SEPTERIBER, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
and larger scale over the same temperature range using the more promising fuel and water gas conversion catalysts.
Aclinow-ledgment The authors deqire to exprev their appreciation of the services of Robert 11. lliller for aid in the experimental work. Grateful acknonledgnient iz made also to Vernon A. Stenger for hi3 pain-taking work in the ash analyses
Literature Cited (1) Amistronr, E. F.. and Hilditch, T. P., Proc. Roil. Suc. (London). 697,265-73 (1920!. ( 2 ) Ihid., -1103, 586-97 (1923). (3) Brantley, L. R., and Becknian, -1.O., J . A m . Chenz. Soc., 52, 3956-62 (1930). (4) Brewer. R . E., and Reyerson, L. H . , IXD.ESG. CHEM.,26, 734-40, 892 (correctioni 11934). ( 5 ) Crittenden, E. D., Canadian Patent 318.433 (Dei,. 29, 1931). (6) Emmett, P H . prlrate communlcatlon. ( 7 ) Engelder, c'. J., and Bliimer, >I., J . P h y s . Chern., 36, 1353 (1932).
IS) Engelder. C . ,J., and Miller. L. E.,I b i d . , 36, 1345-62 (1932).
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(9) Evans. R . M.,and Newton, IT. L.. ISD. ESG. ('HEM., 18, 514 (1926). (10) Fox, D. A , and White, A. H., Ihid., 23, 259-66 (1981). (11) Hillebrand, W.F.. and Lundell, G. E. F., "Applied Inorganic .&nalysis," Part 111,?Jew T o r k , John Wiley & Sons, 1929. and Bauer, .1.D., Bur. Mines, RE&. Incesfiga(12) Horne, J. W., tions 2832 (1927). (13) Logan, L.. Am. Gas d s s o c . Proc., 14, 976-1015 (1932). (14) Marson, C. B., and Cobb, J. IT., Gas J . , 171,:39-46 (1925). (15) Neumann, B., and .Ihlen, A. van, Brennsto,f-Chern., 15, 6 - 4 (1934). (16) Neurnann, R., Kroger, C., and Finpas, E., Z . anorg. allgem. Chem., 197, 321-35 (1931). (17) Reinders, IT-.,Z . p h y s . Chem., 130,405-14 (1927). (18) Schotz, S. P., "Synthetic Organic Compounds," p. 84, Sexv Tork, D. Van Sostrand Co., 1925. (19) Tanimann, G., and Sxvorykin, .I.. Z . anorg. aZ2gern. Chern., 170, 62-70 (1925). (20) Weiss, C. B., and White, .1.H., ISD.ESG. CHEX.,26, 83-7 (1934'1.
RECEIVED 3Zarch 13, 1935. Presented before t h e Division of Gas and Fuel Chemietry a t t h e 89th Xeeting of t h e .Imerican Chemical Society, Cleveland. Ohio, September 10 to 1 4 , 1934.
Canned Meats Effect of pH on the Formation of Ferrous Sulfide V. R. RUPP, Kingan and Company, Indianapolis. Ind.
@A
N O S G canned meat products, tripe ii:
notably subject to iron sulfide discoloration. One of the common methods used in preventing this discoloration is to dip the tripe in a vinegar solution before canning. Lowering the pH of the tripe in this nianiier represses the ionization of the hydrogen sulfide formed during the processing. If the ionization of hydrogen sulfide i; repre.sed t o a degree where the product of the sulfide-ion and the ferrous-ion concentrations is less than the solubility product of ferrous sulfide, precipitation will not occur. The effect of the hydrogen-ion concentration on the ionization of hydrogen sulfide can be calculated from the following equations:
If the concentration of sulfides and the p H are known, the concent'rationof sulfide ion can be calculated from Equation 5 . From Equation 6, (F-+) (S-) = L where L = solubility product of ferrous sulfide
the concent'ration of ferrous ions necessary to produce precipitation with a given concentration of d f i d e ions can lie calculated. L as determined by Bruner and Zawadski is
3.7
x
10-19
($1.
Similarly, if the iron concentration is determined, the pH a t which precipitation of ferrous sulfide mill occur can be calculated. By substituting in Equation 6 the value of (S--)as obtained in Equation 5 , we have:
+
(H+)2 9.1 X ahere c
(H+)
= second ionization constant of hydrogen sulfide (given by Knox, 5 ) = 1.2 X
+ 10.9 X
=
10.9 X 10-*8 (c) (Fc-7) 3.7 X
= molal concn. of sulfides present
k1 = first, ionization constant of hydrogen sulfide (given b y Auerbach, 3) = 9.1 X I:?
(6)
(7)
This equation can be simplified without a material change in the exactness of the expression to
From Equations 1and 3 we obtain: (H') = 1.7 X 10-2
\/m - 4.5 X IO-'
(8)
Experiments with Tripe Substituting Equation 4 in Equation 2:
Equation 8 waq checked experinientally over a limited range of pH which is commonly encountered in vinegardipped tripe. Owing to the unequal distribution of iron in canned tripe, solutions of ferrous chloride and hydrogen sulfide containing no protein were used for the purpose:
INDUSTRIAL AND ENGINEERING CHEMISTRY
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A solution of ferrous chloride containing 10 p. p. m. of iron was prepared by dissolving pure iron wire in iron-free hydrochloric acid. The excess acid was neutralized with sodium hydroxide solution; care was taken not to add an amount sufficient to precipitate iron hydroxide. This iron solution after making to volume contained exactly 10 p. p. m. of iron. Shortly before using the iron solution, washed hydrogen sulfide gas was passed through a measured portion until the ferric chloride was completely reduced. The excess hydrogen sulfide was removed by boiling in an Erlenmeyer flask, and the solution was rapidly cooled. This reduced solution was then made up to the original volume by the addition of recently boiled and cooled distilled water. A series of buffer solutions was prepared from stock solutions of sodium monohydrogen phosphate and potassium dihydrogen phosphate. These stock solutions contained 23.752 grams of sodium monohydrogen phosphate and 18.156 grams of potassium dihydrogen phosphate per liter. To 5 cc. of buffer solution were added 3 . c ~of. ferrous chloride solution and 2 cc. of distilled water. The hydrogen-ion concentration was then determined by means of the hydrogen electrode. Those buffer solutions were selected which had pH values of 4.95. 5.10, 5.25, 5.50, and 5.65, respectively, after the addition of ferrous chloride solution and water. A solution of hydrogen sulfide of known concentration was prepared by passing washed hydrogen sulfide gas into boiled and cooled distilled water. This solution was then diluted with boiled and cooled distilled water. The concentration of hydrogen sulfide in the diluted solution was determined by pipetting a portion into an excess of standardized iodine solution (approximately 0.01 N ) and titrating the excess with standardized thiosulfate solution. The concentration of hydrogen sulfide determined in molal. this way was 1.39 X To 5 cc. of the buffer solutions in test tubes were added 3 cc. of ferrous chloride solution, prepared as described, and 2 cc. of the hydrogen sulfide solution. Immediately after the addition of hydrogen sulfide, the tubes were sealed to prevent the escape of hydrogen sulfide or its oxidation. On standing 24 hours, a black precipitate of iron sulfide occurred in the solutions having pH values of 5.25, 5.50, and 5.65, respectively. No precipitation occurred in the solutions having pH values of 4.95 and 5.10 during an observation period of a month. Since the concentration of iron in the ferrous chloride solution was 10 p. p. m., that in the tubes was 3 p. p. m. or 5.4 X molal. The sulfide concentration in the tubes was 2.8 X molal. Substituting these concentrations in Equation 8, we find that precipitation will occur at a hydrogen-ion concentration of 6.59 X 10-6 or less. This is equivalent to a pH of 5.18 or more. The sulfide concentration in canned processed tripe was determined by the method of Almy ( 1 ) . Tlventy-five gram samples of finely ground tripe were used in each determination. The hydrogen sulfide was liberated with hydrochloric acid and washed out of the solution with carbon dioxide. The liberated hydrogen sulfide was caught by passing it through two successive solutions of zinc acetate. After washing into a volumetric flask, the zinc sulfide was treated with a solution of p-aminodimethylaniline hydrochloride and a 0.02 M ferric chloride solution. The color developed was compared wit,h standards prepared from known concentrations of hydrogen sulfide to which the above reagents had been added. Results were expressed in micromilligrams per 100 grams of sample.
Three samples of uncooked tripe showed no hydrogen sulfide. Ten samples of canned processed tripe showed a sul-
t
VOL. 27, NO. 9
fide concentration of 600 to 800 mmg. per 100 grams of sample. Iron was also determined on the same samples by the micromethod as published by the A. 0 . 4 . C. ( 2 ) . Results are as follows: Sample NO.
HzS
Iron
Sample
HzS
NO.
iMmg./100 o. P . p . m.
M m g . / 1 0 0 Q.
Iron
P. p . m.
The average moisture content of the tripe was 79.2 per cent. On this basis the sulfide concentration in solution was 2.6 x molal, the iron concentration, 8.6 X 10-6 molal. Subiitituting these values in Equation 8, we find t h a t ferrous sulfide precipitation will occur with these concentrations of ferrous and sulfide ions a t a hydrogen-ion concentration of 2.53 x 10-6 molal. This is equivalent to a p H of 5.6. The greatest iron concentration naturally occurs in the tripe in direct contact with t h e can, particularly near seams and corrugations where the tin plate is liable to be faulty. This is where sulfide discoloration will occur if the p H is near t h e critical point. It can be seen from Equation 8 that, with a given sulfide concentration, the hydrogen-ion concentration a t which precipitation of ferrous sulfide will occur varies directly as the square root of the iron concentration. Thus B small increase in hydrogen-ion concentration will compensate for a considerable increase in the concentration of ferrous ions. Determinations of the p H of the liquid in a considerable number of cans of tripe indicate that discoloration is common when the pH is 5.6 or greater. In these calculations it is assumed that all the iron present is in the form of ferrous ion. This assumption does not appear t o be unwarranted in view of the well-known reducing action of meat products when heated out of contact with air.
Acknowledgment The author wishes t o thank Robert Felch of this laboratory for making the sulfide determinations reported.
Literature Cited (1) Almy, L. H., J . Am. Chem. Soc., 47,1381 (1925). (2) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 3rd ed., p. 103. (3) Auerbach, F., 2.p h g s i k . Chem., 49,217 (1904). (4) Bruner, L., and Zswadski, J., Z.anorg. Chem., 65, 136 (1909). (5) Knox, J., Z . Elektrochem., 12,477 (1906). RECEIVED March 30, 1935.
