Extraction of Titanium Thiocyanate with Tri-n-octylp hosphine Oxide Direct Colorimetric Determination in the Organic Phase J. P. YOUNG and J. C. WHITE Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
A simple, sensitive, and selective method for the colorimetric determination of titanium is based on measurement of the absorbance of the titanium-thiocyanate complex extracted from sulfate or chloride acidic solutions by tri-n-octylphosphine oxide dissolved in cyclohexane. The molar absorbance index for this complex at 432 mp, the wave length of maximum absorbance, is about 41,000. The extracted complex conforms to Beer's law up to a concentration of at least 1.7 y of titanium per ml. Neither quadrivalent metal ions nor extremely large amounts of trivalent iron and sexivalent uranium interfere. Niobium in concentrations as much as three times that of titanium will cause a 5% error. Fluoride, phosphate, and oxalate do not interfere; nitrate will cause decomposition of the thiocyanate ion. The titanium complex can be extracted quantitatively into tri-n-octylphosphine
REAGENTS
oxide in a single equilibration at phase ratios, aqueous to organic, of at least 20. The coefficient of variation i s about 2%.
D
Tri-n-octylphosphine oxide, approximately 0.01M in cyclohexane. Dksolve 3.9 grams of tri-n-octylphosphine oxide (TOPO) (Eastman Chemical Products, Inc., Eastman Kodak Co.), in 1 liter of cyclohexane. Standard titanium stock solution in 4M sulfuric acid, approximately 10 mg. of titanium per ml. Add approximately 25 ml. of titanium tetrachloride (Fisher Scientific Co.), purified, to 100 ml. of concentrated sulfuric acid and heat to fumes of sulfur trioxide, Cautiously add approximately 300 ml. of water and dilute to 1 liter with 4M sulfuric acid. Determine concentration of titanium gravimetrically by precipitation with ammonium hydroxide (1). Standard titanium stock solution in 6 M hydrochloric acid, approximately 10 mg. of titanium per ml. Dissolve 25 ml. of titanium tetrachloride (Fisher Scientific Co.), purified, in 500 ml. of concentrated hydrochloric acid, and dilute to 1 liter with water. Determine titanium concentration as above. Dilute both solutions with the respective acids to a concentration of approximately 5 y of titanium per ml.
studies on the effect of thiocyanate ion on the extraction of various cations with tri-n-octylphosphine oxide dissolved in cyclohexane, it was noted that an intensely colored organic phase resulted from the extraction of titanium from an acidic solution which contained ammonium thiocyanate, and that the absorbance of the phase appeared to increase with increasing titanium concentration. Of the numerous metals which form complexes with thiocyanate, titanium was one of the few that was extracted into tri-n-octylphosphine-oxide-cyclohexane as a colored species. The absorbance of this organic phase is the basis for the colorimetric determination of titanium which is described. URING
07
0 6
05
04
u z m 4 (r
2
r
I
\ \
03
t
II
i
PROCEDURE
The solution to be extracted is adI
I
I
I
1
I
I
I
IO0
t i \
U
0.2
f
c
A
CARY RECORDING SPECTROP H O T O M E T E R , M O D E L No. 14 1-cm CELLS REFERENCE SOLUTION - CYCLOHEXANE TITANIUM CONCENTRATION-
-1
\;oMPLEx TITANIUM
THIOCYANATE
j 1
\
\
.-"
d
75
I-
VOLUME OF AQUEOLIS PHASE- 5 m l .
+
P H A S E RATIO I T I T A N I U M CONCENTRATION-2 98 y / m l
-
Y
W
3f
ANION CONCENTRATION
HELD
50
t
01 AMMONIUM
THIOCYANATE
00 300
400 WAVE L E N G T H ,
500
6 00
mp
Figure 1. Absorbance spectrum of titanium-thiocyanate complex in tri-n-octylphosphine oxide-cyclohexane medium
0
2 ACID
4 CONCEllrlbSICN,
6 Moloiily
Figure 2. Extraction of titanium-thiocyanate complex with tri-n-octylphosphine oxide as function of composition of extracted solution VOL. 31, NO. 3, MARCH 1959
393
-
-
0 0 500 BECKMAN
100
a-" F
SPECTROPHOTOMETER, MODEL DU
WAVE L E N G T H - 4 3 2 rnp. I - c r n CELLS T I T A N I U M C O N C E N T R A T I O N - 0 6 y /rnl
0 w
z 4 [L
Lc5
75
0
r W
v)
m
t
I
AQUEOUS P H A S E - 5 m l . 7 M_ HCI PHASE R A T I O - I .
0400 50 0
50
200 AMMONIUM THIOCYANATE, mg. / m i . IO0
I50
250
Figure 3. Effect of ammonium thiocyanate on absorbance of titanium-thiocyanate complex in trin-octylphosphine oxide-cyclohexane
Figure 4. Effect of time of equilibration on extraction of titanium-thiocyanate complex into 0.01 M trin-octylphosphine oxide in cyclohexane
justed to be either 6M chloride or sulfate and at least 1M acid (4M if in a sulfate medium). The solution should not exceed 25 ml. nor contain more than 100 y of titanium. If large concentrations of moderate to strong oxidants, such as quinquivalent vanadium, quadrivalent tin, or sexivalent chromium are present in a sulfuric acid solution, 0.2 ml. of thioglycolic acid should be added to reduce these oxidants.
About 150 mg. of ammonium thiocyanate is then added per ml. of sohtion. (A convenient measure is a microcrucible (-4dolPh Coors co.1, Size 5/0; when filled, it contains approximately 700 mg. of ammonium thiocyanate). The resulting mixture is extracted for 5 minutes with 5 ml. of 0.01M tri-n-octylphosphine oxide in cyclohexane. A reference solution, which contains the reagents but no
Table I. Extraction of Titanium-Thiocyanate Complex with 0.01M Tri-noctylphosphine Oxide TOPO, 0.01M in cyclohexane, 5 ml. Phase ratio, 1 Aqueous phase, 6M in HCl or H2SOd
Table 111. Effect of Diverse Ions on Colorimetric Determination of Titanium Extraction Colorimetric measurement Becliman spectrophotometer, Model DU Aqueous phase, 5 ml. Wave length, 432 mg Phase ratio, aqueous/organic, 1 Concentration of TOPO, 0.01111 Corex cells, 1-cm. Equilibration time, 5 min. Titanium, y/ML Diverse Taken Found Difference Taken Found Difference 6.44 Sulfuric Acid Ion 7Al Hydrochloric Acid Y/M1. AI +3 130 2.35 -0.03 2.38
Ammonium thiocvanate concentration, 150 mg./ml. " Equilibration time, 5 min. Titanium Taken, Extracted, Y
23 60
%
Au f 3 Bi + 3 Ce +a
99.9 99.9
Ce +4 c o+2 920
46
Cr + 3
Table II. Effect of Phase Ratio on Extraction of Titanium-Thiocyanate Complex with Tri-n-octylphosphine Oxide Volume of organic phase, 5 mt. Equilibration time, 3 min.
Ammonium thiocyanate concentration, 100 mg./ml. Titanium Pres- ExtracTOPO, V./ Extraction ent, ted, Mmole V O System y % 0.05 1 7MHCI 14.9 99.3 2 5 10
0.1 0.05
0.1
394
10
5 6MHzSOd 10 10 20
14.9 14.9 14.9 14.9 11.6 11.6 11.6 11.6
ANALYTICAL CHEMISTRY
99.3 98.6 93.2 99.9 99.2 88.0 99.9 95.8
Cr +e Fe+3
Mo+%
Nb+'
Ni Pt +4 Sn +4
260 200 200 200 400 140 100 200 100 200 100 200 112 370 200 400 600 1000 8 16 100 200 400 50 100 100 200 100 100 200 400
titanium, is similarly extracted. The absorbance of the organic phase resulting from the extraction of the sample is measured against the organic phase of the reference so!ution in Carex l-cm. at 432 m p by a spectraphotometer, DU Or B. If necessary, the organic Phase may be diluted with Cyclohexane prior to the absorbance measurements.
2.38 2.98 2.98 2.98 2.98 2.98 2.98 2.98 2.38 2.38 2.38 2.38 2.38 2.38
2.36 3.01 3.04 2.95 2.95 2.99 3.02 3.01 2.34 2.34 2.39 2.26 2.37 2.37
-0.02 0.03 0.06 -0.03 -0.03 0.01 0.04 0.03 -0.04 -0.04 0.01 -0.12 -0.01 -0.01
2.38 2.38
2.50 2.67
0.12 0.29
2.38 2.38 2.38 2.38
6.4 10 2.40 2.37
4 8 0.02 -0.01
2.98 2.98 2.98
2.95 2.96 2.95
-0.03 -0.02 -0.03
2.32
2.41
0.09
2.32
2.23
-0.09a
2.32 2.32 2.32 2.02
2.33 2.25 2.19 2.00
0.01 -0.07 -0.13 -0.02a
2.32 2.32 2.32 2.32
2.39 2.36 2.41 5.94
2.32
2.31
-0.01
2.32
2.23
-0.09a
0.070 0 . 04a
0.090 3.62
After completing the determination, all glassware that has contained the extracted complex should be rinsed with ethyl alcohol. If the complex has remained in the glassware for an extended period, a yellow film may adhere to the surface, which can be easily removed by rinsing with nitric acid. EXPERIMENTAL
Absorbance Spectra. The partial absorbance spectrum of the titaniumthiocyanate complex in a tri-n-octylphosphine oxide-cyclohexane medium is shown in Figure 1. The maximum absorbance of the complex occurs a t 432 mp. At this wave length the abso1bance of a reference solution Khich does not contain titanium is approximately 0.010 or less. The absorbance of the titanium-thiocyanate complex in the tri-n-octylphosphine oxidc-cyclohexane medium conforms t o Beer’s law up to a concentration of 1.7 y of titanium per nd. (absorbance of 1.50). The reciprocal of the slope of the absorbance-concentration plot is 1.15 y of titanium per ml. per unit ab-
sorbance. The molar absorbance index of the complex a t a wave length of 432 mp is about 41,000. Extraction Characteristics of Titanium Thiocyanate. Titanium thiocyanate can be extracted from either acidic chloride or sulfate solutions. For quantitative extraction in a single equilibration, the solution should be a t least 6 M with respect t o the particular anion. The minimum acid concentration should be 1M. As shown in Figure 2, aluminum and magnesium chlorides are suitable reagents for adjusting the chloride concentration and are also more effective than sodium chloride. Sodium sulfate in sulfate solutions parallels the results found for the sodium salt in chloride solution. Effect of Ammonium Thiocyanate. Ammonium thiocyanate serves t w o purposes: as the chromogenic reagent, and also as an effective masking agent hich provides some selectivity as to the cations that will be extracted by tri-n-octyl-phosphine oxide, particularly from hydrochloric acid solutions. Ammonium thiocyanate
a s Thiocyanate Complex in Tri-n-octylphosphine Oxide-Cyclohexane Titanium, y/-Ml.
Diverse Ion Ta +6
Th+4
UO, + 2
v
+4
V+’
w +e Zr +4
F-
POa’
Citrate Lactate Oxalate a
?/MI. 13 26 52 104 100 200 400 108 216 100 200 400 200 600 200 400 50 100 200 100 200 400 600 140 280 380 560 760 400 800 1000 1200 2000 2000 1000 40000 1000
Taken
Found
Difference
2.98 2.98 2.98 2.38 2.38
3.00 2.96 2.96 2.37 2.36
0.02 -0.02 -0.02 -0.01 -0.02
2.98 2.98 2.98
3.00 3.02 2.54
0.02 0.04 0.44
2.98 2.98 2.98 2.98 2.98
2.95 3.01 3.11 3.05 3.10
-0.03 0.03 0.13 0.07 0.12
2.98 2.98 2.98
3.22 2.99 2.98
0.24 0.01 0.00
2.98
2.71
7M Hydrochloric Acid
-0.27
2.98
3.00
0.02
2.98 2.98
3.00 3.00
0.02 0.02
2.98 2.99 0.01 0.2 ml. thioglycolic acid present in aqueous phase.
Taken
Found
Differenoe
2.32 2.32 2.32 2.32 2.32
2.36 2.36 2.36 2.33 2.33
0.04 0.04 0.04 0.01 0.01
2.32 2.32 2.32 2.32
1.67 2.43 2.47 2.66
-0.65 0.11 0.15 0.34
2.32 2.32
2.35 2.36
0.03 0.04
2.32 2.32 2.32
2.34 2.35 2.35
0.02 0.03 0.03
2.32 2.32 2.32
2.36 2.34 2.35
0.04 0.02 0.03
2.32 2.32
2.32 2.35
0.00 0.03
2.32 2.32 2.32
2.37 2.37 2.32
0.05 0.05 0.01
6M Sulfuric Acid 2.32 2.32 0.00 2.31 -0.01 2.32 2.24 -0.08 2.32 2.12 -0.20 2.32
grossly hinders the extraction of sexivalent uranium and trivalent iron from a solution of hydrochloric acid and some data has been reported on this (3). The concentration of ammonium thiocyanate in the aqueous phase has no effect on the absorbance of the titanium complex provided the concentration of this salt exceeds 20 mg. per nil. in the solution to be extracted (Figure 3). As a general rule, the concentration of this salt is usually maintained a t about 150 mg. per ml. to ensure sufficient thiocyanate ion not only to form the colored titanium complex, but also to provide a large excess of thiocyanate ion as a masking agent. In a highly acidic aqueous phase, ammonium thiocyanate slon ly decomposes to form, among other products, polymerized thiocyanic acids and hydrogen cyanide ( 2 ) . The aqueous phase from the extraction of the titanium complex should be discarded within approximately 30 minutes. If not feasible, the extracted aqueous phase should be placed in a suitable hood. There was some variation in the absorbance of reference solutions with various lots of ammonium thiocyanate, but not in the net absorbance of the titanium complex. When the absorbance of the reference solutions exceeds approximately 004, a fresh supply of ammonium thiocyanate should be obtained. Effects of Concentration of Tri-noctylphosphine Oxide. A slight effect of tri-n-octylphosphine oxide on the absorbance of the titanium-thiocyanate complex was noted over the wide concentration range of tri-noctylphosphine oxide in the final test solution, from less than 0.00002 t o 0.04M, to which the method has been applied. For concentrations of less than 0.00211f tri-n-octylphosphine oxide, the absorbance of a given amount of titanium complex is constant. From about 0.004M to 0.04M tri-n-octylphosphine oxide the absorbance is 4% less. In either Concentration range, the absorbance of the titanium-thiocyanate complex conforms to Beer’s law. Csually the variance in calibration curves is insignificant but should be considered in the most accurate determination of extremely low amounts of titanium. Time of Equilibration. The effect of equilibration time on the extraction of the titanium-thiocyanate complex is presented in Figure 4. These data were obtained by mechanical agitation with a Kahn shaker (Kahn & Co., Inc.). Equilibrium conditions are attained in about 3 minutes; however, t o ensure equilibrium, a 5-minute period is recommended. Stability of Titanium-Thiocyanate Complex. Because thiocyanate ion decomposes a t a finite rate in acidic VOL. 31, NO. 3, M A R C H 1959
395
solutions, the stability of the extracted complex in contact with the aqueous phase and in cyclohexane solution was studied by measuring the absorbance of the solutions as a function of time. The titanium-thiocyanate complex in 0.01M tri-n-octylphosphine oxide when kept in contact with the aqueous phase increased only slightly with time, and after 24 hours was only 4%. The in-
IV. Summary of
Table
Effect
of
Diverse Ions
Weight Ratio, IonjTi
Diverse Ion
Al+a
110 70 70 130 60 70 70 40 90
Au +a Bi +a Ce +a Ce +4
c o +2
Cr +a Cr +e
F e +*
160 260 500 4
:Mo +e
40 90
180
Nb+6 Ni +z Pt +b Sn f 4
3 90 40 40 130 10 30 130 170
Ta +6 Thf4
uo
+*
260
Vf' V +)
a
b b
a
b
a (1
a
-5 D
b b
t4 b b
-4c -3 -5 - IC +3 3c +2 4c
t5 + + 1 5 b b
+b4
b
b b a
a b
-4 c a
-5 a 0
-5
-5
40
+5
+b5
170 100
+b5
Zr +4
100
150 300 900
Citrate Lactate Oxalate
b
a
25
20
PO4'
a
a
W+S
F-
Error, yo 6M .~~ HC1 &SO, 7M
900 9000
(I (I
a
*
-5
(I
b
a
(I
0
D
a
b
d
a a 450 a Error less than 2% Not tested in this medium. 0 0.2 ml. thioglycolic acid present in aqueous phase.
crease in absorbance of a 2.5fold dilution with cyclohexane of the extracted complex in a similar period was approximately 4%. No significant increase in absorbance of a 25-fold dilution of the extracted complex with cyclohexane was noted in 24 hours. The somewhat greater stability of the 25-fold dilution of the titanium-thiocyanate complex is not unexpected because any effect from hydrochloric acid, excess ammonium thiocyanate, or tri-n-octylphosphine oxide is virtually eliminated by this large dilution. Reference solutions containing an equal amount of thiocyanate were prepared, extracted, and diluted in exactly the same manner. The absorbance of the reference solutions increased a maximum of 0.006 absorbance unit over a 24-hour period. Cyclohexane should be used if any dilution of the organic extract is required prior to measurement of the absorbance. Absolute ethyl alcohol, as a diluent for the titanium-thiocyanate complex is 0.01d4 tri-n-octylphosphine oxide, was completely unsatisfactory. The absorbance of a known amount of the titanium complex in an 80% ethyl alcohol medium was 60% of theoretical and steadily decreased on standing. Effect of Concentration of Titanium. Data on the extraction of the titanium-thiocyanate complex with 0.05 mmole of tri-n-octylphosphine oxide as a function of titanium concentration are shown in Table I. Up to 100 y of titanium can be quantitatively extracted as the thiocyanate complex with 0.05 mmole of tri-n-octylphosphine oxide; 200 y of titanium is extracted to the extent of 95%. Larger quantities may be extracted by using a more concentrated solution of tri-noctylphosphine oxide in cyclohexane. Effect of Phase Ratio. Data on the effect of phase ratio on the extraction of titanium as the thiocyanate complex are presented in Table 11. For 0.05 mmole of tri-n-octylphosphine oxide, the largest phase ratio ( a / o ) from which quantitative extraction of 15 y of titanium can be achieved is five. By increasing this
Table V. Typical Results for Extraction of Titanium Thiocyanate with Tri-n-octylphosphine Oxide and Its Direct Colorimetric Determination in Organic Phase
Sample S B S KO, la, argillaceous limestone NBS No. 59," ferrosilicon S B S S o . 85a, aluminum alloy Lithium metal Ash from 7-cm. Whatman filter paper KO. 40 0
NO.
Titanium, % Found
Extraction System
Detns.
Present
7 M HC1 6M HzSO,
6 8
0.16 0.105
0.180 1 0 . 0 0 2
7M HC1
7
0.016
6M
2
...
0.0143 1 0 0001 8.3 X
6M HzSOi
5
...
(0.26 y Ti/7-cm.
of
0.102 +0.002
circle)
0.2 ml. of thioglycolic acid added t o aqueous phase prior to extraction.
396
ANALYTICAL CHEMISTRY
quantity to 0.1 mmole, phase ratios as large as 20 are tolerable. With proper choice of the concentration of tri-n-octylphosphine oxide, and proper dilutions where necessary, the absorbance of titanium in concentrations ranging from less than 0.01 to 40 y per ml. can be measured in a 1-cm. absorption cell. Effect of Diverse Ions. An aqueous solution which contained both titanium and the cation to be studied, 7 M hydrochloric acid or 6 M sulfuric acid, was treated according to the procedure described. The results are presented in Table I11 and summarized in Table IV in terms of the highest weight ratio of diverse ion to titanium studied, or the maximum weight ratio that can be tolerated If the error in the determination of titanium is not to exceed 5%. Alkali and alkaline earth ions were not investigated, because they do not extract with tri-noctylphosphine oxide. Of those ions tested which can form complexes with thiocyanate, the uranyl, ferric, and cobalt complexes are not extracted to any appreciable extent by trin-octylphosphine oxide and do not interfere. Niobium, tungsten, and molybdenum form colored thiocyanate complexes that are extracted. Niobium can be tolerated in rveight ratios of niobium to titanium up to 3, with 5$& error. Tungsten can be tolerated in high weight ratios, up to 40, without causing serious interference. The maximum amount of tantalum that can be tolerated can be increased above the 30 to 1 ratio by the addition of lactate prior to extraction to prevent hydrolysis. Zirconium, in a chloride medium only, extracts as a fairly unstable colorless thiocyanate complex (3). Extremely large concentrations of this cation impart a turbidity t o the organic phase. Sexivalent molybdenum interferes rather seriously, but its effect is notably diminished in the presence of a small amount of thioglycolic acid. The reaction of molybdenum with thioglycolic acid probably involves a reduction-complexation reaction n hich proceeds a t a fairly slow rate. At weight ratios of molybdenum to titanium of 90 or more, approximately onehalf hour is required for the complete elimination of the molybdenum-thiocyanate complex. The presence of thioglycolic acid not only eliminates the interference of molybdenum, but also reduces oxidizing agents which tend to decompose thiocyanate, particularly in a sulfate medium. Sulfur dioxide was also useful in this respect. This method is free of interference from anions such as fluoride, phosphate, citrate, lactate, and oxalate. Concentrations of nitrate ion above approximately of 0.1X cannot be tolerated because of thiocyanate ion oxidation.
Applications and Accuracy. This method has been applied t o the determination of titanium in three Kational Bureau of Standards standard samples, t o demonstrate its accuracy and general applicability, and t o the determination of titanium in lithium metal and in filter paper ash (Table 1‘). This method is applicable t o a wide variety of samples and to a wide range of titanium concentrations. For example, it has been applied t o a sample which contained 5.0 mg. of titanium per ml. and somewhat smaller concentrations of several other metal ions. The results, after adequate dilution, were within 5y0 of the expected value and the entire determination was per-
formed in approximately 15 minutes. The precision, as demonstrated in the determination of titanium in the three NBS samples, is to 2% or less. The significant amount of titanium contained in filter paper ash should be considered if the dissolution of a particular sample involves an ignition of filter paper with subsequent addition of the dissolved ash to the final solution. Based on the amount of titanium found in filter paper ash, an absorbance correction of as much as 0.05 might be necessary if the entire quantity of the ash were equilibrated with tri-n-octylphosphine oxide. The titanium concentration in filter paper ash varies with the manufacturer and Riith different lots from the same manufacturer.
ACKNOWLEDGMENT
The authors acknowledge the assistance of J. R. French in carrying out some of the experimental details of this study. LITERATURE CITED
(1) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 582,
Wiley, New York, 1953.
( 2 ) Stokes, H. N., Cain, J. R.,
J. Research Natl. Bur. Standards 3, 15’7,
(1907). (3) Young, J. P., White, J. C., Tulanta, 1, 263 (1958). RECEIVEDfor review June 26, 1958. Accepted October 15, 1958. Work carried out under contract No. W-7405-eng-26 a t Oak Ridge National Laboratory, operated by Union Carbide Nuclear Co., a division of Union Carbide Corp., for the U. S. Atomic Energy Commission.
Determination of Hydroxyl Concentration in PoIy propylene Glycols by infrared Spectroscopy EUGENE A. BURNS and R.
F. MURACA
Jet Propulsion laboratory, California Institute of Technology, Pasadena 3, Calif.
b A rapid method for determination of the hydroxyl concentration in polypropylene glycols by infrared spectroscopy has been developed. The -OH stretching vibration of 14 different polypropylene glycols follows the Lambert-Beer-Bouger law with a coefficient of variation (per cent The standard deviation) of 2.2%. “effective” molar absorptivity of the hydroxyl is calculated to be 6.74 grams per mmole-mm. at a slit width of 0.012 mm.
E
in this laboratory have demonstrated that the usual acetic anhydride acetylation procedure (6) for the determination of hydroxyl concentration is applicable to polypropylene glycols when the time for acetylation is increased to about 4 hours. Because of the amount of time required for this determination, it was considered desirable to investigate the possibility of supplanting the acetylation procedure by a rapid, simple, infrared technique such as has been used with great success for analysis of low molecular weight alcohols (8) and phenols XPERIRIEKTS
(‘J
4)’ EXPERIMENTAL PROCEDURE
The infrared work was performed with a Perkin-Elmer Model 21 doublebeam infrared spectrometer. All absorb-
ance measurements were made with a single, fixed-thickness sodium chloride cell, which was calibrated from its interference fringes (7) and found to have a path length of 0.0405 mm. The spectrometer was operated a t a scanning rate of 0.39 micron per minute, response 1, gain 5, suppression 10; the slit width a t the 2.84-micron -OH peak was calibrated and found to be 0.012 mm. The base-line technique (3, 9) was employed to evaluate Io (effective incident light intensity) , used in calculating the absorbance, A: A log ( l o / Z ) (1) where I is the transmittance a t the 2.84-micron absorbance peak. EXPERIMENTAL RESULTS
A typical transmittance curve of a polypropylene glycol having a molecular weight of about 2000 is shown in Figure 1. The -OH stretching vibration gives rise to a sharp peak a t 2.84 microns (3520 cm.-I) and thus should yield reproducible values for transmittance. The absorbance, A , of this peak, with a cell of constant optical path length, is a function of the amount of hydroxyl and water present in the sample : A = AOH A H ~ O = UOH COH U~OCH,O
+
+
(2)
where U O H arid u H 2 0 represent proportionality constants related to the molar
absorptivities of the species denoted by the respective subscripts, and COH and caz0 represent the concentrations. I n order to evaluate the true molar absorptivity of the hydroxyl group, it was necessary to establish the contribution of water to the 2.84-micron -OH peak, as preliminary experiments had shown that water exhibits some absorbance at 2.84 microns. The water content of a polypropylene glycol sample (sample I, Table I) mas determined by the Karl Fischer procedure, Samples containing different amounts of water were prepared by further additions of water, and the exact amounts were determined by a Karl Fischer titration. A graph of the absorbance of sample I as a function of the per cent water is a straight line. The absorbance value a t the intercept (0.273 absorbance unit) is a function of the hydroxyl content alone. The slope of the line, evaluated by least squares, is 0.075570‘0-1. Hence, for a 0.0405-mm. cell thickness, the following expression for the molar absorptivity of hydroxyl can be evaluated : aOB
AOH -= A COH
- (0.0755) ( % HzO) COB
(3) where uOH is obtained in gram per millimole of hydroxyl when cOH is expressed in millimoles of hydroxyl per gram. VOL 31, NO. 3, M A R C H 1959
0
397
