if a11 ionizing voltage slightly higher than 12 volts is selectrd. the parent masscs of most organic compounds in a coml)lc>smistuit will be observed. On the other hand, if it is dcsired to distiIiguisli b h w n compounds of different functional groups, voltage settings can bo made in the appropriate range. Furthermore, within a homologous scric*s for the same functional group thc ionization potential is found to dccrcssc with incrpasing chain length and uith incrr,asing branching of the side chains. Thew facts aid considerably in tlie identification of compounds when a reference spectrum for the compound is not available. An rssmple of how low ionizing voltage can he employed to simplify the mass spcctrum of a complex mixture is shown in Figure 9. The upper record is tlie mass spectrum obtained a t high 70 volts-of a ionizing ldtage-i.e.,
fraction separated from a coffcc aroma center cut. The complesity of the spectrum is obvious. The lower record shows the spectrum obtained a t low ionizing voltage. I n the low voltage spectrum the parent masses for only nine components of thc mixture are seen. Thus, at mass 32 methanol is indicated; at 44, acetaldchydc; a t 58, propionaldehyde; at 60, methyl formate; at 62, dimethyl sulfide; a t 68, furan; a t 7 2 , methyl ethyl ketone; at 7 6 , carbon disulfide; and at 82, methyl furan. The accurate qualitative determination of the large number of components in this sample from the conventional high voltage spectrum alone would be a timeconsuming operation, but with a low voltage spectrum to signify the parent peaks readily, the compounds present in the mixture can be easily identified. Very little extra work is required to utilize the low voltage technique. Only
one sample introduction is necessary. The high voltage spectrum is obtained first, and when this is completed, one switch activates the low, voltage circuit, and without interruption the lower energy spcctrum can be obtained. LITERATURE CITED
(1) Field, F. H., Franklin, J. L., “Electron Impact Phenomena and the Properties of Gaseous Ions,” pp. 106-15,
Academic Press, New York, 1957.
( 2 ) Lumpkin, H. E., ANAL.CHEM.30, 321 (1958). (3) Merritt, C., Jr., Baeinet, hl. L., Sullivan, J. H., Robertson, D. H., J. Agr. Food Chem., in press. (4) Merritt, C., Jr., Bresnick, S. R., Bazinet. M. L.. Walsh. J. T... Anaehni, P., Zbid:, 7,784‘(1959).’
RECEIVEDfor review March 5, 196%. Accepted June 13, 1962. Presented in part before the Division of Agricultural Food Chemist 135th Meeting, ACS, Boston, Mass., x i r i l 1959.
Determination of the Primary Hydroxyl Group Content of Polypropylene Glycols WARREN B. CRUMMETT Special Services Laboratory The Dow Chemical Company, Midland, Mich.
b An investigationof Schmulyakovskii’s nitrite spectrophotometric method for primary and secondary alcohols showed that it cannot be applied directly to the analysis of polyglycols because of instability of the nitrosation products. Development of a device for carrying out the reaction in absence of air, inclusion of a filtration step, and correction for background absorbance have made possible a satisfactory determination of primary hydroxyl groups in polypropylene glycol.
D
on the primary-secondary hydroxyl ratio in polypropylene glycols are important in reaction rate studies and in establishing specifications for the wide range of materials prepared from propylene oxide and water or alcohols. Experience in this laboratory with the methods previously available (1-4, 10) indicated that while they yield useful information, it would be desirable to hare a more rapid procedure capable of equal or better accuracy. Schmulyakovskii (8) has proposed a method, based on the absorption spectra of the nitrites of the alcohols, for the determination of primary, secondary, and tertiary alcohols. Ile later extended the method to include the determination of primary and secondary .~TA
hydroxyl groups in higher aliphatic alcohols (9). A 45-minute analysis time and a precision comparable to that of other methods made this approach attractive. Preliminary attempts t o utilize the Schmulyakovskii method showed that the primary nitrite of a polyglycol in hexane solution is unstable in the presence of air. The solution becomes ~~
Table 1. Effect of Decomposition Products on Ratio of Net Absorbances at 355 mp and 385 mu
Material 1-Butoxy-Zpropanol 2-Butoxvethanol Voranol CP3000 Voranol CP3001 Voranol CP3500 Polyglycol P-2000 Polyglycol A
R :355 :385 Absence In air of air 1.35 1.36 1.34 1.35 2.31 2.29 2.30 2.31 1.32 1.42 1.28 1.38 1.80 1.80 1.45 1.51
1.33
1.27
1.58 1.56
1.40 1.38
1.33 1.39 1.27 1.42 Polyglycol B 1.26 1.38 1.25 1.38 a No light was transmitted by the sample.
turbid and nitrogen dioxide is evolved. The amount of turbidity varies with the concentration of primary hydroxyl groups and the degree of contact with air. Thus a polyglycol with 40% of its hydroxyl groups primary may produce a solution which is opaque t o ultraviolet radiation. The turbidity cannot be removed by filtration, dilution, or change of solvent. Nitrogen dioxide is produced by both primary and secondary nitrites of polyglycols. The effect of the decomposition products on the ratio of the net absorbances a t 355 and 385 mp is shown in Table I. The low results make i t necessary to use a device for excluding air from the system. With air excluded, a clear colorless solution is usually produced. A few samples will still show a slight turbidity due to a trace of emulsion, but this is easily removed by filtering through a tightly-packed cotton plug. Slight variations in background absorbance are eliminated by a base line correction. EXPERIMENTAL
Apparatus and Reagents. Ultraviolet spectrophotometer, Beckman Model DU or equivalent can b e used. A recording instrument such as t h e Cary Recording Spectrophotometer Model 14 is more convenient. T h e silica absorption cells, 10.00-em., are VOL 34, NO. 9, AUGUST 1962
0
1147
0.80
0.10
0.60
0.50
tfl
a
0.40
0.30
0.20
0.10
0.00 320
340
360 380 WAVELENGTH, mp
400
420
Figure 2. Ultraviolet absorption spectra of nitrites of butoxy alcohols
2-Eutoxyethanol (1 0.3 mg./lOO mL) --..-. 1 -8utoxy-2-propanol (1 2.5 mg./l 00 ml.) 1 0-cm. cells
Figure 1. tion cell
Reaction funnel and absorp-
equipped with adaptors as shown in Figure 1. A 125-ml. separatory funnel is used as the reaction funnel (Figure 1). Hexane, free of alcohols and transparent to ultraviolet radiation from 420 to 310 mp is used. Phillips 95 mole per cent hexane is usually suitable. The hexane is tested by running it through the procedure. The net absorption obtained should be negligible. Standard primary alcohol. 2-Butoxyethanol, purified by gas chromatography, was used in Figure 2. Standard secondary alcohol. 1Butoxy-%propanol, purified by gas chromatography, was used in Figure 2. Calibration. Prepare standard solutions of the standard primary and secondary alcohols SO t h a t the final solutions contain 17.0 mg. of hydroxyl groups per liter of hexane solution. If 2-butoxyethanol and 1-butoxy-2propanol are used as standards, the concentration of the solutions should be 0.118 and 0.132 gram per liter, respectively. From these solutions, prepare 100-ml. samples of mixtures 1148
ANALYTICAL CHEMISTRY
Solvent: hexane
of standard primary and secondary alcohol solutions. Run two 100-ml. portions of each solution through the procedure. Determine €2: 355: 385 for each standard as described in the calculation. Either plot the average R:355:385 os. per cent primary hydroxyl of the total hydroxyl or derive a mathematical expression for the calculation of per cent primary hydroxyl (6, 7 ) . The graph should be a straight line. Procedure. Weigh a sample which contains about 17.0 mg. of hydroxyl groups. Dissolve it in hexane and dilute to 100 ml. with hexane. Mix thoroughly. Pipet a 10-ml. aliquot of the sample solution into the separatory funnel and add 90 ml. of hexane. Add 10 ml. of 1M hydrochloric acid and 2 ml. of 25y0 sodium nitrite solution. Stopper the funnel and shake vigorously for 5 minutes. Allow the layers to separate and discard the aqueous layer without removing the stopper. Remove the stopper and add 10 ml. of -0.25M ammonium bicarbonate solution. Insert the gas-inlet tube with the stopcock closed. Shake the funnel and contents for 5 minutes. Allow the layers to separate and discard the aqueous layer without opening the gas-inlet tube. Insert a cotton plug in the tube immediately above stopcock B and assemble the rest of the apparatus in Figure 1. With stopcocks A and B open and stopcock C in the vent position (opposite to that shown in Figure l), blow nitrogen through the cell until all oxygen is removed from the system. Turn stopcock C to the funnel position (position shown in Figure 1). With
nitrogen flowing around the tube a t stopcock D, open stopcock D. When the cell is filled with hexane solution close all stopcocks in the order C, A ,
D,B.
Disconnect the cell (with its stopcocks
A and B closed) from the funnel, and
transfer it to the spectrophotometer. Scan the solution against hexane from 420 to 310 mu with a slit of 0.1 mm. a t 355 mp. Calculation. GRAPHICAL.Draw a base line through the minimum at about 317 mp ana tangent to the curve a t 410 mp. Subtract the base line reading a t 385 mp from that of the curve a t the same wavelength to obtain a net absorbance (A385). Obtain the net absorbance a t 355 mp (A& in the same way. Divide A 8 5 6 by As5 to obtain a ratio ( R :355 :385). Average the ratios obtained from duplicate determinations and read the per cent primary hydroxyl from the graph. The theory which justifies the base line technique has been discussed by Morton and Stubbs (5) and that for the use of absorbance ratios by Pernarowski, Knevel, and Christian (6, 7 ) . MATHEMATICAL. Pernarowski, Knevel, and Christian derived an expression relating the absorbance ratio to the fraction of one of the two components in terms of molar absorptivities a t the two wavelengths. Using their expression and constants derived from the net absorbances of pure primary and secondary alcohols determined as indicated above, the following equation is obtained:
33.9(R:355:385) - 46.4 21.6 4.2(R:355:385) Primary Hydroxyl, % of Total Hydroxyl
+
0.70
0.60
0.60
0.50
Pa
P 0.40
9
2 0.30
0.30
0.1 0 L
I
o'Po
i I
0.00
'
I
'
3PO
I
340
'
I
'
"
"
"
'
360 380 WAVELENGTH, rnp
400
490
Figure 3. Comparison of ultraviolet absorption of nitrites of Polyglycol P-2000 and 1 -butoxy-2-propanol
- Potygtycol P-2000 (101
mg./100 ml.)
._____ 1 -Butoxy-2-propanol (1 2.5 mg./l 00 ml.) 1 0-cm. cells
Solvent: hexane
3PQ
340
360 380 WAVELENGTH, rnr
Figure 4. Comparison of ultraviolet absorption of nitrites of Voranol CP3001 with mixture of 2-butoxyethanol and
1 -butoxy-2-propanol
- Voranat CP~OOI (1 00 mg./lOO ------ Mixture (48% primary OH) 1 0-cm. cells
RESULTS AND DISCUSSION
Results on known mixtures are summarized in Table 11. Either the graphical or mathematical method may be used. The graphical method is somewhat more convenient after it is set up. However it requires the examination of a series of mixtures
Table II. Results Obtained on Known Mixtures of 1 -Butoxy-2-Propanol and 2-Butoxyethanal
Primary Added, % 0.00 1.9 5.2 5 2 9.8 9.8 10.2 14.4 18.9 19.2 23.3 23.9 33.1 46.6 48.1 48.3 56.6 72.2 73.6 78.8 84.0 89.4
Avg. R:355: 385 1.35 1.38 1.41 1.42
i.47
1.45 1.46 1.50 1.53 1,54 1.57 1.56 1.67 1.79 1.84 1.79 1.89 2.04 2 01 2.07 2.13 2.17
Primary Found, % Graph Equation 0.0 3. 6. 7. 13. 10. 12. 16. 19. 20. 23. 22. 34. 47. 52, 47. 58. 74. 70. 77. 83. 87.
-2. 1.5 4. 6. 13. 10. 12. 16. 19. 21. 23, 23 35. 48. 55, 48.
60. 75. 72. 79. 84. 88.
whereas the mathematical expression requires only examination of the standard materials. By either calculation the precision appears to be about *2%. A further comparison of the two methods of calculation is made in Table 111, which shows that different ratios may be used for the calculation with equally good results. The primary hydroxyl content of some competitive materials is also shown in this table. Figure 3 shows that the spectrum produced with Polyglycol P-2000 is very similar t o that obtained with 1 - butoxy - 2 - propanol. Polyglycol P-3000 produces a curve identical with that of Polyglycol P-2000. Thus the amount of primary hydroxyl groups present in these materials must be very low. The absorption produced by a mixture of 1-butoxy-2-propanol and 2butoxyethanol compared to that from Voranol CP3001 (The Dow Chemical Co.) is shown in Figure 4. The striking similarity of the curves indicates that this method is valid for the determination of primary hydroxyl groups. Table IV compares results obtained on the same samples by the nitrite method with those obtained by the acetic anhydride reaction rate method. The agreement is reasonably good. percentage shown on the results The reported by the acetic anhydride rate
*
4PO
400
rnt.)
Solvent: hexane
Table 111. Comparison of Different Methods of Calculating Per Cent Hydroxyl
Primary Hydroxyl, yo of Total Hydroxyl R:355 :385
Rz343.5~385
EquaEquaMaterial Graph tion Graph tion Voranol CP3001
41. 42.
44. 42.
41. 41.
44. 44.
20. 19.
22. 21, 19.
23. 21. 21.
25. 23. 23.
35.
35. 37.
32. 33.
35. 36.
46.
52. 48.
47. 47.
50. 50.
47. 49.
46. 47.
49.
47.
Polyglycol C 21. Polygl yml D 33. Polyglye01 E 50. Polygly col F 45.
50.
Table IV. Comparison of Nitrite Spectrophotometric and Acetic Anhydride Reaction Rate Methods Primary OH, 70 of Total OH
Material Polyglycol P-2000 Voranol C P 3000 Voranol CP 3001 CP 3000-CP 3001 Blend
Acetic
Nitrite Anhydride 4.
7.
5.
5.
47.
40. & 4
27.
24. i 2
VOL. 34, NO. 9, AUGUST 1962
1149
Table V. Comparison of Results Obtained on Known Blends of Voranol CP3001 and Voranol CP4000
Primary Hydroxyl, yo of Total Hydroxyl Voranol Trityl CP3001 Nitrite ChloAcetic % Method ride Anhydride 0 10 20
80 90 100
0. 3. 9.
34. 43. 46.
0. 5. 7.
34. 37. 41.
A comparison of three methods is shown in Table V. Again good agreement is found except a t the higher concentrations of primary hydroxyl.
for the privilege of including their data.
INTERFERENCES
(1) Bashkirov, A. N., Lodizik, S. A., Kamzolkin, V. V., Trudy Inst. Nefti, Akad. A'auk SSSR 12, 297 (1958). (2) Critchfield, F. E., Hutchinson, J. A., ANAL.CHEM.32. 862 (1960).
,4ny alcohol or glycol present in the polyglycol will interfer with the determination. Ketones, which also would cause high results, are not likely t o be present in polypropylene glycols. The same is true of propenyl unsaturation, which, however, can be eliminated by hydrogenation if it should be present. ACKNOWLEDGMENT
40. f 4
method indicates the uncertainty in drawing the secondary hydroxyl rate line. The higher percentage is probably most nearly correct. However, great care must be taken in interpreting results by this method when the amount of primary hydroxyl is high.
The results by the trityl chloride rate method reported in Table IV were obtained by J. C. Ambuhl, Central Laboratory, The Dow Chemical Co., Freeport, Texas. The results by the acetic anhydride rate method in Tables I11 and IV were obtained by R. A. Hummel of the Special Services Laboratory. The author thanks these chemists
LITERATURE CITED
(3) Hanna, J. G., 'Siggia, S., 'J.Polymer Scz. 56, No. 164, 297 (1962). (4) Hendrickson, J. G., Texas Div., The Dow Chemical Co., FreeDort, Texas,
private communication.
-
( 5 ) Morton, R. A., Stubbs, A. L., -4nalyst 71, 348 (1946). (6) Pernarowski, M., Knevel, A. hf., Christian, J. E., J . Phann. Sci. 50, 943 (1961). (7) Pernarowski, M., Knevel, A. M., Christian, J. E., Ibid., 946 (1961). (8) Schmulyakovskii, Y . E., Khim. i Tekhnol. Topliv i Masel 4, 46 (1949). (9) Schmulyakovskii, Y . E., Zhur. Prikl. Khim. 32, 2513 (1959). (10) Siggia, S., Hanna, J. G., ANAL.CHEM. 33, 896 (1961).
RECEIVEDfor review April 9, 1962. Accepted May 25, 1962.
An Efficient Dynamic Method for Surface Area Determinations H. W. DAESCHNER and F. H. STROSS Shell Development Co ., Emeryville, Calif,
b The measurement of surface area of solids b y the sorption of nitrogen from a flowing mixture of nitrogen and helium first proposed by Nelsen and Eggertsen has been evaluated and further developed. Variables, sources of error, and means for controlling them are discussed. The response of a thermal conductivity detector is shown as a function of the composition of the gas mixtures used in the method. By suitable manifolding of sample cells and use of a special calibration procedure, a complete determination of surface area (three relative pressures and BET computation) can b e made in about 40 minutes. The experimental procedures are described. The revised method has a repeatability of better than 370, and is applicable over a range of surface areas from 0.04 sq. meter per gram to the highest normally encountered.
A
to the problem of determining the surface area of solids was described by Nelsen and Eggertsen (6). Like most methods in common use (3), their method involves NEW APPROACH
1 150
ANALYTICAL CHEMISTRY
the measurement of the amount of gas adsorbed at a temperature near the boiling point of the gas. The novel feature in their technique is to pass a known nitrogen-helium mixture continuously a t atmospheric pressure through the sample, to measure, by thermal conductivity devices, changes in the nitrogen concentration of the gas leaving the sample, and t o show these changes on the chart of a recording potentiometer. The sample cell is immersed in liquid nitrogen; the sample thus cooled adsorbs nitrogen and temporarily reduces the nitrogen concentration in the gas effluent. Upon warming the sample, the adsorbed nitrogen is liberated, temporarily increasing the nitrogen concentration in the effluent. The adsorption and desorption processes are represented as peaks on the recorder chart; the areas of these peaks are direct functions of the amount of nitrogen adsorbed and desorbed by the sample. Improvements leading to the design of an apparatus for routine analysis were discussed by Lee and Stross ( 4 ) ; the commercial version of this instrument was described by Ettre ( 2 ) . This apparatus featured a nitrogen injector valve, which provided convenient means
for frequent calibration of the instrument with amounts of nitrogen comparable to the amount sorbed during an actual determination. The present paper describes the results obtained in evaluating the response of the thermal conductivity detector as a function of gas composition. It discusses errors that can arise from the flow changes consequent to adsorption or desorption of the measuring gas, and describes a method for avoiding such errors. It also outlines a scheme for calibrating the instrument to make frequent use of the injector valve unnecessary. EXPERIMENTAL
The flow scheme shown in Figure 1 is similar in principle to that described by Nelsen and Eggertsen. Helium and nitrogen are introduced from cylinders equipped with pressure regulators and pass through flow controllers. After blending, the gases' pass through the apparatus as follows: a cold trap to remove any condensibles; B mixing chamber; a thermal conductivity cell (reference) ; the sample cell; a nitrogen injector valve; a second mixing chamber; a second thermal conductivity cell (measuring); and finally through a
