June, 1918
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ORIGINAL PAPEFPS THE QUANTITATIVE ESTIMATION OF ANTHRAQUlNONE B y HARRYF. LEWIS Received April 29, 1918
I n the analysis of anthracene by the method of Luck,l an impure anthraquinone is formed. Various methods for the purification of this anthraquinone have been proposed. The most popular common modification is the one which goes b y the name of the “Hochst Test,”z which prescribes a solution of the product in pure fuming sulfuric acid and the resultant separation of a pure crystalline anthraquinone b y dilution with water. This is a long process and with certain types of anthracene i t may be very inaccurate. Basset3 suggests boiling the impure anthraquinone, obtained according t o Luck’s method, for some time with a solution of mixed chromic and nitric acids, for t h e reason t h a t the pure anthraquinone does not lose weight by this treatment, while t h a t obtained from commercial anthracene may lose from I t o 2 per cent. If it is desired t o accurately determine the amount of anthraquinone present in samples contaminated with either large amounts of anthracene or phenanthraquinone, t h e above methods leave much t o be desired. Quantitative determination of anthraquinone based upon the formation of the oxime by the methods described by Goldschmidt,4 Schunck and Marchlewskis and Musenheimer6 have not been found practical for the reason t h a t the manipulation is long and difficult and the yield of oxime could not be made quantitative. A method for the estimation and purification of anthraquinone has been developed based upon the susceptibility of the carbonyl radicals t o reducing agents. Grabe and Liebermann7 described the preparation of a compound, which they call oxanthranol or anthrahydroquinone, formed b y the reduction of anthraquinone by a n alkaline suspension of zinc dust. This compound is quite soluble in hot alkaline solution b u t in t h a t solution is easily reoxidized t o anthraquinone. They recommend the use of 2 parts of zinc dust and 30 parts of a 50 per cent sodium hydroxide solution t o I part of anthraquinone. The anthraquinone is suspended in a small amount of jo per cent alcohol and the hot alkaline solution and zinc dust added. This mixture is heated for half an hour and filtered. On the filter paper are found t h e unchanged portion of the anthraquinone and the zinc dust; the oxanthranol in the filtrate may be oxidized with air, and the anthraquinone formed filtered and weighed. Following out the above directions, a dark green, alkaline solution is obtained instead of the cherry-red Z . anal. Chem., 12 (1873), 34; 13 (1874), 25. Ibzd., 1 6 (1877), 61. a Chem. News, 7 3 (1896), 178. 4 Bey., 16 (1883), 2179. 6 I b i d . , 27 (1894), 2125. 8 A n n . , 323 (1902), 207. Ibid., 160 (1871), 126. 1
2
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solution described by Grabe and Liebermann. The red color is formed on dilution, so i t seems possible t h a t a typographical error in regard t o the concentration of alkali employed was made in the original paper. When the concentration of the sodium hydroxide solution is about 5 per cent or less it has been found t h a t the reduction and subsequent reformation of anthraquinone can be made quantitative. Johann Walter‘ has made use of this process for the separation of anthraquinone from anthracene and phenanthraquinone. He gives no detailed description of the method. The following procedure, if carefully followed, has been found t o give very accurate results. I n addition t o the factor of increased accuracy this method has the added advantage of a substantial saving of time. One part of anthraquinone is wet with a small quantity of alcohol, mixed with z parts of zinc dust, and about 5 0 parts of a hot 5 per cent sodium hydroxide solution added. The mixture is heated just below the boiling point for 5 min., and then rapidly filtered b y suction, and washed once with water. The filter paper with the residue is heated with another equal portion of the sodium hydroxide solution and rapidly filtered into the same flask. A third heating with alkali is sufficient t o eflect the solution of any residue of anthraquinone t h a t may remain unreduced. The combined filtrates are cooled and reoxidized b y shaking in the presence of air. A practical procedure i s t o shake the suction flask under a stream of cold water until the red color disappears. The resulting anthraquinone is filtered upon a weighed Gooch crucible, washed with water, dried a t 1 1 0 ’ and weighed. The following precautions must be observed: If the mixture is boiled for too long a period, there is some formation of the next reduction step, the compound called by Liebermann and Gimel,2 anthranol, which contains one less oxygen atom t h a n does the oxanthranol and does not reoxidize t o anthraquinone by the action of air. The presence of this compound is easily shown by any yellow color in the filtrate from t h e final reoxidation. There is danger t h a t all the anthraquinone may not go into solution through t h e reduction. This is readily determined. If the residue on the third filter paper imparts no red color t o the liquid when boiled again with hot alkaline solmtion, the reduction is complete. A green color showing in the reoxidized anthraquinone indicates t h e presence of a reduced compound anthraquinone, the structure of which has determined. Phenanthraquinone in the original substance causes high results, but when present in amounts less t h a n I O per cent the error is not sufficient t o vitiate the practical value of the method. I t is necessary t h a t the hot solution of thereduced anthraquinone be filtered rapidly in order t o prevent reoxidation on the filter. I 2
D. R. P. No. 168,291 (1904). Bey., 20 (1887), 1854:
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It has been found t h a t certain grades of asbestos are affected by alkali and i t is necessary t h a t t h e asbestos, before use, should be freed from alkali-soluble constituents. For analytical purposes a very good sample of anthraquinone was obtained from a so-called chemically pure commercial sample by recrystallizing several times from hot toluene. This commercial sample was analyzed by the above method and gave the following result: Anthraquinone taken. . . . . . . . . 0.2000 g. Anthraquinone recovered.. . . . . 0.1992 g. Loss.. . . . . . . . . . . . . . . . . . . . . . . 0.008 g. 0.40per cent After purification t h e sample was analyzed as follows:
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TABLEI-ANALYSIS
OF
PURE ANTHRAQUINONE
SAMPLE Zinc Dust Sodium Hydroxide Recovered Gram Gram Cc. of 5% S o h Alcohol Gram 0.2000 0.4 15 To 0.2003 0.3002 0.6 20 wet 0.2997 0 2003 sample 0.2007 0.4 15
Per cent 100.15 99.83 100.19
This method of purification and estimation seems t o be especially adapted t o supplant the “Hochst Test” in the estimation of anthracene because of its greater speed and accuracy. It may also be used with excellent results in estimating t h e purity of anthraquinone which is contaminated with anthracene and less t h a n I O per cent of phenanthraquinone. The examples given in Table I1 illustrate the degree of accuracy t o be expected when no special precautions are taken. Some of these analyses were completed in less t h a n z hours. TABLE11-ANALYSES OF MIXTURES COMPOSITION O F MIXTURES Anthraquinone AnthraAnthra- Phenanthraquinone Recovered quinone cene Gram Gram Gram Gram 0.1803 0.0297 0.0000 0.1805 0.2000 0.0000 0.1975 0.1982 0.1782 0.0000 0.0228 0.1805 0.2018 O.Oi47 0.0000 0.2020 0.2005 0.1342 0.0098 0.2000 0.2004 0.0000 0.0100 0.1999 ~
~
Per cent 100.09 99.65 101.28 - . ~ 100.10 99.75 99.75 ~~
A single analysis may be easily completed in 1’/2 hrs., exclusive of the drying of t h e final product t o constant weight. If analyses must be completed in a short time the drying of the sample may be hastened by washing with alcohol and ether. The sacrifice in accuracy may be as little as I per cent. A modification of this method is being worked out t o increase the accuracy in the presence of large amounts of phenanthraquinone. \
COLORINVESTIGATION LABORATORY BUREAUO F CHEMISTRY WASHINGTON, D. C.
CRITICAL ELABORATION OF QUANTITATIVE PRECIPITATION METHODS EXEMPLIFIED BY A METHOD FOR THE DETERMINATION OF PHOSPHORIC ACID By H. HEIDENHAIN Received December 12, 1917
Of the numerous quantitative precipitation methods comparatively few have had the benefit of a thorough critical investigation. With most of them their authors have been satisfied when “good results” had been obtained. Such, however, are by no means proof of the correctness of a method. A compensation of errors must always be considered a possibility. A s
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long as there exists any doubt in this respect, the scientific analyst will not be satisfied. On t h e contrary, he will desire t o learn how a method will work under various conditions, i. e., what influence t h e quantity of substance employed, concentration of solution, temperature, presence of certain substances, etc., may have on t h e result. T h e task of examining a method as t o its reliability has been p u t u p t o me repeatedly. While a t first such work was lined out by me just for the particular case on hand, I later found t h a t certain methods employed in my researches were applicable in a great number of cases. I might say t h a t in a measure I have found a scheme for this class of work. I t is t h e purpose of this article t o develop this scheme. However, before taking u p my subject proper, I think i t advisable t o show how in one case of my experience the scheme has been successfully applied, as by so doing i t will be easier for me t o make myself clear later on. A PRACTICAL CASE
The method of determining phosphoric acid by precipitating t h e same b y molybdic acid solution and transforming the molybdic precipitate into magnesium ammonium phosphate is generally known. This transformation was necessary as long as we did not understand how t o produce the molybdic precipitate in constant form. This, however, has finally been accomplished. Several articles on this subject have been published, but it was t h e thorough researches by Hundeshagen, Zeikchrift fgr analytische Chemie, 1889, which chiefly aroused my interest. Hundeshagen proved in convincing manner t h a t t h e precipitate contains, for every 3 equivalents of phosphoric acid, 2 4 equivalents of molybdic acid and 3 equivalents of ammonium, if produced under certain conditions, and t h a t t h e precipitate could be determined by titration with standard alkali solution, using phenol,phthalein a s indicator. Testing Hundeshagen’s method I could confirm his findings, but I noticed t h a t t h e end reaction a t titration was lacking in sharpness. Hundeshagen used a solution of ammonium nitrate as wash liquor, a n appreciable amount of which remains in t h e filter and precipitate. This, as well as the ammonium in chemical combination with t h e phosphoric and molybdic acids, evidently is t o be blamed for the uncertainty a t titration, as ammonium salts cannot be titrated with exactness with phenolphthalein as indicator. On the other hand, this indicator seems indispensable t o bring phosphoric acid t o a definite stage of neutralization. There was, however, a way out of this dilemma. After the precipitate had been washed with ammonium nitrate solution, this salt could be removed b y washing with alcohol a n d the ammonium in the precipitate could be gotten rid of by supersaturation and evaporation with t h e standard alkali solution, and determination of t h e excess of alkali b y boiling with a n excess of standard acid solution and titrating back with standard alkali solution. Thus t h e end-reaction was made sufficiently sharp and the results obtained were very satisfactory, b u t t h e method had become rather cumbersome.
