Mag
ANALYTICAL EDITION
IS, 1942
liters of 0.01 N ammonium hydroxide are obtained from the calithus hration curve the obtained per cent as previously of nitrogen described. in the original From sample the data may be calculated. The pH of the solution is dependent upon the amount of boric acid present 88 well 88 the amount of ammonia and thus it is necessary to the amount Of aid In addition, i t is necessary to prepare a new calibration curve whenever a fresh stock of boric acid solution is prepared. Specid conductivity water need not be prepared if the calibration curve is made using the same water for dilution or if a blank is run.
439
represented by the boric acid absorption solution after dilution to a definite volume and estimation of the ammonia content by reference to a calibration curve. This method is more accurate, but involves the use of a standard titration solution. The second method, while somewhat less accurate, is not dependent upon the preparation and storage of sstandard acid and is admirably suited to routine a n a b e s hecause of the saving of time otherwise required for a titration.
Literature Cited
Conclusions ~w~ of the Wagner micr+KjeldaM p r o c ~ u r e avoid the troublesome use of methyl red as indicator. The first method consists in a potentiometric titration to the p H
(1) Eisner and Wagner, IND.E m . Ca~ar.,ANAL.ED.,6. 473 (1934). (2) Meeker and WaKner. Ib& 5,396 (1933) ; 12,771 (1940). (3) Nieded and Niederl, “Micromethods of Quantitative Organia Elementary Analysis’’, New Y z k . John Wiley & Sone, 1938. (4) R-’-------”
--J - ~ - - L - n ’ ~ ~ ‘
Rnf-wtive Index MeasurtiilGii
^^..
^^”
,..”,.~
at. aiiu auv v
the Melting Point of Solids H. A. FREDIANI’ Fisher Scientific Company, Pittsburgh, Penna.
P.
ROBABLY the most widely used method for determining the refractive index of crystalline or solid materials involves use of the “Becke line” phenomenon ( I ) , for which a microscope and a series of standard liquids of known refractive index are required. For optimum conditions the liquid series used should include numerous duplicates, in order to obtain immersion media in which the solid material is insoluble. Even for the crystallographically simplest of compounds, isotropic in nature, the procedure is somewhat tedious and time-consuming, Two courses are possible: (1) Crystals, or fragments, of the solid may be immersed in a progressively increasing or decreasing series of the immersion media of known index until a liquid of similar index is found, or (2) the particle may be immersed in a medium of lower refractive index and a second medium of higher index used for dilution until minimum visibility is attained. In this latter method the index of the medium finally attained must either be computed or he measured with a suitable refractometer. For anisotropic substances the procedure is still more complex. The refractive index varies with the direction of transmission and of vibration of the light in the specimen. Two constants must be obtained for uniaxial and three for biaxial crystals. This necessitates use of a polarizing microscope and proper crystallographic orientation of the material for each determination. The complexity of the measurements required may he understood by referring to the procedure recommended by Larsen (IO). Inasmuch as most crystalline materials may be classified as anisotropic, it is easy to understand the recent statement that the values (refractive indices) for organic solids have not been so well collected as have those for liquids (8). Many organic chemists working in qualitative organic analysis make routine refractive index measurements of liquids as an easily and conveniently determined physical constant to assist in their identification. Because of the complexity of the apparatus, the specialized technique required, and the Preaent xddieaa, Eimer and Amend. New York, N. Y.
FIG
labor involved, investigators (a, LU WUCL rnacena~u. A newly developed attachment for the Fisher refractometer (6) now extends the ease of measurements of refractive indices to a much larger group of organic substances. The apparatus permits the simultaneous determination of two common physical constants, melting point and refractive index of the resultant liquid, as well as $he approximate estimation of a third, less commonly employed value, the dispersion (7). Despite the fact that the method is not applicable to all types of compounds, it is of considerable value to the organic analyst. The substance to be investigated is placed on a heated stage and the temperature raised until the melting point is reached and noted. The heat control may be adjusted to maintain the temperature a t the melting point and the refractive index of the resultant true liquid determined. An alternative procedure is to raise the temperature further to some reference point above the melting point and determine the refractive index and dispersion.
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
440
Vol. 14, No. 5
peep hale and slightly behind the circular optical glass backing plate. The temperature was regulated and controlled by plugging the heater cord from the eyepiece into IL Varitran (continuously variable auto-trrmsformer) and adjusting the voltage applied hetween 0 and 60 volts. Fine, stepless, continuous adjustment is thus possible and the instrument may be brought to, and maintained at, any temperature between that of the room and 200" C. For mechanieal reasons use of higher temperatures is not recornmended. The complete laboratory setup used for obtaining the data reported herein is shown in Figure 2.
FIGURE2. LABORATORY SETUP Apparatus The instrument employed is a Fisher refractometer, which embodies certain modifications of the principles suggested by Jelley ( 6 ) and also reported by Edwards and Otto (8). An accurately ground and polished glass prism is utilized (Figure,.J) for the formation of a "liquid prism" of the substance to he investigated. Simultaneous observation of an illuminated slit and its virtual image, seen through the liquid prism, permits measurement of the vertical displacement of the image from the slit.: This distance is dependent upon the refractive index of the liquid investigated. The scale used is calibrated directly in refractive index units from n = 1.300 to n = 1.900, Measurement8 are possible to *0.002 unit. For measurements at controlled elevated temperatures a modified eyepiece, consisting of 8 nickel-plated brass block into which a Nichrome resistance wire heating unit has been sealed, is substituted for the one ordinarily supplied with the instrument. A thermometer. or thermocouple well, is d80 present. In the ~~~~
~~~~
~~~
permit placing the hulb'ofthe thermometer directly ;hove-the
'ine glass piate ma pnsm ot tne mstmment m e caremlly wiped with a soft cloth, lens paper, or Kleenex, or if necessary, are cleaned by wiping with a cloth wetted with water, benzene, carbon tetrachloride, or any other readily volatile solvent. The prism is then placed on the eyepiece in the position governed by the spring clamp, with the beveled edge forming B well of V-sha ed cross section, the lower edge bisecting the small hale b e h i d t h e permanently mounted optical glass circular plate (see Figure 1). A crystal or fragment (or preferably a few milligrams of finely powdered material) is placed in this well, so that upon melting the liquid will he drawn to the apex and form a liquid prism. The size of sample required depends to a very large extent upon tho volatility of the specimen. Two milligrams of sample will suffice for compounds exerting low vapor pressures at the melting point. The heater is plugged into the Varitran and the voltage turned up so that the temperature is rapidly brought to within 5" to 10" of the melting paint. The Varitran then should he adjusted so that the temperature continues to rise 1- or 2" per minute. At the melting point the fragments will liquify rapidly and the resultant liquid will flow into the prism. The exact temperature may now be noted and recorded. Upon depressing the refraetometer light switch at the rear of the instrument, the refmotive index of the liquid may be read directly from the scale, in the plane of the light slit, by peering through the small hole in the eyepiece. If desired, the heater may be adjusted to maintain the temperature at a predetermined point-i. e., 5', lo", 16". eta., above the melting point-and the index measured under these conditiona. Because the readings may be obtained practically instantaneously, it is not necessary to attempt to maintain constant temperatures for appreciable lengths of time. However, since the thermometer bulb is embedded in the hloek close to the heating element, whereas the sample is separated from the block by a. glass plate 1 mm. thick, temperatun changes should be effected slowly enough to compensate for thermal la(: between the thermometer and sample. It has been found best policy to clean the glass plate and prism immediately after recording the desired data. This may readi1.y he accomplished by wiping while hot. The Varitran may then he turned to zero, and as soon as the thermometer drops helow the melting point of the next specimen to he investigated, the fresh sample may he placed on the eyepiece and studied, A curve indicating the thermometer reading at various settings of the Varitran has been found extremely eonve?i?nt. By its use approximat,e temperature settings may be resioiiy maae.
Discussion E
I. COMPOUNDS INVESTIGATED
ing Point
Handbook (8)
Reiraotivi Index at Melting Point HandDetd. book (#)
Dispersion Nlellsuremellta
Scde
Compound o-Naphthylamine Palmitio aaid
Iletd.
Steeric mid a-Nitroaniline o-Nitrophenol &Naphthylamine
69
n
7u SY
z
p-Nitrophenol
nrNitroaniline *Tolidine dl-Malic a d d Pyiugailol Anthranilic acid p-Nitrohniline Ammonium thiocyanate
Citric acid Tartaric acid Potassium thiooyanate Suoeinio acid
49 62
50 83&4
1.66Y 1.435
Red
Green
1.6703 1.430
.Yo. 4
n G
3
109
110 111
125 1% 134 144 147 150 152 169
153 168-70
1.590 1.460 1.464
174 190
173 189
1.558 1.405
I50
... ... .. . ... ...
1.598 White image White image
1.586
1.555
1.562
White image
1 0
With reasonable cart3 in the adjustment of the n ~ rr a" d heater, melting point" m ..._ ..,v he clotor. mined within 1" or 2' of reported values. The principle is, of course, similar to that used in the Fisher-Johns melting point apparatus (4) which has been used by organic chemists for some four years. Since a regular 110-volt, tungsten filament bulb is used as light murce, the beam employed is not monochromatic but "white". For substances having low dispersion-i. e., where all wave lengths of light are similarly diffracted-the virtual image seen is a white image as narrow as the slit itself. For substances having appreciable dispersive pourers the image obtained is not a line image but rather a multicolored band. The actua1 width of the band-i. e., distance between the far edges of the red and violet portionsdepends upon _ I
1"
May 15, 1942
T tHLE 11.
ANALYTICAL EDITION
DIFFERENCE BETWEEX READINGS TAKEN ON FJDGLS O F I I E D AND (;REEN PORTIONS O F IMAGE
Dispersion Scale No.
0 1 2 3
! h
; Y the dispersive power of the sample. The scale of the instrument has been so constructed that reading of the yellow portion of the spectral band formed indicates the refractive index of the medium as generally determined by using an Abbe or Pulfrich refractometer with light of sodium D wave length. This yellow portion is generally sharply defined as the narrow region between the brilliant red and green portions of the image. The dispersive power of the sample investigated may readily be estimated from readings taken a t the visible limits of the spectral hand formed. \Tit11 carefully performed observations the precision at~tainaMe,irrespective of the dispersion encountered, is 10.002 unit. The temperature coefficient itlay also be ascertained hy adjusting the heater so that the temperature slowly rises over the range desired and taking periodic readings on the refractometer as the thermometer reaches predetermined values. h I I T . 4 T I O X S E i w O U Y r E R E D . Because of the personal safety factor it is recoinniended that temperatures in excess of 175" C. be employed but rarely. I n making a reading the eye must necessarily be brought close to the specimen; foi, this reason lachrymatory compounds will be difficult to study. Compounds that sublime cannot be studied. Data will be difficult to obtain for compounds exerting high vapor pressures a t or slightly above the melting point. Salicylic acid is an excellent example demonstrating this type of difficulty. Severtheless there are many organic compounds which do not fall in the above classes and which lend themselves admirably to study by the method and apparatus described. DATA. The data obtained in this preliminary investigation were chosen, not with any specific class of compounds in view, but with the intention of determining the constants on a representative group for the melting range recommended. A large enough group was chosen to include substances ranging from high to low refractive index and from zero to appreciable clispersion. I n Table I are listed the compounds invest'igatetl, the determined and handbook values for melting point (and refractive index where available), and an indication as to the dispersion observed. The classification of the dispersive power is somewhat difficult to decide upon. Since monochromatic radiations were not used, accurate calculations were not possible (dispersion is usually defined as being proportional to the rate of change of the reciprocal of the velocity with wave length). It seemed likely, however, that' some simple, definite, seniiquantitatire indication of the degree of dispersion would be useful. For that reason the compounds investigated have been classified according to an easily determinable arbitrary method. Readings were t'aken on the lowest edge of the red and the highest edge of the green portion of the band formed. Readings were not taken to the blue or TTiolet because of the difficulty in locating the edge of these regions. The red aiid green edges are usually clearly discernible. These readings are also included in Table I and the clispersion scale nuintier.: :ire based upon the relationships indicated in Table 11. DISCVSSIOX OF DATA. For those few compounds for which cl:it,a could be found in the literature the agreement, between
441
the author's values and those previously reported are satisfactory. The compounds used in this study were stock chemicals of the purest grade obtainable commercially but were not crystallized or further purified before use. The determined and handbook values for a-naphthylamine as well as for stearic acid agree remarkably. The handbook value for &naphthylamine is reported as having been obtained a t 98" C., about 14" below the melting point of the pure compound. The author's sample melted a t log", indicating some impurity, and his value is somewhat less than that mentioned above. The difference between the two values (0.006 unit) may easily be laid to the differences in purity of the two samples and to the temperature difference a t which the measurements were made. Small amounts of irnpurities are apt to have a much larger effect upon the melting point of a conipound than upon its refractive index. For this reason the latter property may well be the better for identification studies. Determination of the specific refraction for such compounds would be an interesting study, although density measurements :it the reference temperature would be required. For many compounds the melting and solidification points may be accurately determined by noting the temperature a t which the image appears and disappears upon slowly raising and 10%ering the temperature. Supercooling must be considered for certain samples. The value of refractive index measurements a t elevated temperatures for identification purposes will be enhanced greatly after sufficient data have been obtained and reported to permit the compilation of orderly data tables. I n the meantime one may make use of the method by alternate determinations on the unknown material and on known compounds. Obviously, because of the scarcity of low-melting inorganic compounds, the method is not apt to find application in this field. It should prove of value in studies of natural and artificial wixes and similar materials, even though these substances are apt to have melting ranges rather than melting points. A compilation of data for this group of substanreq is now under way.
Summary Refractive index irieasurements of organic compounds a t and above the melting point have been proposed for identification studies. Such a procedure necessitates the determination of but a single value, rather than the two or three required on crystalline anisotropic material. This value may be determined more easily and rapidly than similar values for even isotropic materials in the crystalline state. Apparatus for the simultaneous determination of melting points up to 200" C. and refractive indices between 1.300 and 1.900 has been described. The melting points may be obtained with an accuracy of 1" to 2" C., while the index measurements may be made to *0.002 unit. Estimations of dispersion may be made x-ith the apparatus described and a dispersion scale is suggested for classification of compounds.
Literature Cited Chaniot and Mason, "Handbook of Chemical Microscopy", Vol. 1, p. 369, New York, John Wiley & Sons, 1935. Dewey and Witt, ISD. EXG.C H E Y . , kiar,. ED.,12,459 (1940). Edwards and Otto, Ibid., 10, 225 (1938). (4) Fisher Scientific Co., Laboratory, 8, 72 (1937). (5) Ibid.,12,86 (1941). (6) Jelley, E. E., J . R o y . Microscop. Soc., 54, 234-5 (1934). (7) Jenkins and White, "Fundamentals of Physical Optics", p. 286, New York, McGraw-Hill Book Co., 1937. (8) Kirk and Gibson, ISD. E s c . CHEM.,ANIL. ED.,11, 403 (1939). (9) Lange, "Handbook of Chemistry", Sandusky, Ohio, Handbook Publishers, 1937. (10) Larsen, U. S. Geol. Survey. Bull. 679, 22 (1921).
