138 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 18, No. 2

attached to the ultrafilter through a Hoke pressure reduc- tion valve. Nitrogen gas was used instead of compressed air in order to avoid oxidation of the tannin solutions. The total solids of each solution, before ultrafiltration, were de- termined by evaporation of a definite volume of sample in the usual way, and the amount of solute passing through the ultrafilter was similarly determined by evaporation of a measured portion of the ultrafiltrate to dryness. All weights in the table refer to 25 cc. of solution.

Table I-Ultrafiltration of Hemlock Bark Tanning Solutions of Different pH Values

Dry residue Solids held Per cent in 25 cc. of Ultra- backin of total

original soh. filtratec Tannin ultrafilterc solids PH Gram Gram test6 Gram held back0 (1) (2) (3) (4) (5) (6)

Series I , Pressure = 80 lbs./sq. i n . (234 k g . / s q . cm.) - 0.052 42 - 0.089 67

2 . 0 0.125 0.073 3.0 0.132 0.044 3.7‘ 0.130 0 . 0 6 5 + 0.065 50 4.0 0.131 0.050 ++ 0.081 62 4.5 0.131 0.050 $+ 0.081 62 7.1 0.135 0.135 ++ 0.000 0 9.0 0.140 0.141 ++ 0.000 0

Series 11, Pressure = ZOO lbs . /sq. in. (586 kg./sq. cm.) 2.0 3.0 3.70 4.5 7.1 9.0

0.122 0.131 0.132 0.133 0.131 0.138

0.067 0.095 0.046 0.090 0.106 0.094

Series III, Pressure a 120 lbs./sq. in. (352 kg. /sq. cm.) 2.5 3.3 3.8’ 4.1 4.1 4.5 4.5 6.0 7.0 8.0 9.0

0.130 0. A34 0.132 0.132 0.132 0.132 0.132 0.137 0.138 0.140 0.148

0.055 0.055 0.052 0.049 0.045 0.044 0.048 0.064 0.082 0.097 0.118

+ + + + - - - - + ++ ++

0.075 0.079 0 * 080

E: E:; \ ::E 1 0.074 0.056 0.044 0.030

55 72 35 68 81 68

58 59 61 64

65 54 41 31 20

a “Natural“ pH of tannin solution. b The signs in this column refer to the result of a gelatin-salt test upon

c The data in Columns 5 and 6 would be termed “Tannins and Insoluble the ultrafiltiate.

Solids,” and those in Column 3, “Nontannins” by Browne.

Results

In Series 111, a t pH = 4.1 and 4.5, are given duplicate filtrations run simultaneously. A deviation of 8 to 10 per cent is seen. This is an example of deviations due to

the difference in permeability of two membranes as a result of the uncertain technic of drying to a certain “tackiness.”

Series I and I1 were made with the same hemlock bark solutions, but the membranes used in Series I1 were made after a number of others had been prepared from the same 6 per cent collodion solution. This solution, although carefully protected from evaporation, had become con- centrated owing to unavoidable slight evaporation of alcohol and ether during the preparation of membranes, thus pro- ducing less permeable membranes.

In Series I11 a different lot of hemlock bark solutions was employed, but prepared in a manner identical to that of Series I and 11. A new 6 per cent collodion solution was also used in Series 111; consequently, these membranes were practically identical with those employed in Series I.

Examination of the data and curves shows a pronounced difference in ultrafiltration of hemlock bark solutions as a function of the pH thereof. Comparison of the three series reveals large differences in degree of ultrafiltr R t’ ion of identical solutions as a result of differences in the collodion solutions in which the membranes were prepared and in- dividual variations in the membranes. Furthermore, vari- ations in the gelatin-salt test for tannin (Column 4) give additional indication of the unreliability of this method of separating tannins from nontannins. The depressions in the curves of Series I and I1 a t the “natural” pH of the extract solutions may be due to a coincidence of using less permeable membranes in these cases.

The total solids or grams of dry residue in Column 4 are low a t pH = 2 and high a t pH values alkaline to pH = 7. This is because (a) a t pH = 2 hydrolysis on account of the acidity results in the formation of certain volatile organic matter, and (6) in the solutions alkaline to pH = 7 oxidation of the organic matter increases the weight of solids.

It is apparent that ultrafiltration of vegetable tanning solutions as a method of separation of tannins from nontans is of very doubtful analytical value.

Acknowledgment

The authors are indebted to A. F. Gallun & Sons Company for grants in aid of this investigation.

Laboratory Mixing Machine for Solid s1

By R. M. Hixon

JOWA STATE COLLEGE, AMES, Ia.

HIS machine, designed for the laboratory testing of sufficient traction for rotating the bottle. Mixing paddles nicotine dusts, will be found valuable for the mixing

of small quantities of powders or even for the agitation of solids with liquids. It can be made in a few minutes with a Cenco stirring motor, ringstand, and metal rods or glass tubing.

The shaft of the stirring motor is connected by suction tubing to s glass or metal rod of sufficient length to take the 1- or 2-liter bottle used. A parallel rod is rigidly sup- ported from the same ringstand, and provided with a sleeve of glass or metal tubing to rotate with the bottle. The outer end of the rigid rod and the elongated shaft of the motor are connected by a short rod held by Universal clamps. The driving shaft is covered with rubber tubing to give

may be inserted in the bottle if desired.

1 Received December 16, 1925. Laboratory Miring Machine for Solids