April, 1922
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE VII-EFFECT
OF VARIATION OF CONCENTRATION OF SODIUM CHLORAT& IN ELECTROLYSIS BATH Bath number 16 17 18 Concentration of NaClOa, per cent. 4 0.05 Concentration of AszOs, per cent.. o,05 2 o1 , 0 5 Current density, amps./dm.z.. ...... 1.25 I . 25 I . 25 Phenomena.. ..................... Little lead No lead No lead a t cathode Character of precipitate.. Least finely Finely More finely divided divided divided Voltage.. 4 4 4 8.5 5.5 3 Distance between electrodes, cm.. Yield, grams.. 5.926 6.32 6.371 Percent lead 65.6 60.95 60.4 Current efficiency, per cent.. 98.3 97.4 97.2
.....................
...
.......... ........................ ... .................... ..................... .......
,
no lead was deposited at the cathode, although the acidity was allowed to run up as high as 0.2 per cent at times. It was also observed that violent stirring of the electrolyte cau’sed the precipitate to cake on the electrode, while- as long as the bath was allowed to remain quiet it formed in flakes and fell freely to the bottom of the tank. If, however, the electrolyte was not circulated to some extent, the concentration of arsenic acid became depleted between the electrodes, the bath became alkaline to phenolphthalein a t this Point, and lead Was deposited at the cathode. To obtain a slow circulation of the electrolyte, air was slowly bubbled in at one corner of the tank. The I’esults Presented above indicate that the best Conditions are as follows:
................... ................... ............................. .........................
concentration of sodium chlorate per cent Concentration of arsenic acid, pe; cent.. Current density, amp./dm.z. Distance between electrodes, cm.. Electrolyte slowly circulated.
1 to 2 0.05 1.875 2 . 5 to 5 . 0
313
I n order t o confirm the above results three baths were made up of the above concentrations and run according to the best conditions as determined in the preceding experiments. The results are tabulated in Table VIII. TABLE VIII-RESULTS
OF ELECTROLYSES WITH BEST CONDITIONS FOUND Bath number.. 20 21 22 2 2 Concentration NaC108, per cent. 2 0.05 Concentration AszOs, per cent.. 0.05 0.05 Current density, amps./dm.*. .......... 1.875 1.875 1.875 2.5 Electrode distance, cm., 2.5 2.5 3.0 3.0 Voltaee 3.0 6.4038 6.3946 Yield, grams 6.3655 61.9 Per cent lead.. ...................... 61.85 62.55 98.4 Current efficiency, per cent.. . . . . . . . . . . 98.6 98.9 Gain on coulometer cathode, grams.. . . . . . . . . . . . . 1.2374 Total coulombs.. ..................... 3760 Time (sed 4800
...................... ....... ........ .............. ............................. .........................
............. ......... ............ ............ ~ ~ y ~ . ~ ~ ~ n . t ;: ; :, :(: :t :m : : : ~: :. :.: : :::; : : ;:;:: : : : : : : : : : ::$& Kw. hrs. per lb. lead arsenate.. ............................. 0.222 ............................
In addition to the above experiments and in order to determine the effect of allowing the precipitate to stand in contact with the electrolyte the following experimenta,as conducted: A sample of lead arsenate from R~~ 11 containing 65.05 per cent lead was placed in a 0.05 per cent arsenic acid solution and shaken up at intervals for days. It was then iiltered off and analyzed for lead, when it was found to be 59.4 per cent lead (very neap to the theory for lead hydrogen arsenate). This would indicate that by allowing the precipitate to stand for several days in contact with the bath solutions a product could be obtained containing practically pure lead hydrogen arsenate.
The Physical Properties of Commercial Arsenates of Lead’ By R. H. Robinson2 CHEMICAL DEPARTMENT, OREGON AGRICULTURAL EXPERIMENT STATION, CORVALLIS, OREGON
Chemical analysis shows very little variation in the brands of lead arsenate now on the market. Work on twelve commercial brands, which forms the basis of the following paper, shows marked differences in certain physical properties which may afect the eficiency of the spray. Speci-fic gravity varies only slightly, and there is apparently no correlation between this d u e and dry colume. Appreciable differences are observed in the site of particles, in the proportion of large to small particles, and in suspension qualities. I n order to give lead arsenate better spreading properties, it is the custom of orchardmen to add spreaders such as soap solution, oil emulsions, etc. These cannot be used under all climatic conditions, and seoeral commercial arsenates contain harmless organic substances as “deflocculents.” Their function is chiefly to penetrate
and subdivide the clusters of fine particles of arsenate, causing them to remain in suspension for a long time and increasing their spreading properties and also, probably. their adhering properties. I n the course of the experiments herein reported similar deflocculating properties were produced in other commercial arsenates by the addition of small amounts of calcium caseinate, glue, soap solution, tannin, certain sugars, gum arabic, oil emulsions, and dextrin. The amount of lead arsenate, which will remain in suspension in water for a specified length of time, is perhaps the best indication of its eficiency. The practical eficiency of any deflocculent which may be added to a commercial lead arsenate can best be determined by results obtained in jield practice, where only ordinary care is exercised in the application of the spray.
INCE the time that arsenicals were first employed as insecticides in the control of the codling moth, chemical investigations have been carried on in order to learn definitely regarding the composition of the various arsenates of lead and to choose those forms which will be most efficient. A desirable commercial product must be high in total arsenic for optimum killing efficiency, low in water-soluble arsenic in order that foliage burn may not occur, and free from foreign impurities. Investigations to this end have been successful and two forms of lead arsenate are now on the market, namely, the acid or hydrogen arsenate and the “neutral” or basic arsenate. It has been found by experimentation that the killing efficiency of the former is superior to the latter; consequently more than 95 per cent
of the lead arsenate on the market is the lead hydrogen arsenate form. Chemical analyses of samples of lead arsenate submitted to the chemical department of the Oregon Experiment Station during the past several years have shown that manufacturers are producing a very high-grade product, free from impurities, and lotv in water-soluble arsenic. Representative samples were analyzed in 1920. Table I gives the results obtained. The composition of the arsenates indicates only a slight variation between the different brands. Furthermore, this variation is not due to the presence of impurities. All the samples reported are of the lead hydrogen arsenate form, containing small amounts of the neutral lead arsenate, which accounts mainly for the slight variation in composition of the different brands. The low water-soluble arsenic content
S
1
Received September 3, 1921. Chemist, Oregon Agricultural Experiment Station.
* Associate
T l f E JOliRiVAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
314
T ~ LI-Z-ANALYSES OF COMMSRCI~L BRANDS oP L I n o BUND
A
B
C
D
8G
n
i
K
L
Total PbO 63.82
63.96 63.19 64.14 64.41 63.87 65.71
64.86 63.93 63.74 64.41 63.74
TOW ArrOs 31.80
ARS~NITB
Water-Soluble
32.53 31.64 32.19 31.27 32.36
30.46
30.75 32.07 32.94 31.84 31.04
As06
0.06
Trace 0.04 Tr*Ce 0.06 Trace Trace 0.12 0.17 0.07 Trace 0.16
lead arsenate. Care was taken to remove occluded air by placing the pycnometer in an exhausted desiccator. It has been observed that some brands of lead arsenate are light and fluffy and occupy a comparatively large volume, while others are compact and granular. To correlate the apparent dry volume wit,h other data, gram portions were weighed out and transferred to padusted oil tubes of the type shown in Fig. 1. As the lower part of these tubes tapcr, the volume occupied may he read accurately to 0.1 oc. The tubes were tapped gently until the arsenates ceased to settle and the volumes recorded. ?Table I1 gives 6he specific gravity a t 20' C. referred to water at 4" C. and the dry volume for the twelve brapds under ohservation.
A B C
n E
F
G
n I
I , L Fro. l - N ~ r u n a ~SETTLING PROPBRT~ES OP SOMBCOMMBXCIAL
LEADAXSBNAIES
of all samples further emphasizes the high degree of purity of the commercial brands that are now found on the market. We can conclude, therefore, from these and numerous other analyses, that from the standpoint of their chemical composition all brands of lead arsenate are safe to use and equally efficient results are to be expected from any of them. Any apparent differences in brands of lead arsenate now on t.he market may, therefore, be ascribed to their physical properties. A high-grade arsenical that will give best control of insect pests should not only contain a certain proportion of arsenic, but should also be in a very fine sta,te of subdivision so that it will remain suspended in water for a reasonable length of time and, when sprayed, will adhere well and form a thin, uniform fdm of the poison on the surface to he covered. Superficial examination of the various commercial brands indicates that they differ widely in their physicnl properties. Some are finely ground, fluffy and bulky; others are coarse-grained and compact. Again we find some brands which contain a fraction of a per cent of an organic substance which tends to hold the arsenate suspended in water for a long period of time. For the sake of convenience, we shall refer to this substance as a "deflocculent" or "spreader," since it functions chiefly by ,causing deflocculation of the particles when suspended in water and manufacturers claim that better spreading of the arsenical is obtained when this material is added. The efficiency of the spray material is no doubt enhanced by the addition of a deflocculent, provided i t adheres better and causes the drops of spray to flatten ant and spread over a greater area. In order to ascertain whether these differences are of such magnitude that one brand may be classified as superior to another, a study has been made of the physical properties of twelve brands collected in the market in 1920.
SPECIFIC GRAVITX AND DRYVOLUME The specific gravity of a commercial lead arsenate may reflect a favorable or an unfavorable physical condition for spraying purposes. It was determined by ascertaining the weight of toluene displaced by a known quantity of the
Val. 14, S o . 4
5.69 5.87 5.69 5.91 5.82 5.88 6.01
5.91 5.02 5.80 5.91 6.84
4.3 2 5 1.9 3.4 2.6 3.0 3.2 3.4 1.8 2.5 1.9 2.3
The above results indicate only slight differences in the specific gravity of the various commercial lead arsenates. Apparently there is no correlation between the specific gravity and the dry volume. Some brands occupy more than twice the dry volume of certain other brands.
SXTTLINO TESTS The length of t,ime an arsenical will remain suspended in water is an indication to a limited extent of the size of the integral particles. Thelarger granules will settleimmediately while the fine, flour-like particles will remain suspended in water for a longer period of t,ime. A lead arsenate containing fine, flour-like particles is, of course, more desirable. In order to judge clearly between brands, distinction must be made between a lead arsenate that consists mainly of very fine particles and one that contains a high percentage of coarse particles together with a small amount of very fine particles that will remain in suspension a long time and render invisible the larger particles that settle immediately. To determine the relative amounts of fine particles of the various commercial brands of lead arsenates which remained in suspension during a definite length of time, oil tubes sontaining 100 cc. of water and 1 g. of the lead arsenate were prepared. The tubes were aaitated thoroughly and photographs were taken after settling for 5 min. Since all the brands which contained deflocculents were similar and those which did not cont.ain deflocculents were also similar, only one photograph oi a few of the contrasting brands is reproduced io Fig. 1. It will be observed that Brands I and E, which contained deflocculents, remained in suspension while B and G settled. The photographs (not reproduced here) of Brands A, C , D, H, J, and K were similar to B and G. Brands F and L (not reproduced here), both of which contain deflocculents, were similar to E and I. Chemical tests showed that t,hose brands which remain in suspension for a long period of time contained deflocculents. These deflocculents were identified in three of the brands and were found to he harmless as an ingredient of a spray material. Since these deflocculentsseem to penetrate the lead arsenate aggregates and cause them to subdivide instead of clinging together in clusters that settle rapidly, certain physical properties are no doubt improved by the addition of such materials.
April, 1922
T H E JOURNAL OF I N D U S T R I A L A N D ENGIhTEERING CHEMISTRY
e oil tube-marked S in Fig. 1 exemplifies similar susharacteristics that may be prosenate by the addition of certain contains Brand G in water SUI-
315
TABLEIII-SETTI,INC TGSTSO N LEADARSENA? 'E I METHOD METHOD I1 BRAND In Suspension Settled In Suspension Settled A 98.22 2.4 97.6 96.37 96.8 4.2 . -B C98.16 2.2 97.8 97.50 1.7 98.8 9.25 90 2 9.8 28.38 71.5 28.5 91.04 9.6 90.4 H 95.15 5.0 95.0 I 23.40 72.2 27.8 97.36 1.7 98.3 97.15 2.6 97.4 L 87.16 12.7 87.3
!i .%"*
gum arabic, oil emulsions, and dextrin act as deflocculents when used in certain amounts and probably are as good as those added to the four commercial brands. However, it is impractical for the consumer to use some of them, and others react chemically with lead arsenate and would cause burning of foliage; hence, it is not advisable to recommend definitely at this time how they may be employed. Fig. 1 and the photographs of the other brands not reproduced here do not show some particular points that are distinctive of each brand. A low percentage of very fine particles may remain in suspension and tend to obscure the actual physical condition. Close examination during the settling process warrants the following remarks regarding the samples under observation: The small particles of Brands E and I, which contain deflocculents, were held in suspension while the larger granules settled. The former had a much higher percentage of the coarse particles than the latter. Brands A, C, and D contained a low percentage of coarse particles, the relative amounts increasing in the order named. Brand B was very uniform and finely subdivided; none of the larger granules that were plainly visible in most of the other brands could be detected, Brand G was peculiar in that it contained a comparatively high percentage of very coarse sand-like granules while it also showed a large amount of very fine flour-like particles. Brands K and L showed a very high percentage of coarse particles, although the latter contained a deflocculent that held the smaller particles in suspension. Brands F, H, and J contained a medium amount of coarse granules, while the remainder apparently were fine and uniform in size. The photographs show that one brand of lead arsenate will rcmain in suspension in water longer than another, but only the approximate amount can be judged. In order to show the actual amounts that remain in suspension after 5 min., two methods were employed: I-The glass tubes shown in the photograph, containing 1 g. of the arsenate and 100 cc. of water, were thoroughly shaken and allowed to stand 5 min. The particles which remained in suspension were siphoned off with the supernatant liquid, evaporated to dryness at 100" C., and weighed. The amount that settled was calculated by difference. 11-The other method was more rapid but not so accurate. A 1-g. sample was introduced into the graduated oil tube containing 100 cc. of water. After shaking thoroughly the tube was centrifuged for 1min. at 1200r. p. m. and the volume of the settled arsenate was noted. The tube was again shaken, allowed to stand 5 min., and the supernatant liquid and particles in suspension siphoned off. The tube was again centrifuged and a second reading taken of the settled arsenate. From the two readings the percentages of lead arsenate which settled and remained in suspension were calculated. Table I11 gives the results obtained for the twelve brands. The results indicate a perceptible difference in the various brands of lead arsenate. Since only the very fine particles remain in suspension, the percentages reflect to a certain extent the relative amounts of large and small particles. The four brands containing a deflocculent naturally show the highest amounts that remain in suspension. The other brands did not vary greatly in the amounts that settled and remained in suspension. It is interesting to note that Brand G, which showed in the settling tests the prerence of a larger
H
45
46 35
36
h
25
2
