m
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8 sidt ied good absorptiometric practice to make up the oalibiatiiig d u t i o n s with mercuric chloride, but it was found experimentally that varying the amount of mercury present had no effect on the absorptiometer readings. Further experiments were carried out on the determination of copper in the presence of other metallic elements that give coloiless citrate complexes. It was found that copper could be determined in the presence of silver, zinc, cadmium, magnesium, aluminum, and lead. Varying the amounts of these metal ionb had no effect on the copper color. The presence of chloride, acetate, and nitrate ions also had no influence. Iron in amountq of 1 to 2 mg. has no effect on the copper color. Five to 10 mg. of iron will impart a greenish tinge but the correct absorptiometei wading is obtained. Fifty milligrams of iron will give a yelloii green color to a solution containing 20 mg. of copper and obviously interferes. Other colored ions such a t cobalt, chromium, and nickel will affect the copper color. DISCUSSION
The prcscrat work was carried out Kith the addition of 50 nil. (if ammonium citrate and 50 ml. of ammonia to neutral solutions. These amounts are in excess of those required for the immediate complexing of the quantity of mercury present (3.5 grams of mercuric chloride) but it was found that unless a large excess was present the mercury tended to precipitate on standing. The citrate complex of copper has been studied in detatl by Bobtelsky and Jordan ( I ) , who found in their photometric measurements that a tenfold excess of citrate did not further affect the oxidation. For many purposes it would be possible to employ ammonium citrate only, in order to retain other ion. in solution and
t o develop a copper color such the determination of copper in the presence of silver and zinc. It was necessary in the present instance to add a constant excess of ammonia in order t o a w s t in complexing the mercury present. This colorimetric method could be adapted for the rapid determination of copper in a number of light alloys, brasses, and solders and the actual amount of ammonium citrate and ammonia t o be added could best be determined for each particular case. Mehlig ( S , 4 ) found that the copper-ammonium color was always dependable, provided that a constant excess of ammonia was present, and was stable for 6 weeks. By the addition of ammonium citrate it is possible to determine copper in the prrscnce of other ions which would otherwise precipitate in a straight ammoniacal solution. The volatility of ammonia, to which some workers object (@, is not so great that there would be any serious diminution in the concentration during the length of time taken to carry out colorimetric observations on a number of samples, If the solutions are kept in stoppered flasks. By the use of 4-cni. (30-ml.) cells a calibration graph may be constructed which can be read to *0.5 mg. of copper and will give greater accura(v than smaller cells with low concentrations of copper. The I-tm (8-ml.) cells will, however, give a graph which enables the work to be carried over a wider range and can be read with an accuracy of + I mg. of copper. LITERATURE CITED (1)
Bobtelskj, M., and Jordan, J., J. Am. Chem. Soc., 61, 1824
(1945). (2) CrumDler. T. B.. ANAL.CHEM..19. 325 (1947). (3j Mehlig, J. P., IND. ENG.CHEM.,- ~ N A LED., . 13, 533 (1941 (4)Ibid., 14,903 (1942). RECEIVED October 23, 1947
Determination of Hydroxyl Groups in Organic Compounds
,
B. L. JOHNSON, Montuna Stote College, Bozernan, M o n t o m
\CETYLATIOS of alcoholic compounds, followed by isolation, purification, and saponification of the acetylation product, is time-consuming as a quantitative procedure. Simpler and more rapid methods for determining hydroxyl groups by hack-titration of the excess acetyl chloride or acetic anhydride aftcr acetylating a weighed sample of a hydroxylic h a w become well established in recent years (1, 6, 6, ?). The inethod here described oresents a modification of a oreviouslv published acetyl chloride procedure, which has been used succmsfully in this laboratory for a number of years on a variety of alcohols (Table I) and on fats and oils. I t is of doubtful oalue in the case of compounds insoluble in toluene. L
'Tahle I. Number of Hydroxyl Groups in C o m m o n Alcohols Alcohol ri-Butyl alcohol (purified) Benzyl alcohol
No. of
Teste 8
Av. of
Tests 1.04
6
1.12
1,3-Propanediol
5
2.11
Isobutyl alcohol
6
1.26
n-Propyl alcohol
5
1.15
1,2,3-PropanetrioI
3 5
3.06
12-Ethanediol monoethyl ether
5
0.99
2.96
CHEMISTRY OF PROPOSED METHOD
Scetylation in toluene as the solvent: R-OH Of
+ CHZCOC1
=
+ HC1
CH3-COOR
B)
the excess acetyl chloride:
CHSCOCI
+ HOH = CHjCOOH + HC1
21
Back-titration of the acetic acid and hydrochloric acid:
+ XaOH HC1 + YaOH
CH8COOH
= CH,COONa = Sac1
+ H20
+ HzO
31 4)
Reactioii 1 is carried out in an anhydrous system which is closed by a trap containing a measured volume of standardized sodium hydroxide. Loss of volatile hydrogen chloride or acetic acid 1s thus prevented.
__ Individual Results 1.04 1.06 1.15
1.16 2.26 2.06 1.44
1.34 1.22
1.17 2.84 3.11 3.04 0.94
1.11
.
1.18 0.94 1.06 1.06 2.13
0.98
EXPERIMENTAL REAGENTS
i:i6 1.15
1.27 1.23 1.07 1.19 3.09 3.09 3.01
1.25
1.18 1.19
.Icetyl chloride dissolved in toluene to make an approximately 1 S solution, accurately standardized. Approximately 1 K sodium hydroxide, accurately standardized. Phenolphthalein indicator, 1 gram in 100 ml. of alcohol. Recently cooled h i l e d distilled ivat,er, in large quantity.
2.91 2.94
SPECIAL EQUIP!$lE.%T
2.05
0 94 0 92
2.07
1.63
..
Kjeldahl flasks (500-ml.) fitted with a side arm as shown in Figure 1. Calcium chloride tubes partly filled with glass beaQ and indented close to stem outlet to prevent beads from 5ealing
ANALYTICAL CHEMISTRY
778 outlel. Test tubes (1 X 10 inch) fitted with inlet and outlet tubes. These act as condensers in the neck of the Kjeldahl flask (Figure 1). DIRECTIONS
Aboul 0.1 gram of a sample is allowed to fall as drops to the
bottom of the modified Kjeldahl flask from a suitable weighing pipet, and 25 ml. of the acetyl chloride solution are pipetted into the flask. The delivery tip of the pipet should extend almost to the bottom of the flask to localize the acetyl chloride vapor in the bulb of the Kjeldahl flask. The condenser is slipped into the neck of the flask and immediately sealed with melted paraffin. The side arm of the Kjeldahl flask is prepared by attaching the calcium chloride scrubber tube and allowing exactly 3 ml. of the standard sodium hydroxide solution to flow down over the glass beads and into the elbow of the side arm (the trap). A very small flame is lighted below the Kjeldahl flask and the ieaction mixture is allowed to reflux for 2 hours. The gas is then turned off and cold recently boiled distilled water is pourcd rapidly into the calcium chloride tube without overflow. A q each portion of this wash water is drawn into the flask, more I‘ added until nearly one half of the flask is filled with it. AftcS1 the paraffin seal is broken, the side arm is washed by adding water to it as before, but forcing it this time into the flask with a rubber pressure bulb. As the condenser is lifted out of place it is washed ~ i t ha strrv~mof cold recently boiled distilled water. The contents ot t h e flask are now ready for titration; 0.5 ml. of the phenolphthalein indicator is added and the contents are titrated with the standardized sodium hydroxide until a definite pink color permanent for 5 minutes is reached. Because a heterogeneous system csists, shaking must follow every increment of alkali added. Control determinations should be made occasionally to standardize the acetyl chloride solution. The paraffin seal is best tested by allowing the heat of the hands to expand the gases in the flask. A perfect seal is denoted by movement of the alkali solution in the side-arm trap. The fog seen a t first in the calcium chloride tube i j toluene vapor And is ordinarily free from any acid reaction.
Table 11.
Acetyl Values Obtained by Holland’s and Author’s Methods
Sample Caqtor oil
Saponification No. Unacetyl- Acetyl- Acetyl Value ated ated Holland Author
1 2 3 4 5 6 Av.
% OH Holland Author
179.3 185.2 179.1 182.2
...
Cottonseed oil 1
2 3 4 5 6 A v.
Cottonseed oil (used formelting pointa)
185.0
357.8
172.8
217.2 221.3 218.1 221.9 223.4 221.7 220.6 243.1 237.0 235.1 234.6 238.8
250.4 246.0 233.3 241.9 250.0 243.0 246.1 311.5 301.2 299.1 299.9 289.7 283.0 297 4
... ... ... ... ...
..,
A v.
237.7
25.5
. .. ..,
.., ... .. , ..,
59.7
The method as described was applied to the determination of acetyl values of fats and oils of plant and animal origin (Table 11). The method of Holland, used as the reference method, espresses the acetyl value as the difference between the saponification numbers of the acetylated and unacetylated samples. Marks and Morrell ( 4 ) report 5.2% of hydroxyl in castor oil. Glyceryl triricinoleate contains 5.47% hydroxyls. The value of 5.25% hydroxyl obtained by the author using Holland’s method is in close agreement with the value found by Marks and Morrell. The author obtained an average of 5.48% hydroxyl in castor nil k)y the method presented here.
Figure 1. Hydroxyl Group. and Acetyl Apparatus for Determining Value 1. 2. 8. 4. 5. 6.
Glass bead scrubber Side-arm trap Water inlet and outlet 1 X 10 inch test tube Reaction flask Paraffin seal
...
181.4
5.25
j.48
28.3 67.1 65.3 65.8 69.8 66.9
0.77
0.86
Not cornputed ...
66.9
..,
...
...
sot
computed ...
The results for cottonseed oil agree with reported values. For the cottonseed oil that had been used for a long time as a high boiling point bath liquid, results are interesting because of the oxidation that probably occurs a t points of unsaturation. The author here found slightly elevated hydroxyl values, which possibly reflect increased hydroxyls by oxidation. Decreased losses of acetylated sample in comparison with the method of Holland also may be a factor, inasmuch as the author’s method requires no washing. Eight or more acetyl values may easily be determined in 4 to 5 hours, including all operations. LITERATURE CITED
Calculations.
OH groups = Equiv?lents of Dase =
equivalents of base X molecular weight (ROH) sample weight (ROH)
[ml. of NaOH (control) - ml. of NaOH sample)] X normalitv of NaOH ~~
1000
Older methods have certain objectionable features(5,8). The method of Holland (3) requires two saponifications, an acetylation, and a long washing of the acetylated sample. Marks and Morrell(4) showed in 1931 that mixtures of acetic anhydride and pyridine could be used satisfactorily in determining the percentage of hydroxyls in castor oil.
(1) Christensen, B . E., Pennington, L., a n d Dimick, P. K., INB. ENG.CHEM.,ANAL.ED., 13, 821 (1941). (2) Holland, E. B., Mass. Agr. Expt. Sta., Bull. 151 (1914). (3) Lewkowitsch, J., “Chemical Technology a n d Analysis of Oils, Fats, a n d Waxes,” 4th ed., Vol. 1, p. 336, London, Macmillan c o . , 1909. (4) Marks, S., a n d Momell, R. S., Analyst, 56, 428 (1931). ( 5 ) Petersen, J. W., Hedberg, K. W., a n d Christensen, B . E., IND. ENG.CHEM.,- 1 ~ 4ED., ~ . 15, 225 (1943). (6) Peterson, T. L., a n d West, E. S., J . Bid. Chem., 74, 379 (1927). (7) S m i t h , D. AT., a n d B r y a n t , W.41. D., J . A m . Chem. 8oc.. 57. 61 (1935). (8) Terley, A., a n d Bolsing, F., Ber., 34, 3354 (1901).
RECEIVED July 19, 1946.
