May, 1920
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
THE TRUE TANNING VALUE OF VEGETABLE TANNING MATERIALS’ By John Arthur Wilson and Erwin J. Kern MILWAUKEE, WISCONSIN Received March 2, 1920
During t h e past century an enormous amount of energy has been expended in efforts t o devise a method for determining t h e tannin content, or rather t h e t r u e tanning value, of vegetable materials. Numerous methods have been proposed,2 b u t without any indication as t o t h e correctness of t h e results obtained. I n fact t h e methods now in general use, both here and in Europe, were made official without any knowledge as t o their accuracy, but solely because they are of such nature t h a t different analysts have comparatively little difficulty in getting concordant results. Since tanning materials are usually sold on a tannin basis, these methods have proved of very great value in enabling buyer and seller t o agree as t o price, but investigators who have blindly accepted t h e results as reliable have sometimes been led into serious error. I n this paper we present what we believe t o be t h e first successful method for determining t h e true tanning value of vegetable materials. PRACTICAL D E F I N I T I O S O F T A N N N
Work on t h e chemistry of the tannins is still so far from complete t h a t no rigid chemical definition of them as a class can be given, but their extensive use, especially in t h e leather industry, has necessitated defining them in terms of some property of practical value. It has therefore become customary t o apply t h e name tannin t o t h a t portion of the water-soluble matter of certain vegetable materials which will precipitate gelatin from solution and which will form compounds with hide fiber which are resistant t o washing. The remaining portion of the soluble matter is called nan-tannin. N E W METHOD PRINCIPLE O F T H E METHOD-The method aims, of course, t o determine exactly what is called for in t h e definition. A convenient amount of t h e tanning material is shaken with a definite amount of purified hide powder until all tannin has been removed from solution. This point is determined by filtering off a portion of the residual liquor and adding drop by drop, avoiding a large excess, a solution containing I O g. of gelatin and IOO g. of sodium chloride per liter; if t h e solution becomes turbid or a precipitate forms, i t shows t h a t all tannin has not been removed from solution, in which case t h e mixture must be discarded and t h e test repeated, using less of t h e tanning material or shaking for a longer time, until t h e solution after filtration gives no visible reaction with t h e gelatin-salt reagent. The tanned powder is then washed free from soluble matter, including t h e non-tannin removed from solution by t h e hide powder, which is responsible for t h e large errors in t h e methods now in use. It is then carefully dried and analyzed for tannin as in t h e regular procedure for Presented before the Section of Leather Chemistry at the 59th Meeting of the American Chemical Society, St. Louis, Mo., April 14, 1920 2 For a review of work done since 1803, see Procter’s “Leather Iudustries’ Laboratory Book” (Spon, 1908), pp. 168-176.
465
vegetable-tanned leathers, and from this figure t h e percentage of tannin i n t h e original material may readily be calculated. S E N S I T I V I T Y O F GELATIN-SALT T E S T A N D I T S E F F E C T THE METHOD-The gelatin-salt test appears t o be t h e most satisfactory one known for determining whether or not a t a n liquor has been completely detannized. I n t h e present official methods, i t is assumed t h a t t h e hide powder completely detannizes t h e liquor because t h e non-tannin filtrate remains clear upon addition of the gelatin-salt reagent. This reagent, however, gives negative results with extremely dilute solutions of tannin. Another fact tending t o make it difficult to detect t h e presence of small amounts of free tannin in t h e liquor after shaking with hide powder is t h a t these liquors are always turbid and must be filtered, and i t is a known fact t h a t filter paper will remove tannin from solution t o some extent. It was therefore essential t o determine t h e sensitivity of the test for t h e conditions under which t h e analyses were made. If t h e time of shaking, volume of t a n liquor, and quantity of hide powder are kept constant, t h e less t h e concentration of t h e liquor t h e greater will be t h e fraction of tannin removed by t h e hide powder. This made i t desirable in checking t h e accuracy of t h e work t o make several sets of analyses for each tanning material examined, using a different concentration of liquor in each case. The most concentrated solution of each material noted in Table I was used t o test t h e sensitivity of the gelatin-salt reagent b y diluting t o greater and greater extent and then filtering and testing with the reagent under the same conditions as employed in t h e actual analyses. The reagent was added a drop at a time t o I O cc. of t h e clear filtrate in a test-tube. The dilutions a t which t h e test failed, expressed in cc. of t a n liquor diluted t o I O O cc., were as follows: quebracho I , hemlock bark 2 , oak bark 2 , larch bark 2, chestnut wood 0.5, Osage orange 2 , sumac I , and gambier 4. It therefore follows, since t h e detannized filtrates from all these liquors gave a negative test with t h e gelatin-salt reagent, t h a t more t h a n 95 per cent of the tannin of t h e gambier liquor and more t h a n 97 per cent of t h e tannin of every other liquor were removed from solution by t h e hide powder. But t h e analyses were also carried out with much more dilute liquors, from which the hide powder should remove an even greater percentage of t h e tannin. Since the results obtained for any one material, using several dilutions, checked so closely, we may reasonably conclude t h a t t h e hide powder removed practically all of t h e tannin from all solutions which gave no test with the gelatinsalt reagent after t h e shaking process. HIDE POWDER-The hide powder used in this work is a highly purified product known as American Standard and is prepared by the Standard Mfg. Co. of Ridgway, Pa., especially for use in t h e analysis of tanning materials. Since this hide powder is of variable composition due t o changes in water content, i t was necessary t o base our calculations upon t h e protein it contained, t o which we shall hereafter refer simply as hide substance. Von Schroederl found hide substance t o UPON
1
Procter, LOG.cit.
46 6
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
contain 17.8 per cent of nitrogen, which figure is generally accepted. T h e per cent of nitrogen determined by the Kjeldahl method, multiplied by 5.62, should therefore give t h e per cent of hide substance. T h e hide powder used in this work showed by analysis
....................................... ....................... .................... .................................
Water Ash .......................................... F a t (chloroform extract). Hide substance (N X 5.62). TOTAL
Per cent 12.29 0.36 0.82 86.50 99.97
from which i t would appear t h a t the factor 5.62 is correct. M A T E R I A L S EXAMINED-Eight materials were selected for examination which are fairly extensively used and which show great differences in properties, especially in so-called astringency. The solid quebracho extract and the four liquid extracts of oak bark, larch bark, chestnut wood, and Osage orange are typical samples of the best of these materials now on t h e American market. The gambier is the ordinary pasty product from t h e East Indies; the sumac, consisting of ground leaves a n d small twigs, is from a reputable dealer in Palermo; and t h e hemlock bark came from t h e forests of Wisconsin. PROCEDURE-The extracts were simply dissolved in hot water, cooled slowly, and made up t o t h e mark. The bark and sumac were finely ground and leached by percolation, only t h e extracted portions being used after making up t o definite volume. I n each test, 1 2 g. of hide powder (of known hide substance content) were p u t into a wide-mouth, rubber-stoppered, halfpint bottle, the tanning material dissolved in 2 0 0 cc. of solution was added, and t h e whole was shaken in a rotating box for 6 hrs. The amount of material t h a t could be used was limited by t h e amount of tannin t h a t t h e hide powder was capable of taking up in 6 hrs. On t h e other hand i t was desirable not t o use too little, since t h e less t h e amount of tannin fixed per unit of hide substance, t h e less t h e accuracy of t h e method, since the tannin was determined by difference. Whenever t h e liquor after the 6-hr. shaking gave a turbidity or precipitate with t h e gelatin-salt reagent, t h e test was repeated with less material. The smallest amount used for a n y material represents t h e least we were able t o use a n d still get reproducible results. The tanned powder was washed by shaking with 2 0 0 cc. of water for 3 0 min., squeezing through a suitable cloth, and repeating t h e washing operation until t h e wash water showed no color and gave no test with ferric chloride solution. Except for t h e Osage orange and chestnut wood extracts, which are unusual in several respects, not more t h a n 1 2 washings were required t o free t h e powders from non-tannin, which shows t h a t t h e line of demarcation between tannin and non-tannin is fairly sharp for the commoner materials. The wash water continued t o extract coloring matter from the powders tanned with Osage orange until after t h e fiftieth washing, while as many as 2 5 washings were required t o free t h e powders tanned with chestnut wood from soluble matter producing a dark color with ferric chloride. All wash water was tested with t h e gelatin-salt reagent, but in every case t h e test was
Vol.
12,
NO.j
negative. This method of washing was found t o be more efficient t h a n using running water on a suction filter, probably because t h e mechanical agitation assists the diffusion of soluble matter from t h e interior of t h e hide fibers. T h e washed powders were dried a t room temperature for 2 4 hrs. or longer and then analyzed for water, ash, fat, and hide substance. T h e difference between I O O and t h e sum of these percentages was taken as t h e per cent of tannin in t h e leather. The parts of tannin per I O O parts of hide substance in t h e leather divided by parts of tanning material used per I O O parts of hide substance gave t h e fraction of tannin in t h e original material. The results for the eight materials examined are given in Table I. TABLEI Percentage Analysis of Tanned Hide Powder
MATERIAL Quebracho,. , Quebracho Quebracho Hemlock Bark. Hemlock Bark. Hemlock Bark. Oak B a r k . . Oak B a r k . , , Oak B a r k . . Larch Bark.. Larch Bark., Larch Bark.. Chestnut Wood Chestnut Wood Chestnut Wood Sumac. Sumac. Sumac. Osage Orange. Osage Orange. Osage Orange Gambier. Gambier.. Gambier..
..
..... .....
.. . .. ... .. .. ..
....... ....... ....... . ..... ..... ...
-
Per 100 G. of Hide Substance Hide Tan- ---7 Subnin TanMastance (by nin terial ( N X Differ- Found Used Liter per Water Ash F a t 5.62) ence) Grams Grams 18.8 11.56 0.14 0.35 74.92 13.03 17.39 36.8 18.8 11.42 0 0 8 0.35 75.05 13.10 17.46 36.8 11.5 13.81 0.03 0.30 77.53 8.33 10.74 2 2 . 6 150.0 9.94 0.12 0.24 76.54 13.16 17.19 287.9 100.0 10.73 0.13 0.28 79.39 9.47 11.93 191.9 9 . 2 8 147.1 75.0 12.76 0.05 0.28 79.53 7.38 67.5 12.54 0.07 0 1 2 74.76 12.51 16.73 131.6 45.0 11.19 0.09 0.24 79.36 9 12 11.49 87.6 25.0 13.53 0.05 0.34 81.00 5.08 6.27 48.9 67.5 12.59 0.09 0.13 75.61 11.58 15.32 131.6 45.0 13.65 0.09 0.30 77.90 8.06 10.35 87.7 25.0 16.52 0.08 0.25 78.65 4.50 5.72 48.9 67.5 12.43 0.11 0.05 75.76 11.65 15.38 131.6 45.0 12.82 0.13 0.19 78.54 8.32 10.59 87.7 37.5 12.05 0.10 0.21 80.74 6.90 8.55 71.7 93.8 11.39 0.16 0.36 74.92 13.17 17.58 179.3 62.5 12.26 0.23 0.31 78.38 8.82 11.25 119.5 37.5 11.750.120.3781.97 5.79 7.06 73.5 9.53 12.32 95.0 48.8 12.820.130.1777.35 8.42 63.3 32.5 12.83 0.09.0.25 80.09 6.74 26.3 12.430.120.2581.43 5.77 7.09 51.2 6.11 7.50 97.4 50.0 12.080.180.1981.44 11.770.260.2881.70 5.99 7.33 94.7 49.5 4.48 56.5 29.0 1 3 . 0 6 0 . 1 4 0 . 3 5 8 2 . 7 4 3.71
Material Grams
Tannin In Material Per Cent 47.26 47.45 47.52 5.97 6.22 6.31 12.71 13.12 12.82 11.64 11.80 11.70 11.69 12.08 11.92 9.80 9.41 9.61 12.97 13.30 13.85 7.70 7.74 7.93
C O M P A R I S O N W I T H A. L. C. A . M E T H O D
Practically all tanning materials are bought a n d sold in this country on t h e basis of tannin content as determined by the official method of the American Leather Chemists Association. T h e principle of the method is similar t o t h a t of the new method as regards t h e shaking of a solution of t h e tanning material with hide powder, but differs in t h a t t h e drop in concentration of t h e liquor upon shaking is taken as the measure of its tannin content, while t h e tanned powder is simply discarded. T h e details of the A. L. C. A.' method follow: The hide powder is specially prepared by giving it a light chrome tannage with chrome alum, washing i t practically free from soluble matter, and squeezing it until i t contains not less than 71 nor more than 74 per cent of water. The solution of tanning material for analysis must contain not less than 0.375 nor more than 0,425 g. of tannin per IOO cc., as found by this method. To zoo cc. of this solution is added such an amount of the wet hide powder as contains not less than 12.2 nor more than 12.8 g. of dry hide powder and the whole is shaken fcr I O min. The limits defined are to some extent arbitrary, but i t has been found necessary t o set limits of some sort in order to get concordant results. The detannized solution is separated from the powder by squeezing through linen and is then filtered through 1
J . A m . Leather Chem. Assoc., 14 (1919). 654.
May,
==
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
1920
-
4
cc
IO.
U
46 7
,-..
"
n
W
OSAGE ORANGE EXTRACT
WISCONSIN HEMLOCK BARK
-
z ' z
I
z -2
0 w
5 15.
SICILIAN SUMAC LEAVES
a.
5 IO-) W
GAMBIER EXTRACT
15 10
5
IO 15 20 25 30 35 GRAMS WET HIDE POWDER PER I S O c J A N LIQOR
J 70
5
I O 15 2 0 25 30 35 GRAMS WET HIDE POWDER PEP, 150cc.TAN LIWOK
70
FIG.1
FIG.l a
paper, after the addition of kaolin, the solution being returned until the filtrate is quite clear. The amount of -esidue from an aliquot portion of this filtrate, after correcting for the water introduced by the hide powder, is taken as a measure of the non-tannin in the original material. The difference between the total soluble matter and the non-tannin is called tannin. Several outstanding sources of error in this method were emphasized by the authors in an earlier paper.l The most serious defect is t h a t it does not take into account t h e fact t h a t hide powder removes non-tannin of certain kinds, such as gallic acid, pyrocatechol, etc., from solution t o an extent depending upon the concentration. These substances do not make leather and are readily washed out. This explains the fact t h a t where tanners t r y t o keep a rigid cost account of tanning materials, they find no such amount of tannin in the leather as would be expected from t h e amount of tannin used, as determined by the A. L. C. A. method. The analyses of t h e eight materials by t h e A. L. C. A. method are given in Table I1 along with a calculation of t h e percentage errors involved in this method, made upon t h e assumption t h a t the results obtained by the new method are correct.
Although t h e enormous errors in the A. L. C. A. method are nothing short of sensational, we are convinced t h a t they are not a t all exaggerated. The extent of these errors is less surprising, however, when the behavior of certain non-tannins, like gallic acid, in the presence of hide powder is studied. A series of solutions of gallic acid of different strengths was prepared and treated by the A. L. C. A. method as though they were t a n liquors of the required strength. The results for non-tannin should have been T O O per cent in every case, if the method were reliable, but t h e actual d a t a in Table 111 show t h a t a very large proportion of the acid is taken up by the hide powder.
TABLEI1 Percentage Analysis of Material-I,. C. A &lethod--Soluble M a t t e r -*New Insoluble NonMethod MATERIAL Water M a t t e r tannin Tannin Tannin 7.16 6.96 68.01 47.41 Quebracho 17.87 6.71 10.06 6.17 Hemlock Bark 8 . 9 0 74.33 3.68 19.46 24.20 12.88 Oak Bark.. . .. 52.66 5.88 20.90 22.14 11.71 Larch Bark . . . . . . 51.08 1.50 13.80 25.80 Chestnut Wood. 58.90 11.90 17.99 23.56 9.61 S u m a c . . . . . . . 9.25 47.20 3.45 10.63 39.87 13.37 Osage Orange 46.05 5.36 18.57 24.95 7.79 Gambier.. . . 51.12
--
A -
....... ...
.. . . ...... ....
1
.J. Am. Leather Chem. Assoc., 13 (1918), 429.
Percentage Error in A. L C.A. Method 43 63 88 89 117 166 198 220
TABLE111-RESULTS OF TREATMENT O F PURE GALLICACID SOLUTIONSBY
e.
THE A. L. A. MBTEOD Using 47 g. of wet hide powder (73% water) t o 200 cc. of solution Gallic Acid Non-tannin Tannin Per cent Per cent Grains per Liter 54.0 46.0 8.88 47.1 4.44 52.9 43.8 2.22 56.2 40.4 1.11 59.6
TABLEIV-EFFECT
O F ALTERINGPROPORTION OF HIDE POWDER UPON AMOUNT O F GALLICACID REMOVED FROM A 0.888 PER CENT S O L U T I O N Using t h e principle of t h e A. I,. C. A. method Wet Hide Powder Non-tannin Tannin (73% water) Per cent Per cent G. per 200 cc. 91.8 5 8.2 10 86.0 14.0 69.6 30.4 25 52.1 47.9 50 43.7 56.3 7s
Table I V shows t h a t the amount of gallic acid removed from solution is very largely dependent upon the proportion of hide powder employed. Our results by the new method for gambier indicate t h a t this sample contains more t h a n 4 times as much non-tannin as
T H E ' J O U R N A L OF IhTDUSTRIAL A N D ENGINEERING C H E M I S T R Y
468
tannin. When we consider t h a t more t h a n 50 per cent of these non-tannins may be removed from solution by t h e hide powder, errors exceeding 2 0 0 per cent are t o be expected by t h e A. L. C. A. method. The reason for the necessity of arbitrary limits in t h e official method is suggested b y the gallic acid experiments, but is more strongly emphasized by similar experiments upon actual t a n liquors. The effect of altering t h e proportion of hide powder with solutions of the eight tanning materials is shown in Table V and Figs. I and l a . The zero points in t h e figure represent t h e percentages of tannin found by t h e new method. I n none of these experiments did t h e solution after shaking give a test with the gelatin-salt reagent, b u t we nevertheless find enormous increases in per cent of tannin due t o increasing t h e proportion of hide powder. This alone should be sufficient t o show t h a t t h e A. L. C. A. method cannot be considered a n exact one and t h a t the arbitrary limits were set so t h a t different analysts would get concordant rather t h a n more nearly correct results. Table V and Fig. 2 show how t h e percentage error increases with increasing proportions of hide powder.
voi.
5
12, N O .
TABLEV I W e t Hide Percentage Analysis of Mixture of 5 Powder P a r t s of Quebracho Extract t o 9 P a r t s of D r y Gallic Acid (73% Water) Used t o -A. L. C. A. MethodDetannize Soluble M a t t e r New 150 Cc. T a n InsolLiquor uble NonMethod Grams Water M a t t e r tannin Tannin Tannin 4.4 5.80 3 . 9 6 6 3 . 3 4 26.90 16.93 3 . 9 6 53.39 3 6 . 8 5 16.93 8.8 5.80 3 . 9 6 44.07 46.17 16.93 17.5 5.80 3 . 9 6 3 7 . 1 4 53.10 16.93 35.0 5.80 70.0 5.80 3 . 9 6 3 3 . 8 7 56.37 16.93 I
--
__L-
Percentage Error Due t o Hide Powder
- ------
-QuebrachoGamIn Pres- bier Alone encepf (from (from Gallic Table F i r . 11) Acid V) 18 59 72 33 118 121 39 173 190 43 214 229 233 273 44 I
,
which give t h e least errors are most astringent, while those giving greatest errors are least astringent. T h e order of the materials in Table I1 might almost be taken as the order of decreasing astringency, although a n exact parallelism cannot be claimed since we have no quantitative measure of astringency. Quebracho and hemlock bark are generally conceded t o be t h e most astringent,. and sumac and gambier the least astringent of these materials. This suggested a relation between astringency and t h e ratio of non-tannin t o tannin. I n t h e experiments listed in Table I, the hide powder fixed more t h a n twice as much tannin from the quebracho liquors in 3 hrs. as from the gambier liquors
TABLEV
Wet Hide Powder (73%
280
Water) .. ----,TJsrd ----
Grams per Liter 3
MAISRIAL Quebracho
............
Hemlock B a r k . .
Oak B a r k .
...... 20
...........
4.11
.........
4.37
Larch B a r k . .
.....
15
.............
4
........
8
Chestnut W o o d . .
Sumac..
Osage Orange.
Gambier..
...........
4.58
t o Detannize 150 C c . T a n Apparent Percentage Liquor Per cent of Error Due t o Tannin Hide Powder Grams 68.18 44 70.0 67.56 43 35.0 66.61 40 20.0 10.0 64.36 36 57.56 21 5.0 10.98 78 70.0 72 10.60 35.0 9.76 58 20.0 9.35 52 10.0 7.98 29 5.0 25,02 94 70.0 35.0 24.59 91 17.5 24.01 86 22.09 72 8.8 18.77 46 4.4 28.10 140 70.0 24.52 109 35.0 21.97 88 17.5 19.10 63 8.8 16.24 39 4.4 26.87 126 70.0 25 80 117 35.0 74.59 107 20.0 23.52 98 17.5 22.49 89 12.0 70.0 24.98 160 35.0 25.05 161 17.5 24.47 155 8.8 23.45 144 4.4 21.45 123 70.0 40.48 203 35.0 39.47 195 20.0 38.21 186 10.0 36.27 171 7.0 35.67 167 70.0 29.04 273 35.0 25.60 229 17.5 22.56 190 8.8 17.22 121 4.4 13.38 72
As might be expected, t h e greatest errors in the Official method are obtained with those materials containing the greatest proportion of non-tannin t o 'tannin. Quebracho, having least non-tannin, gives t h e smallest error. However, if t h e quebracho is mixed with gallic acid t o make t h e proportion of non-tannin t o tannin about t h e same as in t h e case of t h e gambier, i t gives errors nearly as great as in t h e case of t h e gambier. This is shown in Table VI. Comparison of t h e two methods has brought out a t least one fact of practical significance: Those materials
"I &oo Z
I
I60
z z z
6 k-
z
w n
I
5
10 I 5 2 0 2 5 30 35 GRAMS WET HIDE POWDER PIG.2
IO
PER
15Occ.TAN LIUOS
in 6 hrs. B u t , when enough gallic acid was added t o t h e stronger quebracho liquors t o give them the same proportion of non-tannin t o tannin as in t h e gambier, t h e hide powder did not remove anywhere nearly all t h e tannin in 6 hrs. Upon addition of t h e gelatin-salt reagent t o t h e liquors after shaking, huge precipitates were formed, suggesting a great reduction in astringency. That t h e effect was only one of slowing up, t h e tanning action was proved b y t h e fact t h a t t h e hide powder was able t o detannize t h e solution completely in 24 hrs.
May,
1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
PRACTICABILITY
O F THE N E W METHOD
The new method in its present form requires more time t h a n t h e A. L. C. A. method, b u t this disadvantage must be considered negligible compared t o t h e advantage of greatly increased accuracy. If t h e new method is t o prove satisfactory from t h e standpoint of setting a price on tanning materials, i t must give results which are readily reproducible in different laboratories. We are convinced t h a t i t will do this quite as well as t h e official method when t h e different analysts become used t o manipulating i t . The several results for any one material in Table I were determined days, a n d sometimes weeks, apart. The hide powder used in t h e new method is not chromed, b u t is used exactly as i t comes from t h e manufacturer. If, however, i t is first chromed, i t gives a higher result for Osage orange, apparently due t o t h e chrome acting as a mordant for t h e coloring matter. But this coloring matter does not precipitate gelatin nor does it form a stable compound with unchromed hide fiber and we feel t h a t t h e method is t h e more accurate for not estimating this coloring matter as tannin. The question of putting a value on this coloring matter may have t o be solved, b u t this problem will probably be confined t o very few materials. On t h e whole we believe t h e new method will be found quite as practicable a n d certainly very much more satisfactory from t h e standpoint of accuracy t h a n t h e present official method. What has been said concerning t h e A. L. C. A. method applies equally well t o t h e European methods since t h e y differ only in detail. SUMMARY
A new method of tannin analysis is described which we believe determines exactly what is called for in t h e generally accepted definition of tannin from a practical viewpoint, namely, t h a t portion of t h e water-soluble matter of certain vegetable materials which will precipitate gelatin from solution and which will form compounds with hide fiber which are resistant t o washing. The analyses of eight common tanning materials by t h e new method and by t h e official methodof t h e American Leather Chemists Association indicate t h a t t h e latter method is in error t o t h e extent of from 43 t o 2 2 0 per cent. T h e new method gives reproducible results and is considered entirely practicable. PRACTICAL METHODS FOR THE DETERMINATION OF RADIUM. 111-ALPHA-RAY METHOD, GAMMARAY METHOD, MISCELLANEOUS’
By S. C. Lind BUREAUOF
MINES EXPERIMBNT STATION, GOLDEN, COLORADO Received October 29, 1919
I n two previous papers2 t h e writer has described a form of interchangeable electroscope, t h e details of its construction, and its use in connection with t h e determination of radium by t h e emanation method.
I n t h e present paper, i t is proposed, first, t o discuss briefly some more recent modifications in t h e construction of t h e electroscope, which have contributed materially t o t h e ease and accuracy of its manipulation; second, t o discuss rather fully t h e application of t h e a-ray method t o t h e determination of radium in solids without reference t o any particular form of electroscope; and, third, t o consider briefly t h e y-ray method of determining radium. MODIFICATIONS
IN
THE
IXTERCHANGEABLE
ELECTRO-
SCOPE
Conditions due t o t h e European war necessitated an electroscope of American make. I n devising one, t h e writer had economy and accuracy equally in mind, and sought t o combine as far as possible in a single instrument t h e advantages of t h e various types already in use. The advantages sought have already been mentioned,’ b u t perhaps a somewhat fuller explanation in t h e way of acknowledgments will not be out of place. The “interchangeable” feature, enabling t h e use of a detachable head carrying t h e reading device and leaf system, was first used b y Professor Ebler,’ in connection, however, with t h e Exner type of doubleleaf electroscope. Instead of t h e latter, i t was desirable t o use t h e simpler single-leaf Wilson t y p e so modified t h a t t h e leaf could be readily removed if replacements were necessary. The highly advantageous feature of having t h e reading microscope firmly fixed t o t h e head so there can be no chance of accidentally changing t h e relative positions of leaf and micrometer scale of t h e microscope was adapted from t h e Wulf3 electrometer, thus providing rigidity while preserving complete visibility of t h e leaf system-an advantage not possessed by t h e Wulf instrument. The most far-reaching change in t h e instrument, since its earlier description, consists in t h e substitution of amber or amberoid insulation instead of sealingwax for both insulators (points g and d in t h e original) . 4 This substitution made it necessary, however, t o provide a new means of rendering t h e emanation chamber gastight. This has been accomplished without t h e use of any binding material by an ingenious arrangement, due t o Mr. P. F. Elzi, of t h e SachsLawlor Company, Denver. Fig. I illustrates its construction. The brass collar a is threaded into t h e lower chamber (not shown) a t b and made gastight by means of a lead washer, c. The collar a is hollow and provided with an interior projecting shoulder, d , on which rubber washers rest a t f a n d f’ above and below. On f and f ’rest amberoid discs, g and g’, pierced b y t h e rods h and h’, both of which are threaded into t h e connector i. The holes through g and g’ are made tight b y small rubber washers j and j’. h terminates above in a shoulder, k , resting on t h e washer j and a tip I t o make electrical contact with t h e spring s attached t o the leaf system above. Below, h’ is provided with a similar shoulder, k ’ , which rests 1
1 Published with permission of the Director of the U. S. Bureau of Mines. 2 THISJOURNAL, 7 (1915), 406, 1024.
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2 8 4
Part I, LOC.cit. Chem. Kalendar, 1914, Part 11, 367. Physik. Z , 8 (1907). 246, 527. Part I , LOC c i f . Fig 3
