INDUSTRIAL AND ENGINEERING CHEMISTRY
418
TABLEVII. SEPARATION OF LEADFROM. VARIOUS MBTALS [In each case Pb(N0a)z taken, 162.5 mg. = 101.7mg. of Pb] Salt Metal Pb(N0a)z Error, Added Content Found Pb Mg. Mo. Mo. 162.3 -0.1 162.4 HaAsOa 500 -0.1 162.7 f0.l 162.9 500 +0.2 Bi(N0s)a 163.0 +0.3 162.9 15 Ca(N0a)a +0.2 162.2 -0.2 162.3 500 Cd(NOd2 -0.1 162.2 -0.2 Cu(N0a)z 162.2 300 -0.2 162.4 -0.1 162.5 500 0 Hg(NOs)z 162.4 -0.1 162.4 Pure Pb(N0da -0.1
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Summary Attempts to separate strontium from calcium by precipitation of strontium nitrate with nitric acid in organic solvents resulted in incomplete precipitation and slimy, unflterable precipitates Strontium nitrate can be completely precipitated in a dense, crystalline form from a water solution, and separated from twenty-eight metals by the very slow addition of 100 per cent nitric acid until the resultant solution contains not
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VOL. 8, NO. 6
less than 79 per cent: to separate barium from the same metals, 76 per cent acid suffices; to separate lead, 84 per cent is necessary, but chloride makes impossible a separation of lead from tin and antimony. The precipitate should stand a t least 30 minutes before filtering; if it is very small, the solution should be stirred mechanically for 45 minutes. The nitrate is dried 2 hours at 130" to 140" C. Temperatures up to 70" C. do not increase appreciably the solubility of strontium nitrate but may increase that of other nitrates. The solubility of calcium nitrate decreases rapidly with increasing acid concentrations. A maximum of 80 per cent acid is recommended.
Literature Cited (1) Fresenius, 2.anal. Chem., 32, 189 (1593). (2) Lange, "Handbook of Chemistry," p. 853, Sandusky, Ohio, Handbook Publishers, Ino., 1934. (3) Rawson, J. 8oc. Chem. Ind., 16, 113 (1597). (4) Rose, Ann. Physik. C h m . , 110,296 (1860). (5) Stromeyer and Pfaff, "Handbuoh der analytiaohen Chemie," Vol. I, p. 412, 1821. REICEIVED August 26, 1936. Presented before the Diviaion of Physical and Inorganic Chemistry a t the 87th Meeting of the Amerioan Chemical Society, St. Petersburg, Fla., March 26 to 30, 1934.
Determination of Phthalate Plasticizers FRANCIS C. THAMES, Naval Powder Factory, Indian Head, Md.
W
H E N the diamyl, dibutyl, diethyl, and dimethyl esters of phthalic acid are used to plasticize cellulose nitrate they may be determined by extraction and saponification. However, if the cellulose nitrate plastic also contains aromatic nitro derivatives, such as those of benzene, toluene, and xylene, the determination of the phthalate plasticizer becomes more difficult. These nitro compounds are soluble in the plasticizers and are also mutually soluble with them in solvents. They therefore cannot be separated by difference in solubilities. When the alkali is added for the saponification of the ester plasticizer, in the presence of these nitro compounds, colored additive compounds of the alkali and nitro derivatives are formed (4). These colored compounds cannot be separated from the plasticizer by solvents and only a trace of them is necessary to interfere with the titration of the alkali after saponification. Therefore, a method for the determination of the phthalate plasticizers, in the presence of aromatic nitro derivatives, was sought.
Experimental Unable to determine the phthalate esters in this mixture by saponification, a gravimetric method was developed. Barium phthalate is regularly referred to in organic chemistry texts (1, 2) as an insoluble salt of phthalic acid. It will not precipitate when barium chloride is added to acid solutions of phthalic acid, but phthalic acid itself is precipitated. The solution must be neutral to precipitate barium phthalate. Following the procedure of Ekeley and Banta (S), barium nitrate was added to a solution of sodium phthalate. On boiling, a barium salt was precipitated, which analyzed 51.41 per cent barium oxide compared with 50.89 per cent barium oxide theoretical. In 100 cc. of water a t 20" C., 0.2760 gram of this salt was found to be soluble, in spite of the constant reference to barium phthalate in the literature as an insoluble salt of phthalic acid. Since barium phthalate
precipitates only in neutral solutions, allowing the precipitation of other barium salts, and because of its solubility, it was eliminated as a means of precipitating phthalic acid. Precipitation of phthalic acid as lead phthalate proved to be more favorable than the barium precipitation. Lead phthalate was prepared from phthalic anhydride and lead nitrate, and analyzed 56.55 per cent lead compared with 55.82 per cent lead theoretical. It was also made from saponified dibutylphthalate and lead nitrate and upon analysis gave 55.94 per cent lead. Lead phthalate is a white crystalline compound, of which 0.0020 gram is soluble in 100 cc. of water a t 20" C . It is soluble in nitric acid but insoluble in dilute acetic acid. Because of its low solubility and ease of formation, lead phthalate was adopted as a means of precipitation of phthalic acid. Lead acetate was later used in place of lead nitrate, as the nitric acid left from this reagent interfered with the precipitation of lead phthalate. It was also decided to reprecipitate lead phthalate as lead sulfate to eliminate errors that might be introduced by insoluble organic compounds. The phthalic acid could not be liberated quantitatively by the saponification of the ester in the presence of nitro compounds, therefore i t was liberated from the ester plasticizer by the oxidation of the aliphatic side chains by nitric acid. Crystals were obtained by adding 30 cc. of concentrated nitric acid to approximately 1 gram of dibutylphthalate and evaporating to dryness on the steam bath. These crystals were identified as phthalic acid, according to Mulliken (6),by being converted to o-phthalanil, melting a t 205" C. A trace of nitrophthalic acids was also formed. Oxalic acid was found to be present as a product of the side-chain oxidation of the phthalate esters of the higher alcohols. Oxalic acid must be removed before the lead precipitation because lead oxalate has properties similar to those of lead phthalate. This removal of oxalic acid was accomplished by a second oxidation, using potassium permanganate.
NOVEMBER 15, 1936
ANALYTICAL EDITION
At first, the nitric acid oxidation was applied to the free ester plasticizers and yields of around 55 per cent were obtained. Then, dry cellulose nitrate was also added to the phthalate plasticizer. Oxidation of this mixture increased the yields to around 75 per cent. Yields were determined from the lead precipitation of the phthalic acid after the oxidation of the ester. This increase in yields showed that the decomposition of the cellulose nitrate aided the oxidation of the phthalate plasticizer. The concentration of the nitric acid was changed to 1 to 1, as previous work by the author proved this to be the optimum dilution for the decomposition of cellulose nitrate. The effect of the cellulose nitrate decomposition indicated that the plasticizer would have to be oxidized during the oxidation of the plastic itself. The oxalic acid would be increased, but this could be easily removed with potassium permanganate along with the oxalic acid of the oxidized side chain of the ester. The mere addition of the dry cellulose nitrate to the phthalate ester made a very heterogeneous mixture and hence 75 per cent yields were obtained. B y a careful colloiding of the same amount of cellulose nitrate with the same amount of the plasticizer, thereby increasing the homogeneity of the mixture, approxjmately 100 per cent yields were obtained. This condition likewise simulated the finished manufactured plastic. Quantitative results having been obtained, even when aromatic nitro derivatives were added, the following procedure was adopted for the determination of the phthalate plasticizers in cellulose nitrate plastics. Procedure Place 5 grams of the plastic in a 500-cc. boiling flask, fitted to a reflux condenser by means of a ground-glass joint (Scientific Glass Apparatus Co. No. 1400). Add 40 CC. of 1 to 1 nitric acid to the flask and reflux the mixture at the full heat of an electric flask heater (Gilmer type) until decomposition is comlete and a clear solution is obtained (about 45 minutes). Reflux for about 30 minutes longer to ensure complete reaction This oxidation must not be interrupted or the results will be low. Allow t o cool and add 50 cc. of water. Remove flask from the condenser, insert a glass stopper, and cool further, with shaking, under the tap. Allow to stand for 3 hours. Insoluble nitro compounds will settle out, if present. Filter through a No. 42 Whatman filter paper into a 400-cc. beaker, washing three or four times with cold water. If the volume is more than approximately 100 cc., place on a hot plate and evaporate to this volume. Place beaker on the steam bath and add small amounts of potassium permanganate to remove the oxalic acid. Just a few crystals on the end of a weighing spatula are added each time, and the reaction is allowed to complete itself before the next addition. This is continued until a large precipitate of manganese dioxide persists, and, apparently, does not clear up a t the heat of the steam bath. Filter off the manganese dioxide, while the solution is hot, into a 250-cc. beaker, and wash with hot water. Place the beaker on a slow hot plate and evaporate nearly to dryness; then bring t o dryness on the steam bath to remove the nitric acid. Lead phthalate is soluble in nitric acid. After evaporation, the beaker contains hthalic acid and manganese salts. Dissolve these in 50 cc. ofwarm water, add 20 cc. of 10 per cent lead acetate solution, containing 10 cc. of acetic acid per 100 cc. of solution. Heat on a hot plate, low heat, until the volume is about 50 cc. and allow to stand overnight. Lead phthalate will precipitate as a white crystalline precipitate. Filter, using No. 42 filter paper and washing three or four times with cold water from the beaker onto the filter. The volume of the washings should be kept at a minimum, about 20 cc. In some cases, lead phthalate crystals will adhere to the beaker. Do not attempt t o remove these with a policeman, but allow them to remain in the beaker, as they have been washed free of lead acetate. Add 15 cc. of 1 to 1 nitric acid t o the beaker, dissolving these crystals. Now place a 400-cc. beaker beneath the funnel containing the lead phthalate, and pour the nitric acid solution from the 250-cc. beaker onto the filter paper. When the solution of lead phthalate is practically complete, puncture the paper with the stirring rod and wash the contents into the 400-cc. beaker. Then add 10 cc. of sulfuric acid, precipitating lead sulfate. Evaporate the nitric acid completely
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and take up in 100 cc. of one part alcohol and one part water. Allow to stand 2 hours, filter through a Gooch crucible, and weigh the lead sulfate. The weight of lead sulfate is calculated to the phthalate plasticizer sought, by one of the following factors:
Application of the above method to known weights of the plasticizers colloided with known weights of cellulose nitrate in the laboratory gave the results in Table I. Commercial plastics of known composition were analyzed by three operators, using the method herein submitted, and gave the results in Table 11. ~
TABLEI. KNOWNWEIGHTSOF PHTHALATE PLASTICIZERS COLLOIDED WITH CELLULOSE NITRATE Phthalate Plasticizer Diamylphthalate Dibutylphthalate Diethylphthalate Dimethylphthalate
Added Per cent
Found Per cent
5.82 4.97 4.89 6.81 7.02 5.46 4.00 4.00 4.00 5.97 5.88 5.39
5.75 4.94 4.93 6.78 7.00 5.41 4.06 3.95 3.99 6.00 5.91 5.31
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TABLE 11. ANALYSISOF PLASTICS Plasticizer Added Per cent
Plasticizer Found Per cent
4.85
4.80 4.86 3.96 4.02 2.89 2.87
4.00 2.93
These results show that the phthalate plasticizers in cellulose nitrate plastics may be determined, in the presence of color-forming nitro derivatives, by the direct oxidation of the ester and the precipitation as the lead salt. It was also found that aromatic nitroamines did not interfere. Acknowledgment This investigation was made at the suggestion of W. W. Farnum, chief chemist, Naval Powder Factory, to whom the author is indebted for helpful advice throughout the work. The author wishes to thank J. A. O’Callaghan and S. G. Cook, of this laboratory, for checking the method. The opinions or assertions contained in this article are the private ones of the writer and are not to be construed as official or reflecting the views of the Navy Department or the naval service a t large.
Literature Cited (1) Allen, “Commercial Organic Analysis,” 5th ed., Vol. 111, p. 5 7 6 , Philadelphia, P. Blakiston’s Sons Co., 1923. (2) Bernthsen, A., and Sudborough, J . J., “Text Book of Organic Chemistry,” New York, D. Van Nostrand Co., 1922. (3) E F l e y , J. B., and Banta, C., J. Am. Chem. Soc., 39, 764 (1917). (4) Gma, M.,Guzz. chim. ital., 45, 11, 348-62 (1915). (5) Mulliken, “Identification of Pure Organic Compounds,” Vol. I, p. 84, New York, John Wiley & Sons, 1905.
RECEIVED July 24, 1936.
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