Improved Apparatus for Solubility Determination or for Small-Scale Recrystallization L Y M A N C. C R A I G AND OTTO W. POST Rockefeller Institute for Medical Research, N e w York, N. Y.
s
.i -light niodific~atiouin thtb tl(4gri adapts the apparatw for uae with much larger amounts of material. This iR done hy t m larging tht. crystallization part of R by hloiving it larger from the same test tub(%,a s drown in Figure, 2, or otherwise making a very
f ; \ - b X A U a types of apparatus u.eeful in small-arale recrystallization, where centrifuge filtration is employed, were drwribtvl in a previous publication ( 1 ) . Since then, a number of niodific-atione have been made in the design and, in view of tht, wide potvntial use of such devictbs, a tlesrription of the improvenwnts ~vouldappear advisable. For routine fractional recrystallization, the apparatus shown in Figure 1 is easily constructod, and i,* c~onsidt~rahly superior to the onrs previoiisly drsrribed.
thin-n-alled bulb. The hull) or crystallizat,ion part in this case muiit be very thin, so as not to increase the total weight of the crystallization vessel and the filter stirk much beyond 6 grams'. Bulhs holding from 15 to 20 nil. have hvcsn constructed from this weight of glass and sue( fully used by starting the centrifuge a little more Slonly. This volumc permits rwrystallization from 10 to 15 ml. of solvent. For small-acale work, & t c w t l filters art. unqut4onably useful, both for suction anti centrifugts filtration, but are not always easy to (.lean, and this gives rise to a reasonable distrust of their reliability. They also retain appreciable amounts of mothcr liquor, and completc removal of the crystalh from the rough surface without wratching off glass particles a t thr same time is gencrally clificdt. ;in attempt to dcviie an all-glass filtration apparatus frcbe from thcsr objections has mcxt with surcess in the filter Y1ion.n in Figure 3, which is an iniprovcnient O V C ' ~thc, one previously described ( 1 ) (Fipurc, 2 ) .
.-I ii :in ordinary test tub, 100 111111. long :ind 12 nun. witits. I1 i* in:idc, from R rrnaller, thin-\\allrd ttlst tube 70 or 80 mm. lorig, appioxim:itvly 8 mm. \vide, and nrighillg hctrvcen 2 and 3 grams. The uppt*r half is Ividened by heating in the flame to just the softening point and slowly pushing it over a hot carbon rod a littlr over X mm. in diameter. By steadily turning the tube on uniform shouldvl, having a n angle of about 45' can iinothrxi,tcst tube of the same sizc \ d l t l l t , I l just oiigh t h r enlarged part. C is m:-ide est tuhc 8 mni. in diarnetrr \vith :I uniformly round bottom at :I poin from the end and alloiring the gl When enoueh solid elass has collected a t this point, it r:& be dra& out to form a solid rod 2 to 2..5 nini. in dianietcr, as shoivn in Figure 1, and :ipproximatrly 60 mni. long. The lower end of the rod can be enlarged R' hhorvn, by the usc of a flat carbon n-hile the tip is molten. This gives the rod morc strength so that it can better n-itlist:rnd tht, pressure during centrifuging. Thc, cornhined \wight of B and C should ht of the ordw of 4 to 6 grams. D is a larger teqt tube which ia f i t t c d ii-ith a rubhcar stopper and hacotton on the bottom at E. I n :ictu:il practice, t,he niatcrial to be recrystaC liztd is plnrrd in the modified test tube, R , and solution i-: :ichicved with a minimum volume of hot d v e n t . The apparatus is then assembled ill a po~itiontlrc woerse of that shown in Figurrl 1, and coolcd t o the optimum temp -A tallization. After crystallization, the position zhoivn and centrifuged. fA speed of D approximiitdy 1300 in a S o . 2 International centrifuge ha. been found to give clear-cut filtration without bt,c.akage of the equipment .) E The crystals are caiight, a t the point of enlargcment of t w t tube R , since therc is just enough Flg 1 Fig clearancc b r t r n e ~C~and the shoulder of the enlargemc~rit of B to allon. thcl liquid hut not the crystals to p:iis through. For fine crystals, C may be seatrd :i little more accurately on the shoulder by grinding with rough Carborundum, as described below. After filtration, the inner test tube, A , is first removed and held at an angle of about 45'. B and C can then be removed as one piece by grasping the exposed end of R , since in this position C will bind in t h e largrr part of' R and will not fall out.
It is ronvenient to have B and C tared together, so that the weight lost during rryqtallization, the wet weight, etc., can he obtained. After filtration, C can serve a. a stopper t o prevcsnt, the rntrance of rxtraneous material during storage or during further crystallizations or manipulations. \$-hen properly seated by grinding, thc unit is entirely adaptable to solubility microdeterminatioiis, a s rarried out by Inp and Bergmann ( 2 ) and more riwntly in a greatly improved x a y by Moorrb and dtt,in (.3).
€19 3
Fig
413
4
*
A is a gla,?s funnel of the shape indicated. Tlit, iiarrow part of the funnel is made from glass tubjng approximately 5 mm. in inside diameter, niitl i h approxiniately I5 mm. long. R is madr, by stamping out a molten glass rod or tubing with the appropriate carbon surFRce, so that it is l i p proximately the shape shown and then grinding it to fit more accurately. The narrow part of funnel .1 acts as a sleeve to hold C in position, -0 th:it tho tLvo surfaces at C may be ground \\-ith 120-mesh Carborundum just to the point whc-rt. tht. funnel, when in position, will complete1.v r('illlJVffectfiltration may bt> taken apart for cleaning. For solubility determinations, the authors have modified the excellent apparatus of Moore and Stein (3),as showi in Figure 4, for use T T - i t h thi.? type of filter.
Tube -4is exactly as described (3),except that a ground-glass stopper is substituted for rubber. The stopper is hollow and has a wide, flat top, so that the inverted tube will stand on the stopper during the necessary manipulation. TKO small indentations are on opposite edges of the flattened top for rubber bands t o hold tube in the equilibrating apparatus. The inside surface of the stopper has a button on it to hold the flexiblts Fvire
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
414
spring, E , which in turn holds the filtration apparatus and filter stick in place. Flask B is of very thin glass, for its rests against the filter stick to hold it in place and a heavy flask would break during centrifugation. Filter C is as described above, except that the lower part is as short as possible. I t rests on a soft tin collar,
D.
Vsing this assembly, accurate solubilitr dptcrminwtionq may
Vol. 16, No. 6
be made with organic solvents or corrosive solvents, since no rubber is present to absorb any portion of the solvent. LITERATURE CITED
(1) Craig, L. C.. IND.ENG.CREM., ANAL.ED., 12, 773 (1940). (2) Ing, H. R., and Bergmann, M., J. Bid. Chem., 129, 603 (1939). (3) Moore. S.. and Stein, U’.H., Ibid., 150, 113 (1943).
Semimicrodetermination of Arsenic in Insecticides MARK D. SNYDER AND WALLACE M. McNABB Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pa.
T
HE procedure presented below represents an extension to
insecticidal arsenicals of the method recently described (4) for the semimicrodetermination of arsenic in organic compounds, the arsenic being precipitated as element by action of hypophosphorous acid and determined iodometrically with the aid of Koppeschaar’s bromide-bromate solution. This method is believed to be applicable to any properly prepared solution of arsenic free from interfering substances, such as organic material or metals precipitatable by hypophosphorous acid. It was found that arid-soluble arsenicals (Paris green, lead arsenate, calcium arsenate) could be analyzed thus, following dissolution of the samples in aqueous hydrochloric acid, I n presence of organic material, a preliminary decomposition similar to that described for the analysis of organic arsenicals (2, 4) may be necessary. This was the case with the commercial insecticide currently marketed under the name “Victory 76”, stated to contain calcium arsenate together with sulfpr, nicotine, organic compounds with carbon contents from CIOto Cia, and inert material. Decomposition by nitric and sulfuric acids (4) was shown to be a suitable preparation for the determination of arsenic in this insecticide. An alternative decomposition by bromine was found to be more rapid and to lead to acceptable results, but is judged to be less satisfactory because the decomposition liquid contained suspended dark-colored material, prewmably organic bromination products, the presence of which interfered visually a t the time the arsenic was reduced and precipitated. PROCEDURES
SUBSTANCES SOLUBLEIN HYDROCHLORIC ACID, Dissolve a weighed sample (0.5 to 2 grams) of dried material in a minimal volume of 6 N to 12 N hydrochloric acid, transfer the solution to a 500 ml. volumetric flask, and dilute to the mark. Transfer an aliquot portion containing about 15 mg. of arsenic to the flask of an all-glass decomposition apparatus with reflux tube, such as that described for use in the determination of arsenic or mercury in organic compounds (9, 4). If a sufficiently sensitive balance is available-e.g., a semimicrobalance-weigh out the whole sample, of such size as t o contain about 15 mg. of arsenic, and dissolve in hydrochloric acid. To the solution in the flask add and dissolve rapidly 3 grams of sodium hypophosphite (XaHzPO2.HzO), and then add concentrated hydrochloric acid sufficient to increase the acid concentration to about 6 N . Attach the condenser and heat the flask with a small flame, completing the analy*is as described (4). ARSENICALMIXTURESCONTAINISG ORGAXICMATTER. Decomposiiion by il‘itric and Sulfuric Acids (recommended procedure). Weigh accurately a sample of suitable size (to contain about 15 mg. of arsenic; 0.5 gram of Victory 76) and transfer to the decomposition flask. Add 25 ml. of concentrated nitric acid and warm t,he mixture for several minutes, Add 20 ml. of concentrated sulfuric acid, evaporate the mixture to fumes, then add more nitric acid and again evaporate to fumes. Allow the liquid to cool partially and introduce 1 gram of ammonium sulfate. When evolution of gas ceases, heat the liquid gently for 5 minutes. Cool, add about 50 ml. of water, and heat until the solution (7learsor is slightly opalescent. Add 3.5 ml. of concent,rated hydro-
rhloric acid, then 3 grams of sodium hypophospite, and continue as described (4). Decomposition of Victory 76 by Bromine. Tranqfer weighed sample to the decomposition flask, add 2 ml. of liquid bromine. and swirl the mixture for about 5 minutes. Add 50 ml. of 6 AV hydrochloric acid and heat moderately until nearly all the excess bromine is expelled (hood). To the cooled solution add 10 ml. of concentrated hydrochloric acid and 3 grams of sodium hvpophosphite, and complete the analysis as indicated above. RESULTS
Analytical iesults for the four materials mentioned are presented in Table I, which includes also comparative results obtained by the familiar distillation procedure (1) selected as an umpire method. DISCTJSSIOK. Results by the reduction method show satisfactory levels of precision and accuracy, and are substantially identical with results by the distillation method. The reduction procedure is the more rapid, requiring about 40 minutes (exclusive of any needed preliminary decomposition), as compared 6 t h the 2 to 3 hours required for the distillation procedure.
Table
I. Determination of Arsenic i n Some Insecticides
Material
Araenic Found Reduction method Distillation method
%
%
Paris green
42.81 42.77 Av. 4 2 . 7 9
Lead arsenate
20.25 20.18 20.15 Av. 2 0 . 1 9 26.65 26.72 26.73 Av. 2 6 . 7 0
42.75 42.72 42.57 Av. 4 2 . 6 8 20.23 20.27 20.30 Av. 2 0 . 2 7 26.84 26.77 Av. 2 6 . 8 1
Calcium amenate
HNOs-H:SOr Bromine decomposition decomposition 3.57 3.64 3.50 3.55 3.60 3.60 Av. 3 . 5 6 3.59 Prelimmar> decomposition by HNOrHaSO4.
Victory 76 insertiriae
0
3.62“ 3.57” Av. 3 . 6 0
ACKNOWLEDGMENT
Grateful acknowledgment is made to J. J. McGlynn, who executed a series of confirmatory analyses by the reduction and distillation methods. LITERATURE CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analygis, 5th ed., pp. 44-5, 1940. (2) Levine and McNabb, IND.ENG.CHEM., ANAL.ED.,15, 76 (1943). (3) Sloviter. McNabb, and Wagner, Ibid., 13, 890 (1941). (4) h2.. 14. 516 (1942).
