April, 1924
I N D U S T R I A L A X D ENGINEERIXG CHEMISTRY
I n the same way, the relationship between silicon and hardness is shown in Fig. 2, and can be expressed by the equation :
-
where
W P
Si = 0.092 X ( N 86)'J.177 = the Brinell hardness number, or P / A = load, in kg., on a 10-mm. diameter steel ball
4 = the area, in sq. mm., of the impression made by the 10-mm. ball on the sample
From the foregoing equations it will be seen that the electrical resistance of pure iron is 9.6 microhms and its hardness number is 86. Of tho two methods of analysis the electrical resistance probably is the more accurate, although either one should give
367
the silicon content within 0.1 per cent. If samples are available in the form of rods of uniform cross section, electrical resistance determinations can be made a t the rate of one or two per minute, each determination giving the average resistance for the rod. Methods have also been developed for determining the resistance of ring test pieces. The hardness method has the advantage that any form of sample can be used provided it has sufficient body. It should be a t least 6 mm. thick and 12 mm. wide, with a flat, smooth surface where the impressions are to be made. Impressions, a t the rate of one per minute, can be made on different portions of the sample, thus giving information both as to the average composition and to the homogeneity of the sample.
Bacteria in Tannery Soak Waters' Similarity in Effect of Salts upon Bacterial Counts and upon the Stability of Colloidal Dispersions By J. A. Wilson and C. J. Vollmar A. F. GALLUN& SONSC o . , MILWAUKEE, Wxs.
W
.
Studies of bacterial life in tannery soak waters indicate that the necessary to bring the numHEE bacterial bacteria are distributed throughout the water, not as indioiduals, but ber of Colonies on each Plate counts were made rather i n groups, and that the aoerage number of bacteria in each Within the required limits. of water used for group tends to oary with the composition of the solution in a manner Duplicate series were run s o a k i n g salted calfskins, analogous to the oariation in aoerage size of particle in colloidal in every case With the Soak much h&r results were obtained by substituting dispersions. Bacterial counts, as ordinarily made, represent only water diluted both 10,000 the number of groups per unit uolume of culture medium and not the and ~0~~~~~times before sterilized water from the Planting. soaks for distilled or tap total number of bacteria planted in the Petri dish. The bacterial count of any soak water was found to be a function of both the kind As culture medium, Bacto %'ater in making the netessarY &utions. At first and concentration of salt presenf in the culture medium; with nutrientagarwasused. No this Was attributed t o the increasing concentration, the count increased to a maximum, in attempt Was made to differcreation of an environnlent entiate between the comeoery case, and then decreased gradually to zero. more nearly like that in bined and uncombined in which the bacteria had organic components of this originally developed, but further investigation showed it t o medium, but the same sample was used in all experiments. be due to the salt present in the dilution water.2 The salts, previously dissolved in water, were added directly to It waF found that salts may cause either an increase or the medium before making up to final volume and sterilizing. decrease in bacterial count, depending upon kind and con- The importance of controlling the hydrogen-ion concentration centration present in the culture medium, whether added was appreciated, and determinations were made in every directly or carried in by the sample being tested. TI7here case after incubation as well as before. I n no case did the counts are used in the study of the effect of antiseptics upon pH value lie outside of the range 6.90 to 7.10. Microscopic bacterial life in water of variable salt concentration, it is ob- observations were made of many pure cultures prepared viously of great importance to know the extent to which the from samples of soak waters, but the variety was so great salt may vary the count. For this reason, an investigation that detailed descriptions would be out of place here. was made of the effect of the addition of different salts to RESULTS the culture medium upon the bacterial count of water used for soaking salted calfskins, and a few typical results are here The effect of increasing concentration of sodium &loride given. is shown in Fig. 1. All counts are for the same sample of PROCEDURE soak water, the only difference being that of salt concentraAll counts were made by the official method of the American tion in the culture medium. Using the official method, Public Health Association,3 except for deviations herein- with no added salt, the count was 610,000 per cubic centimeafter noted. The soak waters whose counts are given in ter, but with increasing concentration of salt the count rose this paper contained an average of about 0.03 mol of sodium to a nm&~um of ~ ~ , ~a t ~0.05o mol , ~Peroliter ~ of added chloride per liter, but this was reduced to a negligible value salt, and then fell steadily, becoming Practically Zero above by the dilution, before planting, with sterilized distilled water, molar strength. The addition of 0.05 mol of sodium chloride per liter to the medium caused the appearance of eighteen 1 Presented in part under the title "Effect of Salts upon the Bacterial times many in the medium' That Count of Tannery Soak Waters," before the Division of Leather Chemistry a t the 6 6 t h Meeting of the American Chemical Society, Milwaukee, Wis., thls was l l O t due merely to an accelerated growth WaS inSeptember 10 t o 14, 1923,and in full before the New York Section by J A dicated by the fact that incubation for a week, instead of Wilson on January 4, 1924. 48 hours, did not alter any of the counts. Care was also 2 The preliminary work for this study was carried o u t by Guido Daub. exercised to insure against effects of manipulation in planting Standard Methods for the of Water and Sewage, 5th ed , 1913. a large number of aliquots from the same diluted sample of
Iil'DUSTRIAL , 4 5 0 ENGINEERING C H E - I S T R Y
368
1
T'ol. 16, No. 4
24-
12
e 0:1 0:2 0:3 0;4 M o l e s Sodium C h l o r i d e per L i t e r i n Culture l'adium
0.05 l ' o l e s Salt p e r L i t e r i n Culture I'edium
1-EFFECT OF C O N C E N T R A T I O N O F S O D I U M
FIG. Z-EFFECT O F C O N C E N T R A T I O N O F P O T A S -
CHLORIDE UPON THE BACTERIAL COUNT OF WATERUSED FOR SOAXING SALTED CALFSKINS
SIUM HALIDES UPON THE B A C T E R I A L COUNT OF W A T E R USEDFOR SOAKING SALTED CALFSKINS
!?IC.
0.10
0.15
0.20
Wales Salt per L l t e r i n Culture Medium-
FIG.3 - E F F E C T OF C O N C E N T R A T I O N O F S O D I U M S U L F A T E O R C A L C I U M C H L O R I D E UPON T H E BACTERIAL
C O U N T O F W A T E R U S E D ROB SOAKING
SALTED CALFSKINS
soak water, by making alternate plantings in salted and unsalted media. The effect of repeated pipetting from the same sample was a gradual increase in count in the unsalted medium from 610,000 to 960,000-quite negligible compared with the increase to 11,100,000 due to 0.05-molar sodium chloride. Numerous repetitions showed the effect to be general. The curves in Fig. 2 furnish a comparison of the effects of the different halides of potassium upon the bacterial count of another sample of soak water. At concentrations of 0.01 mol per liter, or less, the salts all increase the count, the order of effectiveness being K F > K I >KBr >KCl. Points of maximum occur in all curves, and at higher concentrations the salts all decrease the count, the order of effectiveness being exactly the same as for increasing count a t lower concentrations. Where the initial rise in the curves is steepest, the value of the point of maximum is lowest and occurs a t the lowest concentration of salt. A third sample of soak water was used to determine the effects of sodium sulfate and calcium chloride upon the count. The results are shown in Fig. 3. The general shape of all curves is the same and all seem to show curious points of inflection to the right of the point of maximum.
THEORETICAL It was at first suggested that the effect of the salt was to produce a condition more or less favorable to the growth of certain types of bacteria, but this appeared improbable when the following facts were brought to light and considered collectively: 1-When the experiments were repeated for short portions of the curves, making successive increments of salt very small, it was found that the curves were practically continuous and that each small increment caused a corresponding increase or decrease in count. 2-Increasing the period of incubation caused no change in the number of colonies on any given plate. 3-A violent shaking of the diluted sample immediately before planting in the medium caused a large increase in count. An example of this follows: A sample shaken 15 seconds gave a count of 280,000 per cubic centimeter; 1 minute, 670,000; 2 minutes, 1,220,000; 3 minutes, 1,790,000; 5 minutes, 2,430,000.
The count was increased more than eight times simply by shaking the sample vigorously for 5 minutes before planting.
A more logical explanation is that probably all the bacteria present develop upon incubation, a t least in the more dilute salt solutions, but that differences in count are due to differences in the average number of bacteria (originally present in the sample) responsible for one colony on the Petri dish. The bacteria in the sample would thus be pictured as existing in groups or clusters of many individuals. The effect of a small amount of salt would be to increase the degree of dispersion of the bacteria, which would result in an increased number of colonies on the Petri dish, even though the total number of individual bacteria had not been altered by the salt; still larger amounts of salt would cause agglutination and correspondingly lower counts. Violent shaking causes an increase in count by breaking up the clusters, a t least temporarily, into smaller groups. This view likens the behavior of suspensions of bacteria to that of colloidal dispersions of simple materials, such as metallic gold, in the presence of electrolytes. Loeb4 studied the effect of concentration of different types of ionogens upon the electrical potential difference a t the surface of the particles in various kinds of colloidal dispersions. The effect of concentration of sodium chloride upon a colloidal dispersion of gold, for example, was to increase the negative potential difference to a maximum a t a concentration of about 0.005 mol of sodium chloride per liter, above which the value of the potential difference fell, Korthrop and De K r ~ i f on , ~ the other hand, showed that the electrical potential diffwence a t the surface of bacteria was markedly affected by changing concentration of electrolyte, just as in the case of colloidal dispersions. Loeb found that when the absolute value of the potential difference a t the surface of particles of gold, graphite, collodion, and some other materials, in colloidal dispersion, fell below about 15millivolts, flocculation occurred, while Northrop and De Kruif found a similar critical potential difference, about 15 millivolts, for the agglutination of certain types of bacteria. Still further confirmation of the 4
J. Gem Physiol.. 6, 215 (1923).
8
Ibzd.
4, 639 (1922).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1924
view that bacteria behave towards electrolytes much like ordinary colloidal dispersions is furnished by the work of Winslow, Falk, and Caulfield6 on the electrophoresis of bacteria. Apparently, in both bacterial suspensions and colloidal dispersions there are opposing forces at work tending to regulate the average size of the particles, whether these be groups of bacteria or aggregates of simple molecules. Cohesive forces act to increase the size, while like electrical charges and attraction of the molecules of the particles for water tend to decrease the average size of the particles. Since it has been shown that the potential difference at the surface of bacteria 8
J . Gen. Physiol., 6, 177 (1923).
369
is altered by change in the kind and concentration of salt, other things remaining constant, one would expect the average number of bacteria per group, and therefore the bacterial count, to be a function of the kind and concentration of salt present in the suspension, and this the writers have just shown to be the case. It seems likely that all counts indicated in the curves are lower than the true values, and it may be questioned whether the full count of individual bacteria in a sample is ever obtained by use of the official method. I n making comparative counts, the importance of maintaining constancy of pH value in the culture medium is now generally appreciated, but this work indicates that constancy of composition in other respects is also essential.
The Preparation of Arsenic-Free Reagents’ By George D. Beal and Keith E. Sparks UNIVERSITY OF ILLINOIS, U R B A N A , ILL.
HE chemist about to engage in the examination of toxicological material to determine the presence of metallic poisons is frequently embarrassed by the presence of arsenic in many of the reagents available. Reagent salts may usually be freed from this element by simple processes of precipitation or recrystallization. The purification of certain of the acids involves more difficulty. The very frequent occurrence of arsenic in the sulfur or sulfide ores used in the manufacture of sulfuric acid accounts as well for its presence in hydrochloric acid. Sulfuric acid made by the contact process is more likely to be free from such contamination than, chamber acid, and accordingly a pure reagent acid may be prepared by the dilution of fuming acid, when this is available. Since an oxidizing condition exists during its distillation, nitric acid is generally free from this impurity. While it is possible by careful blank experiments to determine the degree of contamination, reagents so contaminated should never be used when investigating cases of suspected poisoning. Hager2 purified hydrochloric acid by reducing the arsenic to the metallic state with potassium hypophosphite; Habermann3 by aeration in the presence of potassium chlorate, thereby obtaining a dilute acid; Autenrieth4 recommends that dilute hydrochloric acid, 12.5 per cent, be saturated with hydrogen sulfide and kept thus until required, with the disadvantage of having only dilute acid available. Rohmer5 calls attention to the fact that the presence of hydrogen bromide expedites the volatilization of arsenious oxide from a solution when it is distilled in a current of hydrogen chloride and sulfu~dioxide. Ling and Rendlea recommend treatment of the dilute acid with methanol and zinc in the presence of copper foil, volatilizing the arsenic compounds through a reflux cortdenser and finally redistilling the acid. Thorne and Jeffers’ have purified the acid by a modification of the Reinsch test, using a tin-copper couple and causing the evolution of the arsenic in the form of arsine. The arsenic may be removed from sulfuric acid as sulfide by means of sodium thiosulfate. Many manufacturers treat the diluted acid with hydrogen sulfide, afterwards subjecting the product to re-
T
Received December 6, 1923. 2 Chem. Zentr., [3] 45, 98 (1874); Pharm. Zentralhalle, 16, 27 (1874). 3 Z.angezu. Chem., 10, 201 (1897). 4 “Detection of Poisons,” translated by Warren, 4th ed., p. 144. 6 Ber., 34, 33 (1901). 8 .4naZysl, 31, 37 (1906). 7 I b i d . , 31, 101 (1906). 1
concentration. I n both procedures some of the acid is consumed and a great deal of sulfur precipitated.
PURIFICATION OF HYDROCHLORIC ACID After considering the properties of the arsenic compounds, it seemed that hydrochloric acid could be best freed from this metal by distillation with the arsenic in the pentavalent form, in which condition it is least volatile. Accordingly, the acid was saturated with chlorine by, the addition of a small quantity of potassium chlorate and distilled in a current of chlorine gas. This gas was conveniently generated by means of manganese dioxide and commercial hydrochloric acid and allowed t o bubble at a moderate speed through the acidin the distilling flask. The degree of purification was found to depend in part upon the ratio of the volume of distillate to the original acid and also upon the total quantity of arsenic originally present. The quantity of arsenic in the original material wag determined by the Marsh method using comparison tubes, and in some instances known amounts of arsenic trioxide were added to-the acid to be purified. The quality of the distillate was determined by the Marsh test. The results of a typical series of distillations are given. TABLE I-PURIFICATION OR HYDROCHLORIC ACID Sample 1 2 3 4
5 6
Acid Cc. 500 500 200 190
AS203
Present Grams 0.001
“1% Distillate
No. 3 2.000 Distillate No. 5
200
190
Distillate cc. 495
“1:
Rate of Distillation Rapid Rapid Moderate
180 190
Moderate Moderate
Negative Positive
180
Moderate
Negative
Arsenic in Distillate Positive Negative Positive
TABLE 11-PURIFICATION OR SULFURIC ACID
Sample
Acid Cc.
As203 Added Grams
Time of Distillation Minutes
Arsenic in Residual Acid
Note.-Sample 3 showed no arsenic a t the end of 1 hour’s heating. Samples 4 and 5 contained a trace of arsenic at that time, but 15 minutes additional was sufficient for its complete removal.
PURIFICATION OF SULFURIC ACID Since arsenic trichloride is readily volatile in the presence of sufficient hydrochloric acid to prevent its hydrolysis, the arsenic can be readily removed from sulfuric acid by passing
