January, 1926 IND USTRIBL AND ENGINEERING CHEATISTRY 47

Destructive Action of Sulfuric and Hydrochloric Acids upon Leathers’

I C

By John Arthur Wilson , \ ‘ \ , /

A. F. GALLUN & SONS Co., MILWAUKEE, WIS. J

OR several decades it has been generally appreciated that the life of leather is greatly shortened by the pres- F ence of free sulfuric acid, and specifications for vege-

table-tanned leathers usually place a maximum value upon the amount of acid which such leathers may contain. How- ever, in spite of the great importance attached to the sulfuric acid content of leather, no satisfactory work has heretofore been done showing the quantitative relation between the de- structive action o? the acid and its amount and con- centration. Because of the sharp variation in strength over the area of a tanned skin, earlier investigators were handicapped by not being able to tell what the strength of any given piece of leather would have been had it not besn subjected to the action of acid.

Method

Recently the writer made exte:isive measurements of the variation in strength and stretch of chrome- and vegetable-tanned calf leath- ers over their entire areas.2 Starting from a given point and proceeding in a straight line, it was f o u n d t h a t

strength exactly as described in the earlier paper. The even- numbered ones were wet with solutions of different hydrogen- ion concentration, ranging from 0.5 N sodium bicarbonate to 5.0 N sulfuric acid. The sodium bicarbonate solutions were used to get strips of leather containing less than the usual amount of sulfuric acid. The strips were blotted, air-dried, and kept for 6 months to age. Then their tensile strengths were measured and plotted along with those for the untreated

strips in Figure 1. The

Chrome leather has a greater capacity for com- bining with sulfuric acid and is much more readily destroyed by the free acid than vegetable-tanned leather.

The resistance of chrome leather to the destructive action of sulfuric acid can be increased by retanning it with vegetable tanning materials. Very thin sheets of leather can be made by coating the surface of chrome leather with vegetable tannin and then soaking it in a 6 N solution of sulfuric acid, which will dissolve away all except the tannin-coated film.

It is only the volatility of hydrochloric acid that makes its presence in leather less harmful than sulfuric acid.

Chrome leather is easily destroyed by either hydro- chloric or sulfuric acid but more quickly by hydro- chloric acid in solutions stronger than 3 N and more quickly by sulfuric acid in solutions weaker than 3 N .

the variation in tensile strength followed a fairly smooth and continuous curve, but one which might show many points of maximum. The charts obtained made it possible, however, to select portions of a skin where the variations in strength in a given direction showed very few points of maximum. Thus it was found possible to select areas which could be cut into series of strips such that the curve for variation in strength from one end to the other could be obtained simply by testing every other strip. If the odd-numbered strips were used to get the curve, the strength of the even-numbered strips could be obtained from the curve without the necessity for actually breaking them. The even-numbered strips could then be used for experiments and their loss in strength measured after any special treatment. It is further possible to reduce errors to a very small value by running duplicate or triplicate series.

This method is illustrated for a concrete case in Figure 1. d piece of finished chrome calf leather, 75 X 17 em., was cut into twenty-five strips, each 3 X 17 cm., numbered from 1 to 23. The odd-numbered ones were measured for tensile

1 Resented under the title “Effect of Sulfuric Acid upon the Life of Leather” before the Division of Leather and Gelatin Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925; publication delayed to permit the accumulation of further data. Laboratory assistance was rendered by G. 0. Lines, A. W. Bear, G. Daub, and E. J. Kern.

3 Wilson, THIS JOURNAL, 17, 829 (1925).

broken strip in each test was analyzed for water and sul- furic acid; the latter value is recorded on the graph.

Up to an acid content of about 10 per cent the two curves practically coincide, but they diverge sharply with further increase in acid c o n t e n t , the acid-treated strips finally losing all meas- urable strength. The dis- t a n c e be tween the two curves gives a measure of t h e p rogres s ive loss of strength with increasing per cent of acid in the leather. For example, take strip No. 16 with a tensile strength of 79. A line drawn directly upward to the upper curve intersects it at the value

168, which may be taken as the strength of No. 16 before the acid treatment and aging. We may therefore conclude that the treatment has caused the leather to suffer a loss of 53 per cent in strength. This procedure enables one to deter- mine the percentage loss in strength of leather resulting from any special treatment.

Acid Determination.

I n each test the acid content of the leather was determined in the strip used for measuring tensile strength. For vege- table-tanned leather the Procter-Searle method3 was used. The sulfuric acid content of chrome leather was taken as the difference between the total amount of sulfate in the leather and the sulfate left upon ashing. Total sulfate was deter- mined by the Thomas method.‘ Another portion of the sam- ple was ashed a t as low a temperature as possible and the ash was boiled with a solution of sodium peroxide to oxidize any neutral sulfate reduced in the ashing and then analyzed for sulfate. This method, like the Procter-Searle method, does not differentiate between free acid and acid held in a state of chemical combination with the leather, as in the form of collagen sulfate. Similarly the content of hydrochloric acid was taken as the difference between total chloride and chloride present in the ash.

* Procter, “Leather Chemists’ Pocket Book,” p . 189 (Spon, 1912). 4 Thomas, THIS JOURNAL, 12, 1186 (1920).

48 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 18, No.

l0OJ

90-

80-

5 70-

2 60-

'50.

5 v)

a

s 40- c f 30. k 20-

Leather Samples Tested

The experiments were made upon three kinds of finished calf leather: (1) colored vegetable-tanned; (2) colored chrome- tanned; and (3) uncolored chrome-tanned. The analyses of these leathers on the dry basis follow:

VEGETABLE- ~CHROME-TANNED- TANNED Colored Uncolored Per cent Per cent Per cent

Protein (N X 5.62). ............. 48.9 Fat (chloroform extract). ........ 1 0 . 6 Water-soluble organic matter. .... 11.0 Other insoluble organic matter

(combined tannin). .......... 28 .7 Sulfuric acid.. .................. 0 . 5 Hydrochlonc acid. . ................. Chromic oxlde.. .................... Aluminium oxide.. .............. 0 . 3 Ferric oxide ........................ Sodium sulfate.. .................... Sodium chloride.. ...................

74.0 6 . 3 2.0

4 .6 4 .2 0.6 6 . 0 1.1 0.3 0.7 0 . 2

82.6 4.2 0 .8

5 . 1 0.1 5 . 1 0 . 9 0 .2 0.6 0.4

...

Effect of Total Sulfuric Acid Content

The curves in Figure 1 give the results of one of four series of experiments performed with the colored chrome calf leather. Four series of tests were made simultaneously with the vege- table-tanned calf leather. After wetting with solutions of sulfuric acid or sodium bicarbonate and drying, the test strips of each series were allowed to age for a different length of time, the periods selected being 1.5, 3, 6, and 9 months. The strips were exposed to the atmosphere subject to fluctu- ations in temperature and humidity until one week before they were to be broken, when they were transferred to closed containers with atmospheres kept constant a t 50 per cent relative humidity. The results of the 8 series are shown in Figure 2.

For low acid values, the acid-treated strips showed some- times a gain and sometimes a loss whose absolute value varied from zero to 20 per cent of the calculated strength of the leather, indicating that observed values below 20 per cent were unreliable for this particular experiment. Hence we have plotted only the values greater than 20 per cent and

Numbers give grams aulfuric acid associated with 100 grams dry lea4her i n men

r*numbered- strips. /

Odd nLmbered strips ;, contained 5.13 grams 1,s grams d r y l ea the r . s u l f u r i c ac id per 100 s +

- 1 I

1 I I I 1 I I 1 I , I I I S , ' 8 9

Leather S t r i p Nunber

I - : ? % : ! ' 5 10 15 eo 25

Figure I-Illustrating Method Employed t o Measure Destruc- t i o n of Chrome Calf Leather by Sulfuric Acid

obtained the curves shown in Figure 2. They indicate a progressive destruction of leather both with increasing acid content and with time. They show that acid contents greater than 4 per cent for vegetable-tanned leather and 10 per cent

for chrome leather, as determined by the method of analysis described, will cause marked destruction of leather.

The figures do not indicate the maximum amount of acid which the leathers may contain without danger of deteriora- tion with time. However, the writer was fortunate in se- curing two vegetable-tanned calf skins tanned in the same way and having the same composition as the leather being tested, one of which was 13 and the other 20 years old. The former contained 0.60 and the latter 0.53 gram of sulfuric acid per 100 grams dry leather. Tests made of corresponding parts of the butts of these skins and also skins recently tanned and finished by the same method showed that all had practically the same tensile strength, about 400 kg. per square centimeter

"1 2 4 6 8 10 12 14 16 18 20 22 24 26

.Crens H2304 Associated with 100 D m 8 Dry leather

Figure 2-Destruction of Chrome- a n d Vegetable-Tanned Calf Leather a s a Funct ion of Time a n d A m o u n t of Sulfuric Acid, Free a n d Combined, Associated with t h e Leather

cross section. For vegetable-tanned leather the maximum acid value that may be considered harmless would seem to lie between 0.6 and 4 per cent. It will require more time t o narrow this range and further series of tests are now aging for the purpose.

Effect of Concentration

The curves in Figure 2 show that chrome leather has a greater capacity for holding acid, combined or uncombined, without causing destruction than has vegetable-tanned leather. However, chrome leather normally contains a large amount of acid held in chemical combination against practically none for vegetable-tanned leather. Furthermore, when the acid enters into chemical combination with leather, it ac- tually ceases to be sulfuric acid, even though so indicated by the method of analysis employed. Thus, when one compares strips of the two kinds of leather showing equal destruction in a given time, an element of doubt is raised as to whether the true acid content of the chrome leather is really the higher.

In order to clear up this point, test pieces of both kinds of leather were kept under acid solutions of various strengths and the rather surprising fact was revealed that the chrome leather was much more sensitive to destruction by a given concentration of acid than vegetable-tanned leather. I n fact, the tests suggested a novel method for preparing very thin sheets of leather. The surface of chrome-tanned leather is usually given a light vegetable retanning before coloring t o act as a mordant. When a strip of the chrome leather 1.1 mm. thick was immersed in 6 N sulfuric acid, the interior containing no vegetable tannin was dissolved away in the course of a few days, leaving behind two strips which appeared to be in perfect condition after washing and drying: a grain strip 0.1 mm. thick and a flesh strip 0.2 mm. thick, corre-

January, 1926 INDUSTRIAL AND ENGINEERING CHEMISTRY 49

sponding to the depth of penetration of the vegetable re- tannage.

In order to simplify the experiment, the chrome leather selected for these tests was not given a vegetable retannage nor was it colored, but it was fat-liquored and worked to give it the required suppleness and strength. I ts analysis is given above. Strips of this leather and of the vegetable-tanned leather were kept under sulfuric acid solutions of strength ranging from 0.25 to 12.00 N . The containers were sub- merged in a constant-temperature bath a t 2.5' ~ 0 . 0 1 ~ C. and kept for 46 days. Then the strips were washed in run- ning water for 48 hours and thus freed from the added acid.

I

I CBBO1IE TLVNED CALF 9 0 - 1 --*-*-*+-*-

VEGETABLE T A h T D CALF

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 Nornality of S u l f u r i c A c i d S o l u t i o r i

Figure S D e s t r u c t i o n of Chrome- a n d Vegetable-Tanned Calf Leather by Contact wi th Solut ions of Sulfuric Acid of Different Strengths

Analyses were made of all strips after the tensile strength measurements in order to make sure that any weakening of the leather was not due to acid not removed in washing. All strips of vegetable-tanned leather were found to be free from sulfuric acid and all chrome-tanned strips contained less sulfuric acid than that present in the original, untreated sample.

At the end of the 46 days the vegetable-tanned strips in the 10 and 12 N acid solutions had broken into several pieces, but all the others appeared intact to the eye. But all of the chrome-tanned strips in the solutions 0.4 N or stronger were dissolved or completely disintegrated. Figure 3 shows the loss in tensile strength of the strips which had not dis- integrated. In 0.2 N acid, the chrome-leather suffered a loss of 83 per cent in strength whereas the vegetable-tanned leather was not measurably weakened.

These experiments show that the large amount of acid nor- mally found by analysis in chrome leather is not present as free acid; on the contrary, the great sensitivity of chrome leather to destruction by acid indicates that only a very small amount of the acid of chrome leather is present in the free state. However, the acid found in chrome leather is capable of neutralization, and as fast as the small amount of free acid is removed more is liberated to take its place. Chrome leather may be looked upon as a sulfuric acid reservoir having a high capacity factor and a low intensity factor.

The apparently much lower capacity of vegetable-tanned leather for acid seems to be due to the tannins combining with the protein groups which are otherwise capable of combining with acid. Wilson and Bear5 have found that an increasing degree of vegetable-tannage renders hide substance less capable of removing acid from solution. They have also

' Advance note.

found that the sulfuric acid content of chrome leather is de- creased materially by vegetable retanning.

Action of Hydrochloric Acid

Series of strips of the vegetable-tanned calf leather were treated with hydrochloric acid solutions ranging in strength from 0.1 to 5.0 N , dried, and kept for 46 days. Regardless of the strength of acid applied, all strips finalIy contained approximately 3.3 grams hydrochloric acid per 100 grams dry leather, and all showed a loss in tensile strength of approximately 25 per cent. The volatility of the acid had apparently been responsible for the setting up of an equi- librium ratio of acid to leather, practically constant for all strips. The ability of the leather to lose hydrochloric acid in this way explains why this acid has not been found responsible for the destructive action so often attributed to sulfuric acid.

When the effect of difference in volatility was eliminated by keeping the leather samples under acid solutions of definite concentrations, hydrochloric acid proved to be more destruc- tive of both kinds of leather than sulfuric acid in solutions of relatively high concentration.

Figure 4 shows the comparative destructive action of hy- drochloric and sulfuric acids upon chrome calf leather. This leather was neither colored nor fat-liquored and con- tained 8.5 grams of chromic oxide per 100 grams hide sub- stance. Strips of the leather were put into test tubes, cov- ered with acid solutions of different strengths, corked, and kept in a thermostat at 25' C. Constancy of the concentra- tion of acid in each tube was guaranteed by replacing fre- quently with fresh solution. The time required for complete disintegration of the leather was recorded. It is noteworthy that the end point in each test was very sharp. The strip would appear intact up to the day of its disintegration, when it would suddenly start to go to pieces and the destruction would be complete within the day.

HYDBOCXLOBIC ACID

SULFURIC ACID 50

1 I.( 7- 7 2 3 4 5 6 i B i

Iiomality of Acid Solu t ion Figure P T i m e Required for Complete Destruction of Chrome

Calf Leather by Acid Solut ions of Different Strengths

In solutions 3 N or stronger, hydrochloric acid is more ac- tive than sulfuric acid, while in weaker solutions the reverse is true. The greater activity of sulfuric acid in the weaker solutions seems to be due to its greater detannizing power. The more powerful destructive action of hydrochloric acid in stronger solutions may be due to the fact that it is a stronger

50 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 18, No. 1

acid than sulfuric or it may be the result of the specific ac- tion of the chloride ion, which the writer has found to be very destructive of protein matter in concentrated solution even a t the point of neutrality, while sulfates appear to have a pre- servative action.

The test tube experiment was repeated for vegetable-tanned leather, but no destructive action was apparent to the eye

with solutions weaker than about 8 N . The degree of chrome tannage appeared to have no appreciable effect upon the ac- tion of the acid so long as the leather had been sufficiently well tanned to stand the boiling test.

There are many and obvious ways in which this work can be extended so as to make possible intelligent specifications for acid content of all kinds of leather.

The Relative Toxicity of the Arsenates of Calcium' By S . B. Hendricks, A. M. Bacot, and H. C. Young

DELTA LABORATORY, U. s. BUREAU OF ENTOMOLOGY, TALLULAH, LA.

ALCIUM arsenates were first used as insecticides by Bedford and Pickering2 and by Smith.3 Since that C time, especially through the experimental work of

C ~ a d , ~ calcium arsenates have' become extensively used as insecticides. Some work of a fragmentary character has been carried out comparing the toxicity of calcium arsenate with that of other arsenicals. Ricker5 found calcium arsenate (commercial) to be as effective as crude AsmOs or Paris green. Lovett and Robinson6 compared the toxicity of calcium ar- senate with lead arsenate. Cook and McIndoo7 made a similar comparison with some other arsenicals.

The present research deals with the comparative toxicity of the various arsenates of calcium, representing an endeavor to further stabilize the manufacture of calcium arsenates and to obtain a closer relationship between the analysis of a definite commercial product and its toxicity to certain in- sects. Quantitative results can be obtained in this type of research, but very little data of any value can be found in the literature.

Experimental

Locusts (Melanoplus femur Rubrum) and boll weevils (Anthonornus grandis Boh) were used as experimental insects. The locusts were adults that had been collected in the field. They were free from disease and parasites, the check mort'ality never exceeding 5 per cent and seldom being that great. Ten locusts were placed in a wire cage 30.5 X 30.5 X 3.8 cm. and were fed on a standard bran mash containing 24.0 grams of wheat bran, 12.5 grams of sugar, 1.0 gram of the poison to be tested, a few drops of amyl acetate, and sufficient water to moisten 'the mash thoroughly. The check deter- minations were fed on a similar mixture with the exception of the poison. Observations to determine the resulting mor- tality were made a t &hour intervals. Clean paper was placed beneath the cages in order to catch the feces. Bt t'he end of an experiment the locusts were treated with dilut'e acid, dried, and reserved for analysis. The feces were care- fully collected and kept for analysis.

In the case of the boll weevils three tests were made- dust on wet leaves, dust on dry leaves, and spray. The wet leaves were fresh cotton leaves from which the petiole had

1 Presented as a part of the Insecticide and Fungicide Symposium be- fore the joint session of the Divisions of Agricultural and Food Chemistry and Biological Chemistry at the 70th Meeting of the American Chemicdl Society, Los Angeles, Calif., August 3 to 8 . 1925.

2 Lead Arsenate, 8th Rept. Woburn Expt. Fruit Farm, 1908, pp. 15 and 17.

8 Rept. N. J. Agr. Expt. Sta. , Entomology Dept., 1907, p. 476. 4 U. S. Defil. Agr., Circ. 162 (1921); 875 (1920). ' J . Econ. Enfomol., 12, 194 (1917).

J . Agr. Research, 10, 199 (1916). I U. S. Depl. Agr., Bull. 1147, 30 (1923).

been removed in order to necessitate surface feeding. These leaves were dipped into rain water and immediately dusted with the preparation to be tested, the poison being shaken from a bottle over which several thicknesses of cheese-cloth had been placed. Only the upper surfaces of the leaves were treated. In the case of dry leaves the leaves were dusted after wetting and then allowed to dry before the introduction of weevils. The sprayed leaves were prepared as directed above, the poison being applied by dipping into a suspension of the arsenical. The suspensions contained the equivalent of 2 pounds of poison in 50 gallons of water.

Lantern globes, covered with one thickness of unbleached domestic and placed upon an unglazed earthenware dish, which contained about 1.5 inches of sand, were used for ex- perimental cages. The sand was kept moist at all times. The leaves were placed in the globes, the base of the petiole being pressed into the sand, and ten field-collected weevils were released. Observations were made a t frequent inter- vals to determine the resulting mortality. Dead weevils were removed after each observation and reserved for analy- sis. The tests extended over a period of 48 hours.

Table I-Analysis of Compounds

Free CaO in Ratio CaO AszOs Cot CaO comb. CaO: Per Per Per Per Per

Expt. COMPOUNDS As201 cent cent cent cent cent

CaHAsOd. HzOo CasHz(As04)4.4HzOb Car(AsO4)z.EHzO b Caa(AsOa)z.HzOc and 3Ca3(AsO+)z.Ca(OH)zd Ca(0H)z

Ca(Ca0H)AsOi 6 - 0 Ca(0H)zf

0 Soh. containing 0.268 gram AszOs per cc. Solution

{containing 10.3248 gram AszOs per cc. 2 28 .20 57.85 0.00 0.00 28.20 2 . 5 34.80 56 .25 0.00 0.00 34 .80 3 31.22 42 .00 0 . 4 0 0.00 30 .72

3 51.00 40 .00 0 . 4 2 21.16 29 .30 3 . 3 3 40 .35 41.20 5 . 3 2 0.00 33 .58 4 45 .00 40 .88 0 . 6 2 4 . 7 1 39 .50 a 9 6 . 7

0.0757 gram CaO per cc.

Methods of preparation of the compounds: 0 Smith J . A m . Chem. Soc. 42 259 (1920). b Lime-kater and dilute HaAsOn, alkaline to phenolphthalein. allowed

to stand for several days and filtered. c Caa(AsOa)z.EHzO, prepared by method (2). dried a t 200' C., lime

~~

added. d Tartar, Wood, and Hiner, J . A m . Chem. SOC., 46, 809 (1924). e To be described in a later publication by Smith. I C. P. lime. o Analysis made by the method of Smith and Hendricks, THIS JOURNAL,

16, 950 (1924).

N O . Expt. expts.

A 10 B 10 c 10 D 5 E 10 F 10 G 5 H 10

Table

Av. longevity

Hours 32 .9 =t 0 . 5 31.6 t0.8 3 7 . 2 =t 0 . 9 3 4 . 0 =t 1 . 3 4 1 . 9 1. 1 . 0 4 3 . 7 =t 0 . 9 5 8 . 9 1. 2 . 0 7 7 . 5 =+ 1 . 8

11-Toxicity of th Bran mash AszOs con- in

sumed body Gram Mg. 0.0050 0 .132 0.0071 0.102 0.0026 0.038

0.107 0.0649 0 .052 0.0033 0.034 0.0036 0.037 0.0087 0.067

e Locust

AS205 in

feces Mg.

0.0020 0.0013 0.0013 0.0021 0.0031 0.0015 0.0031 0.0279

Ratio AszOs in body As205 in feces

65.4 8 1 . 0 3 0 . 4 51 .0 17.0 -. .- 2 2 . 2 12 .0 2 . 4