172
ANALYTICAL CHEMISTRY
of-nickel and cobalt with 2 arid 10 mi. of reagent are shown in Figures 2 and 3. Although 1% nickel would correspond to 0.02% copper s h e n 2 ml. of reagent are used, the erroaiu usually less, because of the equilibria existing with a-benzoinoxime, nickel, and copper. Tht? following results were obtained on Bureau of Standards steels anti ironh: Saliqilr
34 107 3211 111 33a I2fi
% ' Xi
%cu
1 .125% cu
7c C a F o u n d Beckman Klett 0.126 0.066 O:O& 0.114 0.122 0:iiE 0.068 0.072 0.026 ... 0.058 0.060 0.062 0.063
0.30 0.122 0.81 0.074 1.21 0.117 1.75 0.122 3.24 0.080 36.42 0.096 Synr tirtir" 3.0 0.077 2.5 0.071 2.0 0,065 0.065 1.5 0 . os9 0.056 1.0 0.053 0.062 '' 3Iixtures of Bureau of Standards steels No. 55 (0.041% Cu, m d Xi). 33a (0.080% Cu, 3.24% Hi).
0.066 0,059 0.064 0.021% Si)
Figure 1..
resultb indicate that the method is useful only t'oi routine control analyses of ferrous alloys containing over 0.3% 11ickel. I n the case of cobalt, the iiiterfrrenrr is more seriouh.
400 450 WAVELENGTH IMILLIUICRONS)
00
I C
4bsorbency of Copper-a-Renzoinoxin~e
' h t w
Sainpl? % co 5% c u Cu Foiinri Syntheticn 0.585 0.063 0.071 0.064 0.28 0.062 Synthetica 0.036 8.45 0.099 153 ' I Mixtures of Bureau of Standards steels No. fi5 (O.Ofi0q Cu) a n d \ , I , I R R (n.ng97c CU, 8 . 4 CO). ~ ~
These results indicate that the method is useful for ferroub alloys containing up to 0.25% cobalt.
Slr\f.\IARY
'Thr method dewribed is useful for the rapid deterniirlatioil of ('upper in many ferrous allo) q. The chief iriterfererices are nickel and cobalt, limiting the application of the method to plain and Ion-allov steels and cast iron-. LITERhTURE CITED
(1) E'eigl, F.,Ber., 56B,2083 (1923). (2) Jennings, J., ef d , J. Chem. SOC.,138,818(1935). (3) Kar, H. A., IND. ENG.CHEM.,ANAL.ED., 7, 193 (1935) (4) Silverman, L., Ibid., 12, 343 (1940). RKCEIYED July 19, 1949.
Modified Molecular Weight Apparatus ROBERT .\IATTESON Caltfornia Research Corporation, Richmond, Calg.
In laboratories handling a large volume of molecirlar weight determinations the rapidity with which the test may be performed is second in importance only to the accuracy. A solution to the problem in these laboratories has been effected through the redesign of the water-filled, differential vapor pressure thermometer of Menzies used in conjunction with a specially designed ebulliometer permitting a determination to be made with as little as 100 nig. of material. The operation is simple enough to be made by xwntechnically trained personnel, and the time required for a test is of the order of 25 minnteh.
I
CONSECTIO,?; with many research problems, it is necessary to determine accurately the molecular weights of large numbers of samples. Often, particularly in industrial laboratories, speed is essential; costs must be held down and nccuracx maintained a t a high level. Older methods have not been able to fill all these requirements: hence, the Menzies-Wright ( 4 1 iiiethod was re-examined to ascertain if i t could he adapted to tlw colditions mentioned above. As a result of this investigation. a modification of XIenzies (5') differential thermometer has been successfully developed and a new ehulliometer designed for us(' R ith this thermometer. _";
C sing chloroform as a soltent, practically all of the high boiling types of materials encountered in petroleum research may be handled. The normal boiling point of the unknown must be a t least 150" C. higher than the hoiling point of the solvent. Changes in barometric pressure during the test do not affect the results. The equipment is standardized by four pure compounds covering the range from 128 to 890 in molecular w-eight. BJ a short extrapolation, molecular weights LIP to 1000 niaq be determined with an accuracq only slightly less than that ohtainable using henzene in the cryoscopic method.
Yubaequent to the development oi the apparatus described below, two excellent papers by Kitson and co-workers (1, 2 ) appeared. They cover essentially the same ground as this paper, but their solutions of the problem are somewhat different mechanically. It is this difference that should prove of interest to norkers in the field of molecular weights. For example, Kitson's apparatus is more complicated to fabricate, the heater is not e:isily replaced, and experience in these laboratories has shown that a traveling microscope rigidly attached to the frame holding the ebulliometer permits more accurate reading of the position of the meniscuc: of the liquid in the thermometer.
173
V O L U M E 22, NO. 1, J A N U A R Y 1 9 5 0 THERMOMETER
The Pyrex thermometer (Figure !)is eonstructedbysealingthelower hulh t,a the drawn end of the 1.0% mm. inside diameter capillmy. This bulb is approximately 30 mm. long and is made from 11-mm. outsidr diameter tubing. These dimensions nre not, critical, however. The t,otxl length of the capillary is 160 mm. In fahrieating the thermometer a 6 mm. outside diameter tube is seded to the upper end of the capillary. This tube is made a convenient, length far handling. Several liquids were tried in experimental thermometers before water was selected &s the preferred filling material. Water is siphoned inbo the lower bulb, capillary, and upper bulb and then boiled out. uniil the lower bulb is half full. At a point 25 mm. above the juncture of the citpillary and the 6-mm. outside diameter tube the larger tube is sewled off to form the upper bulb. The 6mm. tube is then cut off Figure 1. Differand a solid rod of equal diameter ential Vapor Presis sealed to the upper bulb. Thr: sure Thermometer male portion of the ground joint and the handle are sfcled to the upper bulb as shown in Figure 1. With both bulbs of the thermometer a t the same temperature, tho upper meniscus of the liquid should stand about 50 mm. above t.he lower meniscus, the natural head due to capillarity. It is possible to adjust t,his height by cooling t,he lower bulb and bringing d l the liquid into thi8 bulb, then permitting the two bulbs to attain the same temperature, preferably in the ehulliometev cont,niningthe boiling solvent.
Before constructing the ebulliometer, there was some concern t,hat possibly superheating of the liquid would result, and make the design inoperable. Experience has proved that this is not the case, as evidenced by the negligible rise of the water column in the capillary of the thermomet,er when the pure solvent is boiling as compared with the height. of the column when both bulbs are at room temperature. COMBINED BQUIPMENT
Figure 3 is a photograph of bhe equipment in operation. A small I-watt lamp operating a t 10 volts is placed behind the boiler in order to illuminate the upper menisous in the thermometer. A white trmslucent paper shield is placed bet,ween the
EBULLIOMETER
Figure 2 shows the ehulliometer designed specifically for use with the new thermometer.
f
Figure 2.
""'"
Ebulliometer
The 10-watt beater is made from 30 om. (12 inches) of KO. 26 Ni,hrome wire wrapped ,n the lower leg of the ,oiling section. Voltage moss the heater is 10 iolts supplied from R ixed-ratio transformer >ra Variae. Where line ioltage variations are ,roublesome it may he tdvisahle to employ a ioltage regulator, al,haugh such a step bas lot been necessary in ,his laboratory. An unsilvered heat ihield mote& the boiler 'rom dkfts. The top of ,he boiler is fitted with L ground-glass joint into which is fitted the thermc#meter,thus affording COIistant positioning of tht3 thermometer, a factor' found ta be imnortant in attaining reproduoihility of results. A removable Drieritefilled drying tube is placed in the top of the condenser to prevent
t6e bottom of the return leg serves in draining the solution a t the end of a test.
F i g u r e 3.
Apparatus in Operation
lamp and the boiler t o diffuse the light. Motion of the meniscus is observed by means of a Gaertner 32 X microscope mounted in a traveling micrometer slide capable of memuring to 0.01 mm. OPERATION
Solvent. In a petroleum research laboratory the preponderant type of sample for analysis is hydrocarbon or allied material. The preferred solvent is carefully dried chloroform of C.P. grade, which possesses the desired characteristics of relatively low boiling point and the power to dissolve an extremely wide range of substances. Far over 4 yeam of operation in these laboratories, it has been found necessary to resort occasionally to only one other solvent, acetone. When acetone is used it is necessary to circulate ice water to the condenser; otherwise, tap water a t room temperature (ahout 25' C . ) suffices. The present equipment uses 10 ml. of solvent in a determination of molecular weight. Calibration. Each new batch of solvent is calibrated by determining the A S os. mass curves for four compounds of known
174
ANALYTICAL CHEMISTRY
molecular weight. The preferred materials used in these laboratories are naphthalene (crystallized from alcohol), diphenyl bismuthine, tetrachlorobenzene, and tristearin. All chemicals are C.P. grade and cover the molecular weight range suitable for calibrating from niw = 100 to 1000.
-----
5 U
5a 3w
I
I
U Y
I
0 A
200-
1
1 '
I
n
\I
$50-
I
\
100
a Table I. Comparison of Freezing Point Molecular Weight with Boiling Point Molecular Weight Determinations Sample No.
1 3
4 5 6 7 8 9 10 11
MASS-MG.
Figure 4.
CalibrationCurves for Differential Thermometer
In Figure 4 are plotted the A S us. mass curves for a typical calibration. A S is the change in scale reading, or meniscus rise, for the addition of a given mass of solute to the solvent and is a measure of the increase in boiling point of the solution. I n classical methods (j), (AT/m), where T is temperature and m is mass, is plotted against m to zero concentration, and the molecular weight is calculated from
(z) tn
=
0
and the proper constant.
However, experience with the new apparatus has proved it is better to plot, from the calibration curves, values of
As
m (taken a t
m = 100 mg.) against molecular weight, as shown in Figure 5 , and to use this curve in determining molecular weights of un: knoivns. I t has the advantage that the value for
is obtained by interpolation, whereas
($)
m - 0
(2) m
100
is obtained by
extrapolation, with the possibility of introducing much larger errors. Determination of Molecular Weight. The first step in making a determination is to boil approximately 10 ml. of pure solvent in the ebulliometer for a reasonable length of time to clean i t out. About 5 to 10 minutes have been found by experience to be sufficient time for this when working with clean stocks. Occa-
Molecular Weight Freezing point Boiling point
c.
c.
192 194 218 237 233 251 257 264 278 416 422
195 190 218 245 230 250 250 270 280 425 415
Difference, % f1.6 -2.1
0 t3.2 -1.3 -0.4 -2.7 +2.3 tO.7 f2.2 -1.7
sionally refractory materials require a longer cleaning time. The solvent from this cleaning run is drained through the ceck a t the lower end of the boiler and vacuum is applied a t this point to hasten drying. Throughout the operation, the Drierite drier is in place as shown in Figure 3, and is removed only during introduction of solvent. With the ebulliometer clean, exactly 10 * 0.01 ml. of solvent are charged and the heater is turned on, bringing the solvent to a boiling temperature. Upon initial boiling the meniscus in the capillary of the thermometer rises immediately to the upper bulb, which is filled completely except for a tiny bubble that remains a t the top. T h e n the boiling vapors reach the upper bulb and the contents reach the temperature of the vapor, the upper meniscus is depressed into the capillary where it comes to rest a t its equilibrium position. Boiling of the pure solvent is permitted t o continue until the meniscus ceases moving, a t which time the first portion of sample material may be introduced to the return leg of the ebulliometer In determining the molecular weight of a particular sample, approximately 50 to 60 mg. of the material are loaded into the end of a piece of 2-mm. inside diameter Pyrex tubing about 20 em in length, all material adhering to the outside of the tube is wiped off, and the tube and contents are weighed to 0.1 mg. -4 short piece of rubber tubing connected to the end of the glass tube opposite the sample facilitates handling and introduction of the sample into the ebulliometer. The tip of the glass tube holding the sample is lowered into the refluxing solvent which washes out the sample. With the thermometer indicating that equilibrium has been obtained in the boiling solvent, the position of the meniscus in the c a d l a r v is noted bv means of the traveling- microscope and is recorded as SI. The drier is removed temporarily while this sample is introduced to the amaratus and is then reolaced. The meniscus rises and comes to ie'st a t a new position, which is noted and recorded as Sp. A S is then S2 - SI.A second 50 to 60-mg. portion of sample is next introduced into the ebulliometer and the third position of the meniscus is recorded as SI. The value of A S
175
V O L U M E 2 2 , NO. 1, J A N U A R Y 1 9 5 0 corresponding to the sum of the two masses is then SS - SI. The AS us. m curve is then plotted on rectangular coordinates. This method of plotting has the advantage that the origin gives an additional point through which the curve must pass and it becomes easier to detect flaws in the data. is From the curve thus constructed the value of
(2)rn
1w
found and by reference to the standardized rurve, such as Figure 5 , the molecular weight is obtained.
A single determination of molecular weight can be made in a few minutes under ideal conditions. Where readily soluble compounds are being measured, and no cleaning difficulties are encountered, the average time required for a complete determination is about 25 minutes. DISCUSSION
The change in the position of the upper nieiiiscub 15 the only one followed in these measurements. This is done for purposes of simplification and is permissible, because the drop in the lower meniscus is a constant function fJf the rise in the upper meniscus. The method of plotting automatically takes care of the fact that A S is not Z A S . hctually, A 8 equals approximately 9iYc of Z A S in practically all measurements made on this apparatus. The usual precaution of maintaining an adequate difference between boiling point of solvent and solute must be observed, and in this laboratory no attempt is made to measure molecular weight by the ebullioscopic method of samples boiling below 210" C.i.e., 150" C. above the boiling point of chloroform. With careful attention to details, the accuracy attainable x i t h this equipment is only slightly less than that obtained on the cryoscopic apparatus using benzene as a solvent and a Beckman t,hermometer for temperature measurement. A romparison of
results obtained by the two methods on duplicate samples appears in Table I. Certain types of compounds yield anomalous results, but this is common to the other widely used methods for determining molecular weight. I n particular, the molecular weights of stearic acid and azoxybenzene are far too low when measured in the apparatus described here. A word of caution is in order relative to the grease used in the ground joint of the thermometer. The silicone greases cause excessive foaming in the boiling solution and render the apparatue inoperable. A satisfactory grease is Celvacene Heavy, a product of Distillation Products, Inc., Rochester, K. Y . Experience over the past 4 years has shown that this equipment meets all the requirements of speed, quality, and ruggedness in a laboratory where a large volume of molecular weight work is constantly being turned out. ACKNOWLEDGMENT
It is
and ideas contributed by W. R. Doty in the construction of the glass equipment. a pleasure to acknowledge the careful workmanship
LITERATURE CITED
(1) Kitson and Mitchell, - k i . k L . CHEM., 21, 401 (1949). (2) Kitson, Oemler, and Mitchell, Z b i d . , 21, 404 (1949). (3) Mensies, J . Am. Chem. Soc., 43, 2309 (1921). (4) Meneies and Wright, Ibid., 43, 2314 (,1921). ( 5 ) Reilly, J., and Rae, W. N.,"Physico-Chemical Methods," 3rd ed., Vol. 1, p. 600, London, Methuen & Co., 1939. RECEIVED September 11, 1947. Presented before the Division of lnalytical CHEMICAL and Micro Chemistry at t h e 112th >feetine of t,he AMERICAX SOCIETY, X e w York. PIT. Y.
Long-chain Alkyl Sulfates Colorimetric Determination of Dilute Solutions FRED KARUSH' A R D MARTIN SONENBERG' iVew York TJniversity College of Medicine, New York, N . Y A simple colorimetric method has been developed for the determination of long-chain alkyl sulfates. Solutions as dilute as 5 X M can be analyzed with an accuracy of about 2YG. The method depends on the formation of a complex between the detergent anion and the cationic dye rosaniline hydrochloride. This complex is extracted into a mixed organic solvent, 50% chloroform and 30% ethj-l ace-
I
N C O S N E C T I O S with a study of t,he interaction between bovine serum albumin and sodium octyl, decyl, and dodecyl sulfates ( C,,H1,+ lSOaSa), the need arose for an accurate analytical method for the determination of these alkyl sulfates in very dilute solutions, of the order of 10-j 111. h search of the literature revealed that no adequate method had been published. The development of a colorimetric method w-as therefore undertaken, following the previous work of Brodie, Udenfriend, and Levj- ( 1 ) . As the basis of methods for the analyses of certain strong organic acids, these investigators utilized the formation of organic soluble complexes between the acids and the cationic dye rosaniline hydrochloride. The method described here is based on the formation by this dyestuff and pararosaniline hyclroI
Present addrws. Sloan-Kettering Institute for Cancer Research. S e w
York.
x. T.
tate, and its spectral absorption is read with a KlettSummerson colorimeter. Pararosaniline hydrochloride is also suitable as a dye reagent. The molar sensitivity of the method increases from octyl to decyl to dodecyl sulfate, because of increasing efficiency of extraction of the dye-detergent complex with increase in chain length. The molar ratio of d?-e to detergent in the extracted complex is 1. chloride of complexes with alkyl sulfates and their extraction into an organic phase. It has been applied to the determination of the alkyl sulfates noted above and is sufficiently sensitive to permit the analysis of solutions as dilute as 5 X ill (involvmole) 1% ith an accuracy of about 27,. Its ujefulness ing 2 X is not limited to alkyl sulfates and it can undoubtedly be applied to other strong organic acids which contain relatively large nonpo1:tr groups. MATERIALS
The rosaniline hydrochloride was obtained from the National .hiline Division, Allied Chemical and Dye Corporation, and was of 82YGstrength according to the manufacturer, the impurity being largely salt. A solution of the dye a t a concentration of 4 x 10-4 31 was prepared in 0.025 M phosphate buffer, pH 6.1 This solution was extracted several times with the same solvent
