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THE JOURNAL O F INDUSTRIAL AND ENGINEERING CHEMISTRY
to 2 parts per lo00 were obtained.* The reaction takes place according to the following equation: BKaFe(CN)s
4- Na&O4 4-
2H20 = S&NaFe(CN)a
+-
tervals in a bottle with an excess of air for 3 hrs. and was then exposed in a thin layer to the air for 16 hrs. The temperature was 82" to 87" F. and the humidity was fairly high at the time of this experiment. Even under these conditions the purity dropped only slightly (1.3 per cent). Another sample of the same lot was ground in a mortar and then subjected to the foregoing treatment; in this case the purity dropped 19 per cent. These figures bring out the relation between the coarseness and the stability of the material very effectively. ACKNOWLEDGMENT
2H1S03
As this method of analysis gives results that are a few tenths of a per cent low, the products obtained in this work are nearly 99 per cent pure. A number of purified samples gave positive tests for small amounts of chloride. A sample was oxidized in alkaline solution with C. P. sodium peroxide, and sulfate equivalent to 36.69 per cent sulfur in the hydrosulfite was found as against a calculated value of 36.78. I n determining the stability of the product a fresh sample andyzing 98.9 per cent pure was shaken at frequent in-
The writers wish to acknowledge their indebtedness to Dr. Reid Hunt for his interest in the work and to Mr. Henry W. George for his invaluable assistance in constructing the apparatus.
After we had been using this method for some time, we found that G. Bruhns, J. Chem. SOG.(London), l l 8 A (1920),11, 395, had used a similar one. 9
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Vol. 14, No. 12
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Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Materials' By John Arthur Wilson and Erwin J. Kern GABORATORIES OF A. F. GALLUN & SONS COMPANY, MILWAUILEE, WIS.
AMERICAN LEATHER CHEMISTS ASSOCIATION METHOD
SERIES of solutions of solid quebracho extract was prepared according to the official method of the American Leather Chemists Association,4 except for the addition of hydrochloric acid or sodium hydroxide to produce approximately the desired pH value before making each solution up to the required volume. The pH values were finally elecdetermined a t 20' C. by means of the hydrogen trode, and the solutions were analyzed according to the official method. The results, which are shown in Table I and Fig. 1, were calculated on the basis of the original extract, no allowance being made for the added acid or alkali, so that the percentages recorded for nontannin are higher than they should be, but this was done so as to record the more significant values for tannin and insolubles on 6he basis of the original extract. I n order to show the effect of valency, we ran a second series of determinations, like the first except for the fact that sulfuric acid and lime were used to alter the pH values. The results of these are shown in Table I1 and Fig. 2. The solution receiving no addition of acid or alkali had a pH value of 4.60. As the pH value was lowered from this by the addition of either sulfuric or hydrochloric acid, there was an increase in the percentage of insoluble matter found and a corresponding decrease in percentage of tannin found. Sulfuric acid proved slightly more effective than hydrochloric
A
in increasing the percentage of insoluble matter. With increasing pH value, there was f i s t a decrease in the amount of insoluble matter, and the unfiltered solution gradually became more nearly transparent. I n the case of NaOH this continued without a break, but with Ca(0H)a an abrupt change occurred a t the neutral point, and with further rise of pH value the tannin was precipitated in increasing a m o u n t s The curve for insoluble matter in Fig. 2 furnishes an interesting proof of the accuracy of Procter's limewater method6 for determining the percentage of acid in tan liquors. Since tanning extracts often undergo fermentation during shipment, or upon storing, with a corresponding increase in hydrogen-ion concentration, it is not surprising that analysts sampling a shipment at different times fail to obtain concordant results by the A. L. C. A. method. Since the chief aim of this method is the ability of different analysts to get concordant results, the method would serve its purpose
At a meeting of the Society of Leather Trades' Chemists in London, September 8, 1921, an international commission was appointed, under the chairmanship of H . R. Procter, to consider suggested revisions of the officialmethod of tannin analysis. Among the things which the commission was called upon to investigate was the ej8ect of change of p H d u e , the importance .of which had previously been pointed out by Procter.2 Later Thompson, Seshachalam, and Hassan3 made a preliminary study of the eflect of adding acetic and hydrochloric acids to extracts of quebracho, mimosa, mangroue, gambier. myrobalans, chestnut and oak wood, and found that the addition of small amounts of acid dected nearly all the determinations made. The present inuestigation represents an attempt to deal with the subject somewhat more systematicalIy. and was confined to a study of the behavior of quebracho extract at diflerent p H values.
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TABLEI-EFFECTOF HCl PH
Value
OP QU~OSRACRO NaOH UPON THE ANALYSIS THE A. L. C . A. METHOD Insoluble Nontannin Tannin Per cent Per cent Per cent
14.72 12.76 10.71 10.06 8.80 6.67 4.15 3.65 3.52 3.50 2.55 2.70 1.58 0.98 0.35
1 Presented before the Division of Leather Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September
4 to 8, 1922. 8 J . SOC.Leather Trades' Chem., 6 (1921), 219. 8 I b i d . , 6 (.1921),389. 1 J . Am. Leather Chem. Assoc., 16 (19211,113.
OR
EXTRACT BY
6
6.70 6.63 6.58 6.02 6.62 6.92 7.55 8.00 8.21 8.13 8.78 8.58 17.70 32.42 70.55
61.63 63.24 65.09 65.70 66.95 09.96 71.05 71.63 72.15 72.07 72.65 72.60 67.80 68.53 30.35
'!Leather Industries Laboratory Book," Spon, London, 1908.
Dec., 1922
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
more efficiently by requiring that the analysis be made on a solution of definite pH value.
TABLE11-EFFECT
O F HzSOd OR Ca(0H)z UPON THE ANALYSIS OF QUEBRACHO EXTRACT BY THE A. L. C.A. METHOD Insoluble Nontannin Tannin PH Value Per cent Per cent Per cent 1 98 17.10 7.97 61.61 2:97 13.23 7.28 62.69 H2SOdI 3 66 11.65 6.95 64.50 9.53 6.60 66.32 k 4:27 8.80 6.62 66.95 4.60 5.77 70.15 5.90 6.63 3.42 6.13 73.02 6.80 3.08 72.95 7.03 2.70 72.90 7.17 9.58 11.65 62.15 8.45 11.93 63.10 34.50 13.92 40.28 52.75 25.61 17.17 10.55 19.65 63.84 19.98
RATE OF FILTRATION Thompson and his coworkers found that the rate of fltration of tan liquors was increased by the addition of acid. While filtering, for the determination of insoluble matter, the various solutions mentioned in this paper, we kept a record of the time required to collect 100 cc. of filtrate, after the paper had been tanned for exactly 1 hr. In order to show the change as accurately as possible, we selected filter paper of uniform thickness and repeated each measurement several
FIG. 2 FIG.3 HOW THE ADDITION OF HYDROCHLORIC ACIDOR SODIUM HYDROXIDZ AFFECTS THE ANALYSIS OF QUEBRACHO EXTRACT FIG. 2-~IiOWING HOW THE ADDITIONOF SULFURIC ACID OR CALCIUM HYDROXIDE AFFECTSTHE ANALYSIS OF QUEBRACHO EXTRACT FIG. 3-sHOWTNG HOW THE RATEO F FILTRATION OF A SOLUTION O F QUEBRACHC EXTRACT I s ALTERED BY THE ADDITION OF ACTDS AND ALKALIES
FIG.1
FIG. l-fhiOWING
times. The resulting values are plotted in Fig. 3. It will be of general interest to note that the untreated liquor was one of the most difficult to filter. The differences in rate of filtration may be,ascribed largely to differences in the degree of dispersion of some of the solid matter, which we followed by means of the Tyndall cone. A very fine or very coarse dispersion filtered much more quickly than one of intermediate degree. The effect was very marked in the solutions to which lime was added. With the first appearance of turbidity, after a pH value of 7 was passed, there was a tremendous increase in time required to filter 100 cc., but as the precipitate grew coarser the liquor filtered much more quickly.
WILSON-KERN METHOD In earlier papers6the authors have shown that the A. L. C . A. method gives values for tannin that are much too high, because of the inclusion of a variable fraction of nontannin that can easily be separated from the hide powder by washing. They proposed a new method for determining the true tanning value of tanning materials that is receiving some attention from the present international commission. In studying the effect of change of pH value upon this method, we prepared a series of liquors from the sample of solid quebracho extract as described in the paper, "The Determination of Tannin,"' except for the addition of HCl or NaOH t o give the desired pH values. The results of a series of determinations are given in Table 111, and the values for tannin are plotted in Fig. 1 for comparison with those obtained by the A. L. C. A . method. A practically constant value for tannin is obtained over the wide range of pH values 3.6 to 7.3. Where the falling off in percentage of tannin occurs a t pH values higher than @
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THISJOURNAL, 12 (1920), 465, 1149; 13 (1921), 772. I b i d . , 13 (1921), 772.
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7, the results should not be considered as found by this method, because in each case the residual solution gave a test for tannin, by the gelatin-salt test, whereas the method specifies that the determination is to be discarded whenever such a test is obtained. Apparently the hide-tannin compound is unstable, or possibly does not form a t all, a t pH values higher than 8. The fact that some combination seemed to take place at higher pH values may be apparent only, because the powders, wet with liquor, were washed with dist,illed water having a pH value of about 5.8. The stability of the hide-tannin compound a t different pH values is being studied as a separate work. The falling off in percentage of tannin found a t pH values below 3 may be the result of the great swelling of the hide powder in liquors of low pH value. A complete investigation of this point has not yet been made. T PLBLE 111-EFFECT O F ADDING HC1 OR NaOH UPON THE DETERMINATION O F TANNIN IN QUEBRACHO EXTRACT BY WILSON-KERN METHOD -pH ValueGelatinTannin Initial Final Salt Test Per cent 1 1.86 2.57 Negative 4.20 Negative 35.50 HCII 4.62 Negative 37.73 3.60 5.05 Negative 44.35 Negative 44.96 3.85 5.35 I 4.60 5.57 Negative 44.83 I 5.03 5.70 Negative 44.87 6.52 6.50 Negative 44.68 7 31 7.002 Positive 44.65 NaOHI S:04 Positive 31.87 8.18 Positive 29.30 .... Positive 15.55 Positive 10.88 11.10 9.16 Positive 7.93 1 Hide powder could not be washed. 2 Hydrogen electrode became poisoned very quickly.
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I
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The pH values of the liquors were determined both before and after the 6-hr. shaking with hide powder. The curve representing tannin is plotted against the initial pH value of the solutions because not all the final pH values could be determined. When alkali is added to a tan liquor so as to produce a pH value of about 9, there is no difficulty in measuring the pH value immediately after the addition, but after a few hours the determination can no longer be made because the liquor then poisons the hydrogen electrode. This change in tan liquors occurring a t a pH value of about 9 has been noted before.*
CONCLUSION The percentage of tannin in quebracho extract indicated by the A. L. C. A: method increases with pH value to a maximum at 8, and then decreases rapidly toward zero. On the other hand, pH value appears to have no effect upon the determination of tannin by the Wilson-Kern method over the range 3.0 to 7.3, but the rate of tanning of hide powder decreases rapidly with increasing pH value above 7. The rate of filtration of tan liquors is markedly affected by change of pH value, which may be attributed to changing degrees of dispersion of some of the solid matter. The addition of lime to tan liquors causes a precipitation of tannin, but only a t pH values above 7.2. 8
THISJOURNAL, 13 (19211, 1025.
