Vol. 5, No. 5
ANALYTICAL EDITION
302
the fluoride solutions in one case contained 0.1 gram sodium sulfate, and in the other, 0.1 gram of the same salt and also 0.1 gram sodium nitrate, 0.04 gram sodium chloride, and 5 cc. saturated silicic acid. The colorimeter readings in the two experiments agreed within experimental limits and are therefore plotted as one curve. Curve 1 shows the results when the aliquots contained no impurities and when they contained 0.1 gram sodium nitrate, 0.04 gram sodium chloride, and 5 cc. saturated silicic acid. Although the effect of sodium sulfate is to reduce the degree of fading of ferric acetylacetone caused by fluorine, and to render the procedure less sensitive, the accuracy of the results is not materially affected, as is evident from Table 11. Later experiments have shown that the sulfate collected ia the receiver of a
simplified silicon tetrafluoride evolution apparatus (1) is not sufficient to alter appreciably the fading action of fluorine on the colored substance.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
Armstrong, W. D., IND.ENQ.CHEM.,Anal. Ed., 5, 315 (1933). Armstrong, W. D., Proc. SOC.Ezptl. Biol. Med., 29,414-15 (1932). Fairchild, J. G., J. Wash. Acad. Sci., 20, 141-6 (1930). Foster, M. D., J.Am. Chem. Soc., 54,4464-65 (1932). Greef, A., Ber., 46, 2511-13 (1913). Guyot, M. P., Compt. Rend., 71, 274-5 (1870); 73, 273-4 (1871). Pulsifer, H. B., J. Am. Chem. SOC.,26, 967-75 (1904). Treadwell, W. D., and Kohl, A., Helv. Chim. Acta., 8, 500-7 (1925); 9,470-85 (1925).
RECEIVBD May 29, 1933.
Determination of Pyridine Bases in the Presence of Ammonia F. H. RHODESAND K. R. YOUNGER, Cornel1 University, Ithaca, N. Y.
T
HEPrude gas from by-product coke ovens contains considerable quantities of the vapors of pyridine, quinoline, isoquinoline, and their homologs. The “tar bases” of higher boiling points are, for the most part, condensed with the tar, but considerable quantities of the vapors of the more volatile bases are carried forward into the ammonia saturator and are there absorbed as their sulfates. The concentration of the sulfates of the organic bases in the saturator bath liquor finally attains a concentration a t which the rate of removal of the tar bases as impurities on the crude ammonium sulfate becomes equal t o the rate of introduction into the bath. The accurate determination of the tar bases in saturator bath liquor and in crude ammonium sulfate is rendered difficult by the presence of ammonium sulfate in large excess and by the wide variation in the basicities of the individual bases. Some of these compounds are so very weakly basic
0
/O
20
30
40
Cc. o f NaOH added. FIGURE 1. TITRATION OF HYDROCHLORIDES OF PYRIDINE BASES 1 2: 3.
Pyridine h drochloride a-Picoline Xydrochlorjde ,?-Picoline hydrochlorlde
that their salts are largely hydrolyzed in neutral solution; others are almost as strongly basic as is ammonia and can be liberated only by making the solution so strongly alkaline that most of the ammonia is set free. One method that has been used to determine pyridine bases in saturator bath liquor is to treat a measured volume of the solution with an excess of sodium hydroxide, distill the ammonia and tar bases into a dilute acid solution, render this solution alkaline, and add
an excess of a solution of sodium hypobromite to oxidize the ammonia, distill the tar bases into standard acid, and titrate the excess of acid with standard alkali, using methyl orange or some other indicator sensitive to weak bases. This method is not very satisfactory. A large amount of the hypobromite solution must be added to oxidize the large excess of ammonia, the recovery of the bases is often incomplete, and the end point in the final titration is not a sharp one. Various expedients for minimizing these disadvantages have been suggested (I,$) but no really satisfactory method has been described for determining tar bases in the presence of large amounts of ammonium salts. The authors have found that the various tar bases can be determined by electrometric titration and that this procedure eliminates some of the disadvantages inherent in the older methods.
PRELIMINARY EXPERIMENTS One hundred cubic centimeters of an approximately 0.25 N solution of pyridine in dilute hydrochloric acid were titrated with approximately normal standard solution of sodium hydroxide. After each addition of the standard alkali the pH of the solution was measured, using a quinhydrone electrode and balancing against a saturated calomel electrode. Preliminary experiments had shown that the standard hydrogen electrode is rapidly poisoned by pyridine and other organic bases and by some of the impurities that are normally present in saturator bath liquor. For this reason the quinhydrone electrode was used, despite its liability to error in strongly alkaline solutions. Within the range of pH encountered in the authors’ work the quinhydrone electrode gave results that were entirely satisfactory for the accurate determination of the tar bases, although in a few cases the absolute values of the pH a t the extreme alkaline end may be in error. The results obtained in the potentiometric titration of pyridine by sodium hydroxide are shown by curve 1 on Figure 1. Free pyridine begins to be liberated when the pH of the solution reaches 2.8; the liberation of pyridine is complete a t a pH of about 8.5. The break that indicates the beginning of the liberation is not extremely sharp. Pyridine is such a weak base that its hydrochloride is hydrolyzed to a very considerable extent. The difficulty of determining pyridine ac-
September 15,1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
curately by titration with an indicator is evident. Few indicators show color change a t the point corresponding to the liberation of pyridine and a t this point the rate of change of pH with addition of alkali is so small that no sharp end point can be obtained. By potentiometric titration, satisfactory results can be secured. In one series of experiments, the results were as follows: PYRIDINE TAKEN
PYRIDIXE FOUND
Grams
Grams
1.920 1.920 1.920 2.179
1.91s 1.917 1.920 2.183
Similar titrations were made with solutions of pure apicoline, @-picoline, lutidine, symmetrical collidine, and quinoline. The lutidine used was a mixture of various dimethyl pyridines. The results were as follows: COMPOUND or-Picoline @-Picoline Lutidine Collidine Quinoline
TAKEN
FOUND
Grams
Grams
2.105 2.093 2.106 1.888 4.247
2.104 2.090 2.107 1.885 4.250
303
strongly alkaline of the compounds is liberated. Mixtures of the hydrochlorides of the various bases with ammonium chloride were titrated potentiometrically with a standard solution of sodium hydroxide. The total volume of the solution in each case was 125 cc. The solutions used contained the following amounts of materials: SOLUTION
1 2 3 4 5
Pyridine 1.920 grams' ammonium chloride 1.257 grama Lutidine' 2.1Ch7 grams: ammonium chloride' 1.257 grams Collidind, 1.888 grams'; ammonium chloride: 1.257 grams Quinoline, 4.247 grams. ammonium chloride, 1.257 grams Pyridine, 0.3866 gram.' a-picoline, 0.4216 gram; @-picoline, 0.4019 gram. lutidine, 0.4211 gram. collidine, 0.3776 gram: quinoline, 0.d494 gram; NHIC1, 0.50n'6 gram
The results of these titrations are shown on Figures 4 and 5. It is apparent that solutions that contain pyridine or quinoline but none of the higher homologs can be titrated potentiometrically even in the presence of ammonium salts, although
8
6
The titration curves for these compounds are shown by Figures 1 , 2 , and 3. The basicity of the homologs of pyridine increases with the number of the substituted methyl groups. Collidine is almost as strongly basic as ammonia. In order to liberate the higher homologs completely in a solution containing large amounts of ammonium salts, the solution must be made so strongly alkaline that a very considerable amount of ammonia is also set free. Several methods of analysis have been suggested in which the tar bases are liberated and sepa-
4
-
"b
,I
/O
I
1
I
I
I I
I
I
I
20 30 eo Cc. o f NcrOH added
I
60
FIGURE3. TITRATION OF QUINOLINE HYDROCHLORIDE the point at which the liberation of the organic base is complete and the liberation of ammonia begins is not extremely sharp. I n the presence of even small amounts of the higher homologs the break between the organic base and the ammonia practically disappears. I n the analysis of a solution containing lutidine or collidine the ammonia must be removed before the tar bases can be titrated electrometrically.
PROPOSED METHOD OF ANALYSIS
Cc.
of
&OH a d d e d
FIGURE2. TITRATION OF HIGHER BASESAND AMMONIA
OF
1. 'Lutidine hydrochloride 2. Collidine hydrochloride 3. Ammonium chloride
rated from the ammonia by making the solution very faintly alkaline and then distilling. It is evident that no such method will separate the higher homologs of pyridine sharply from the ammonia. Quinoline is a weaker base than pyridine, and shows a less abrupt rise in pH a t the point a t which the liberation of the free base begins. In the titration of a solution containing a mixture of the various bases, the initial break in the curve comes a t pH corresponding to the beginning of the liberation of the weakest base that is present. Since the liberation of the weaker bases is not complete a t a pH below that a t which some of the higher homologs begin to be set free, the titration curve for the mixture does not show a series of distinct steps corresponding to the successive liberation of the various individual compounds. There is a continuous rise in pH from the neutralization point for the weakest base to the point at which the most
Based on these preliminary experiments, the following method was developed for the determination of the pyridine bases in the presence of ammonia. Transfer a portion of the solution t o be analyzed, sufficient to contain about 2.5 grams of pyridine bases, to a separatory funnel, add 25 cc. of xylene, and add a considerable excess of a concentrated solution of sodium hydroxide. Shake well, let settle, and separate the layer of xylene. Extract the aqueous layer with four more successive 25-cc. portions of xylene. Extract the combined xylene solutions with six successive portions of 25 cc. each of 10 per cent hydrochloric acid. Transfer the combined acid extract to a 1000-cc. Kjeldahl flask, make alkaline with a concentrated solution of sodium hydroxide, and add 100 cc. of a solution of sodium hypobromite prepared by adding 25 CC. of bromine t o a solution of 100 grams of sodium hydroxide in one liter of water. Distill and collect the distillate in a beaker containing about 250 cc. of approximately 2 N hydrochloric acid. When 150 cc. of distillate have been obtained, discontinue the distillation, transfer the distillate to a 500-cc. measuring flask, and make up to the mark with distilled water. Titrate 100-CC. portions of this solution with approximately normal standard alkali, using a quinhydrone electrode balanced against a saturated calomel electrode. Plot the values of the pH as ordinates against the corresponding amounts of the standard solution of sodium hydroxide that have been added. The volume of the standard solution equivalent to the pyridine bases present is equal to the amount added between the break corresponding to the complete neutralization of the free acid present and the break corresponding t o the complete liberation of the organic bases. In the analysis of any particular set of samples containing a mixture of bases, the weight of the mixed bases corresponding t o 1 cc. of the solution of sodium hydroxide should be determined in a separate experiment. The bases from a comparatively large amount of the material should be liberated, collected, purified,
ANALYTICAL EDITION
304
and dried, and a standard solution containing a known concentration of the pure dry bases should then be titrated with the sodium hydroxide solution in the usual manner. I n the method described, the extraction with xylene serves to collect all of the organic bases and at the same time eliminates most of the ammonia set free upon the addition of alkali to the original solution. The small amount of ammonia that is carried through into the solution obtained by the extraction of the xylene solution with hydrochloric acid is readily and completely oxidized by the sodium hypobromite. To prove that the elimination of the ammonia is complete, a portion of a strong solution of ammonium sulfate, free from pyridine bases, was subjected to the method of analysis outlined above. In the final electrometric titration the amount of alkali required was exactly equivalent to the free acid in the
Vol. 5 , No. 5
tration of the purified bases from the bath liquor was titrated with the standard solution of akali. About 3 liters of the bath liquor were made strongly alkaline with sirupy sodium hydroxide and the layer of liberated bases was separated. The aqueous layer was extracted five times with separate portions (100 cc.) of a mixture of benzene and toluene. The combined extract was shaken with five successive 50-cc. portions of 15 per cent hydrochloric acid. The combined acid solution was made strongly alkaline with sodium hydroxide, and the separated bases were added to the original lot of bases that was obtained from the original bath liquor. The aqueous layer was again extracted with the mixture of benzene and toluene, and the resulting solution was shaken with five portions of 20 cc. each of dilute hydrochloric acid. The acid solution was made strongly alkaline and the small amount of separated bases was combined with the bases previously obtained. The extraction with benzene and toluene and with acid was repeated and a final and very small amount of free base was obtained. The combined bases were distilled to dryness and the distillate was dried with solid sodium hydroxide. The dry material was then again distilled through a fractionating column. The results obtained in this distillation were as follows: TEMPERATURI
c.
20
/O Cc.
of
30
NcrOHadded
40
" OF HYDRoCHLoRIDES OF ORGANIC BASESIN PRESENCE OF AMMONIUM CHLORIDE 1. Pyridine 2. Lutidine 3. Collidine
known volume of standard hydrochloric acid in which the final distillate was collected. None of the ammonia was therefore carried through into the final solution that was titrated. To prove that all the pyridine contained in the original sample is carried through into the final solution that is titrated, a known volume of a solution of pyridine hydrochloride containing a known concentration of pyridine was subjected to the regular method of analysis. The following results were obtained: PYRIDINE TAK~N Grams
Grams
1.920 1.920
1.909 1.914
Start 15 55 74 79 81 87 92 .
From the distillation it appears that the mixed bases contained about 50 per cent of true pyridine and about 25 per cent of picoline, the remainder being composed of a mixture of the higher bases.
/O 6 fH
2
0 '
/O
20
Cc.
PYRIDINE FOUND
%
90 115 120 135 150 185 185 189 (dry)
50
DISTILLED
of
30 40 NaOH a d d e d
50
FIGURE5 . TITRATION OF HYDROCHLORIDES OF ORGANIC BASESIN PRESENCE OF AMMONIUM CHLORIDE 1. Quinoline 2. Mixture of various bases
ANALYSIS OF SATURATOR BATHLIQUOR
A sample of the bath liquor from the ammonia saturator of the By-product Coke Oven Plant of the Rochester Gas and Electric Company was analyzed by the method described above. A sample of 200 cc, of the original solution was taken for analysis. On the addition of sodium hydroide, some solid sodium sulfate separated. This was redissolved by the addition of water. In the extraction of the alkaline solut.on with xylene, a layer of sludge was formed between the xylene and the aqueous layer. This was alwaysdrawn off alongwith the aqueous layer, sludge formed in the extraction of the xylene with hydrochloric acid was allowed to remain behind with the xylene layer. In each of two duplicate determinations 17 cc. of the standard alkali were required in the final titration. In order to determine the equivalent of the standard solution of sodium hydroxide in terms of the bases actually present in the saturator bath, a solution containing a known concen-
Of the redistilled bases, a sample weighing 20.079 grams was transferred to a Kjeldahl flask, 180 cc. of water and 100 C C * of a solution of sodium hYPobromite were added, and the ammonia-free bases were distilled and collected in dilute acid. The solution was diluted to 500 cc. and portions of 100 cc. each were withdrawn for titration with 1.1925 N sodium hywere droxide solution. The bases Present in the equivalent to 35.1 cc. of the alkaline solution. Thus 1 cc. of a normal solution of sodium hydroxide was equivalent to 0.09595 gram of the mixed bases. Since 200 cc. of the original saturator bath liquor required 17 cc. of the standard alkali, the bath liquor contained 0.9726 gram of bases in 100 CC-9 Or o*08116 pound Of bases per gallon* LITERATURE CITED (1) Houphton, J. IND.ENQ.CHIM.,1, 698 (1909). (2) Tallantyrep J. S O C . C h e m Ind.9 4% 466T (1930). RECEIVED May 15,1933.
6Lm
