present as an impurity in the materials is small compared with the amount added to make up the prepared solutions, and becomes negligible in ratio to the added amounts in the range of the larger per-
LOG RATIO FIGURE4. EFFECTOF VARIATION CURRENT
IN A R C
The exposures are made ordinarily without a condensing lens between the source and the slit of the spectrograph. When very pure caustic liquor was analyzed, however, so long a time was required to photograph the lines to a sufficient density for accurate measurement that the material on the carbons was depleted. To shorten the exposures for surh cases a pondensing lens was used, so placed that the convergent beam formed by it had a cross section of about 2 em. a t the slit of the spectrograph, and an image of the arc was formed a t the collimator lens. It was found best not to form an image of the arc on the slit, because such an image forms an erratic source on account of flickering as the arc wanders. The determination of the working curves presented a most interesting problem. I n some specimens of caustic liquor, impurities are present in extremely minute quantities; no purer solutions could be prepared, even from the best materials obtainable. I n order to extend the range of analysis to include these very pure specimens, it was necessary to make allowance for the presence of several elements in the prepared solutions used in determining the working curves.
LOG.Si 2881 Mo3158
FIGURE 6
J
INDUSTRIAL AND ENGINEERING CHEMISTRY
164
centage abundance where the line is straight. If now the straight portion is extended through the smaller percentage range, the amount of the departure of the curve from the straight line gives a measure of the amount of the element present in the original materials. Thus, a correction can be made for residual impurities and applied throughout the range of the working curves. This procedure was followed in determining the working curves for all the test elements.
termined in this way when present in amounts down to
0.1 mg. per liter. The same spectroscopic source has been successfully applied in the analysis of steels and other metallic alloys. Glass, carbon, and graphite products have been analyzed for metallic elements by modifications of this method, using the type of source described. Experience indicates that this source and general procedure are capable of extremely wide application.
Literature Cited
TABLE I. COXP.4RATIVE RESULTS Impurity
Spectrographic
Chemical I
70
%
SI01
0 050 0 0004 0 0106 0 0011 0 0005s 0 00004 0 000155 0 000054 N o trace
Fe&s A1203
Ca 0
AIR0 A-1
cu
Xl n
Cr
Chemical I1
% 0.054 0,0003 0.012 0.0003 0.0005 None
The method of analysis described above has been repeatedly tested for uniformity and acciiracy of results. Repeat analyses of the same specimen show a deviation from the mean of not more than 5 per cent of the amount presentwithin the deviation that might be caused by errors in measuring the densities of the spectral lines on the photographic plate and by the nonunifornlity of the plate. Check analyses of different specimens were made by chemical methods. I n Table I the results of the spectrographic method are compared with those of two independent chemical analyses made by analysts in different laboratories. The widest yariations are for those elements which are most difficult t o determine by chemical methods. TABLE 11. Impurity A1 a3 Ah08 C a BS CaO h l g as MgO Si as Si02 Cr cu Fe Mn Ni Pb Sr
VOL. 10, NO. 3
SPECTROGRAPHIC d X A L Y S I S Spectral Lines Range of Analysls Impurity M O 25% Caustic I.iquid A. ’ A. % 70 0 0001 - 0.014 3158 3092 0 000054 - 0 005 3903 4226 0 00005 - 0 036 2816 2795 0 001 0 10 3158 2881 0 00002 - 0 010 3158 2835 0 000010 0 005 3247 3158 0 00001 - 0 010 3188 3020 0 000002 0 00052 2816 2798 0 000075 - 0 010 3158 3414 0 00002 - 0 0034 2816 2833 0 00001 - 0 010 3903 4077
-
The spectral lines used for the determination of the several elements, those of the control element, and the ranges of percentage abundance for which working curves have been plotted are given in Table 11. Experiments indicate that it should be possible by modifications of the spectroscopic source to extend the ranges of analysis to still lower values, as well as to high values. The spectrograph employed was a quartz instrument of medium dispersion giving a ospectrum 20 cm. (8 inches) long for the range 7500 to 2100 A. Eastman polychrome plates were used. A metal step diaphragm (4) was used for producing the calibration patterns on the plates and a clear glass gas-filled 250-watt tungsten lamp was found satisfactory as continuous source for the production of these patterns.
Applications The method described for caustic liquors has been used successfully for determining traces of impurities-nickel and chromium plating solutions, various inorganic and pharmaceutical chemicals, and plastics. Urine and saliva have been analyzed for sodium, potassium, calcium, and magnesium by a similar procedure @),and the technic has recently been perfected for determining lead in urine, beverages, and other liquids, as well as organic tissues. Lead can be de-
(1) D u f f e n d a c k a n d T h o m s o n , PTOC. Am. SOC.Testing Jfuteriuls, 36, Part I1 (1936). (2) D u f f e n d a c k , Wolfe, and S m i t h , ISD. ENG.CHEM.,A n a l . Ed.. 5 , 226-9 (1933). (3) G e r l a c h , W., 2. anorg. allgem. Chem., 142, 389 (1925); Gerlach and Schweiceer, “ C h e m i s c h e Emissionsspektralanalyse,” Leipeig, Leopold VOSS,1930. (4) T h o m s o n a n d D u f f e n d a c k , S. Optical SOC.Am., 23, 101-4 (1933).
RECEIVEDDecember 7 , 1937. Presented before the Division of Physical and Inorganic Chemistry, Symposium on Quantitative Spectrographic Analysis, a t the 94th Meeting of the American Chemical Society, Rochester, N. Y.. September 6 to 10. 1037.
Melting Point Determinations under Mercury JlYRON A. COLER Paragon Paint & Varnish Corp., Long Island City, N. Y.
T
HE relative inertness and high density of liquid mercury
have been ingeniously employed t o determine the melting points of resins and waxes (8). The solid is fixed below the surface of the mercury either by melting the material and then allowing it t o solidify on the walls of a glass tube or by packing the finely ground solid into a capillary; heat is applied and the temperature a t which the material rises to the surface of the mercury or the mercury falls through the bottom of an open tube is noted. Since many resins are thermoreactive, premelting of the resins may change their melting points (1). The operation of stuffing such materials as soft waxes into capillary tubes may become tedious. Moreover, these methods do not seem t o take full advantage of the fairly high thermal conductivity of mercury. The method proposed here consists essentially of affixing a small piece of the original solid t o the bulb of a thermometer by means of a piece of wire. The thermometer bulb is then
-
50 CC.
BEAKER
MEACURY SAMPL P
W/R€ HOLDER
FOR MELTINQ POINT FIGURE 1. ARRANGEMENT DETERMINATION
