Nitric Acid Method. For the quantitative determination of petroleum, the turpentine oil is oxidized with fuming nitric acid and the residual mineral oil is weighed. According to Burton2) 100 cc. of oil are transferred into a spacious flask connected with a reflux condenser. From a measuring funnel 300 cc.
1) According to H. S. Shrewsbury the method of Frey can only be used if the petroleum content varies between 30 and 70 p. c. Correct values within these limits can be had only by making the necessary corrections. Analyst 36 (1911), 137; Chem. Zentralbl. 1911, I. 1560.
2) Americ. chem. Journ. 12 (1890), 102; Chem. Zentralbl. 1890, I. 882.
of fuming nitric acid are allowed to drop slowly and the mixture kept well cooled. The residual oil is washed with hot water and weighed. Control experiments have revealed that the higher boiling petroleum fractions yield better results. The lower fractions are attacked somewhat by nitric acid. When a petroleum product boiling at 250° was used, 34,1 p. c. were found in place of 35 p. c. added; when a petroleum product boiling at 75° was used 17,9 p. c. were found in place of 20 p. c, and in an other experiment 28 p. c. in place of 30 p. c. A like method is described by Allen1), who, however, employs 400 cc. of fuming nitric acid in place of 300 cc. for the same amount of oil.
For this assay H. Herzfeld2) has constructed an apparatus (Fig. 1) which admits of the use of smaller amounts. To 10 cc. of turpentine oil contained in the lower vessel 15 cc. of fuming nitric acid, contained in the measuring cylinder, are added drop by drop. At the same time the mixture is well shaken and water is passed through the outer jacket of the apparatus. The operation being ended, the reaction mixture is transferred to the graduated cylinder and the volume of petroleum product read off. If the latter is to be weighed, it is washed several times with a few cc. of fuming nitric acid and finally with water. Marcusson and Winterfeld 3) regard it advantageous to reduce the temperature of the acid to - 10° and to use a special apparatus. This consists of a round bottomed flask with a long calibrated neck and a 10 cc. measuring funnel fitting into the ground glass neck. (fig. 2).
"30 cc. of fuming nitric acid (sp. gr. 1,52) are transferred to the flask a and reduced to a temperature of - 10°. As a cooling medium a 15 p. c. solution of sodium chloride in a small tin jar is used. This jar is placed in a mixture of ice and coarse salt to bring about the proper reduction in temperature. The measuring funnel b is then inserted and filled up to the mark d (10 cc.) with turpentine oil which is added drop by drop to the nitric acid while the mixture is being shaken. The time required for the addition varies from 1/2 to 1 hour according to the benzin content. The larger the latter, the more rapidly the oil can be added.
1) Zeitschr. f. off. Chem. 8 (1902), 446; Chem. Zentralbl. 1903, I. 258. 2) Chem. Zentralbl. 1903, I. 258. Later on the apparatus was supplied with a thermometer by Herzfeld. 3) Chem. Ztg. 33 (1909), 987.
The reaction being ended the product is allowed to remain for another quarter of an hour in the cooling mixture. The measuring funnel having been removed, sufficient concentrated (but not fuming) nitric acid (d 1,4) cooled to - 10° is added with the aid of an ordinary glass funnel to force the insoluble oil into the calibrated neck c. After room temperature has been reached, the volume can be read off directly. Meanwhile, however, the bulk of the flask is allowed to remain in the salt solution in order to prevent side reactions by the nitric acid. If the specific gravity and boiling temperature of the portions insoluble in the nitric acid are to be determined, these can be withdrawn by means of a pipette. For this purpose a rather long rubber tube should be attached to the pipette. If but a small amount of benzin is present 20 cc. of oil can be added to 60 cc. of nitric acid"1). If no oily layer separates on the acid, the sample in question may be regarded as free from benzin.
1) The German Customs Authorities regard the oil as pure if the upper layer amounts to less than 0,2 cc. If, however, the index of refraction of the original turpentine oil exceeds 1,48 (at 17°), it is to be regarded as containing mineral oil if the layer exceeds 0,5 cc. (Funfter Nachtrag zu der Anleitung fur die Zollabfertigung. Published by the Reichsschatzamt.. Berlin 1910, p. 57).
The acid is poured into 150 cc. of water and the mixture is heated for about 1/4 hour on a steam bath in a hood. If the turpentine oil was pure no separation takes place. If, however, petroleum hydrocarbons are present, the separation of an oil is accompanied by turbidity and evolution of red vapors. After cooling, the oil is shaken out with ether. The ethereal solution is first washed with water, then with an approximately 8 p. c. potassium hydroxide solution (made from 50 g. potassium hydroxide, 500 g. water, 50 cc. alcohol), again with water, then dried with calcium chloride. After evaporation of the ether the residue is weighed. Note should, however, be taken of the fact that certain constituents of the benzin are attacked by the nitric acid and converted into water soluble compounds. Hence the amount of adulterant is not necessarily the same as the amount of the separated and weighed oil. Moreover, the different kinds of petroleum oils and benzin do not behave alike when treated as directed above. Inasmuch as analysts have not yet come to an agreement as to the correction which should be made, it becomes necessary to refer to the original literature1).
For the examination of the nitric acid liquors for nitro compounds (from benzene hydrocarbons) see p. 50.