VOL. 10, KO. 3
INDUSTRIAL AND EKGINEERING CHEMISTRY
134
excess of hydrogen chloride was removed by triple evaporation with alcohol. The residue was treated with four successive 50-ml. portions of ammonium hydroxide to dissolve the organic matter. This treatment excluded the sodium sulfate. A good deal of the sodium chloride crystallized out during concentration of the ammoniacal solution. The sulfuric acid treatment of the lead precipitate had no doubt hydrolyzed the gum found in fraction B to alcoholsoluble products. These treated with nitric acid yielded saccharic acid. The bronm substance, CZ,was apparently identical with da and BS. I n all, about 3 grams of this material were obtained. It was found to contain 60.4 per cent of carbon and 5.28 per cent of hydrogen. It decomposed without melting, darkened upon exposure to air or to acid vapors, and was only slightly soluble in cold water but readily soluble in alkaline solutions and in alcohol. Its alcoholic solutions imparted a green color to ferric chloride. A test tube distillation with zinc dust a t 330" C. yielded a brown substance with the odor of anthracene. ,411 this behavior is consistent with the conjecture that the brown substance is a phlobaphene (1, 7 , 9 ) .
fractions A, B, and C. Fraction A yielded malic and citric acids and an oily liquid yielding a 2,4-dinitrophenylhydrazone of m. p. 95" C. A carbohydrate gum was detected in fraction B and its hydrolysis product in fraction C. Phlobatannins were indicated in all three fractions.
Literature Cited -illen, A. H., "Commercial Organic Analysis," 5th ed., Vol. 5, pp. 13 ff., London, J. & A. Churchill, 1927. Assoc. Official Agr. Chem., Methods of Analysis, 3rd ed., p. 393 (1930). Findlay, G. H., and Snell, J. F., Can. J . Research, B13, 269-75 (1935).
Fowler, D. E., and Snell, J. F., ISD. EXG.CHEU.,bnal. Ed., 1, 12-13
(1929).
Heide, C. van der, and Steiner, H., Z . LTntersuch. Nahr. Genussrn., 17, 291 (1909). Nelson, E. K., J . Am. Chem. Soc., 50, 2006-8 (1928). Nierenstein, bl., and Webster, C. A , , Ber., 41, 80 (1908). Price, P. H., -4naZyst, 49, 25-9 (1924). Rosenthaler, L., "Chemical Investigation of Plants," London, C. Bell 8: Sons, Ltd., 1930. Schmalfuss, H., and Keitel, K., Z . physiol. Chem., 138, 1 6 6 6 3 (1923).
Snell, J. F., J. ISD.ESG. CHEX.,8, 144-8 (1916).
Summary
RECEIVED December 31. 1937. Contribution from the Chemistry Depart-
By treatment with hydrogen sulfide, cold and hot, and with dilute sulfuric acid, the precipitate produced from maple sirup by treatment with basic lead acetate 11-as separated into
ment of 1Iacdonald College, 11cGill University. RIacdonald College Journal Series KO.91. Based on the 3 1 . 9 ~ .thesis of Mr. Puddington, entitled "A Study of the Carbonaceous Matter of Maple Syrup K h i c h Is Precipitated by Basic Lead Acetate," presented t o 1IcGill Vniversity, March, 1936.
Rapid Colorimetric Determination of Lead in Maple Sirup J. L. PERLAIAN State Food Laboratory, Department of Agriculture and lIarkets, Albany, N. Y.
T
HE colorimetric method described has been developed primarily to facilitate the analysis of sirups for lead content during the short harvest season and to provide a rapid sorting test which could be employed in field stations. Results obtained during preliminary work indicate a n accuracy to within 0.001 grain per pound (0.143 p. p. m.) and good adaptability to routine laboratory work. Checking by a longer and more accurate method (8) would seem necessary only in isolated cases. The method is similar to the procedure (1) employed for several years to estimate spray-residue lead on fruit, particularly apples. I t s accuracy depends largely upon the ability of the analyst to distinguish small differences in color tints. With a fair amount of practice in matching the colors involved, little difficulty should be experienced in obtaining good results. Principles The index of the lead content of the sirup is the amount of red lead-dithizone complex produced when a solution of diphenylthiocarbazone (dithixone) in chloroform is shaken with the ammoniacal sample solution. The dithizone solution, normally green in color, gi\-eq color changes dependent upon the proportion of the reagent Tvhich is changed to the red complex (PbD2) by the lead present. Increasing amounts of lead produce changes from the original green through a series of intermediate blues and purples to a cherry red when all the dithizone is in combination. These colors are readily matched against colors produced from known amount. of lead in solution under identical conditions.
Reagents A11 reagents should be as free from lead as possible. 1. HYDROCHLORIC ACID, approximately 35 per cent HC1. Dilute 180 cc. t o 1 liter with dist'illed water. 2 . AMMOSIA-CYANIDE-CITRATE REAGEST.Dissolve 20 grams of potassium cyanide and 10 grams of citric acid in 500 cc. of ammonium hydroxide (approximately 28 per cent SH3) and dilute to 1 liter. Preserve in a manner to minimize loss of KHa. 3. DIPHENYLTHIOCARBAZONE (DITHIZONE).This reagent must be purified according to the A. 0. A. C. methods or equivalent. Dissolve 30 mg. in 1 liter of chloroform (a) and dilute a portion of this solution with an equal volume of chloroform to give a solution containing 15 mg. per liter ( b ) . These solutions should be kept in a cool dark place when not in use. 4. STANDARD LEADSOLCTIOS. Pure lead nitrate, twice recrystallized and dried to constant weight at 100" to 110" C., should be used. A stock solution containing 10 mg. per cc. of lead in about 0.1 per cent nitric acid can be prepared and further dilutions made as needed. Lead tends to precipitate (probably as silicate) from very dilute solutions in the absence of acid. Prepare the standard solution in hydrochloric acid (reagent 1) to contain 3.864 mg. per liter of lead.
Standards Introduce into each of ten 50-cc. tall-form Sessler tubes the quantities of blank acid (reagent 1) and standard lead solution (reagent 4) indicated in Table I. Since the total quantity of each solution used is 50 cc., the buret readings are given for convenience. Add to each tube 10 cc. of ammonia-cyanide-citrate reagent and 10 cc. of dithiaone solution (3b) accurately measured.
MARCH 15, 1938
ANALYTICAL EDITION
TABLEI. Lead Grain/ 1b.a 0.000 0.003 0.006
0.009 0 012 0 015 0 018 0.021 0 024 0.027 0 143 p. p. m.
PREP.4RATlOX OF ST.4XDARDs
Standard rsed
.
CC 0.0 1.1 2.2 3.4
6 7 7.8 8 9 10 0
Buret Reading Cr. 0.0 1.1 3 3
(i 7 11 1 iG 0
Blank Used CC.
10.0 8 9 7 s 6 6 5 0 4 3
23 3 31.1 40.0
50 0
I 1 0 0
Buret Reading
CC .
10.0 18 26., 33.3 38 9 43.4 46 7 4S.9 30 0
;
0.0
Shake the tubes vigorously (35 to 50 times) and alloJv the layers to separate. The standard tubes should be kept in a dark place n-hen not in use anti rhonld be freshly prepared daily. Determination
the method outlined, indicating that any tin take-up was noninterfering. Maple products (3) may contain appreciable quantities of zinc derived mainly from the galvanized equipment. The use of hydrochloric acid in the cold and the double dithizone extraction procedure of the method successfully eliminates interference from this metal. The former use of nitric acid and heat in the preliminary treatment (if the sample ( 2 ) produced n-ith the sugar present active groups which combined with the cyanide, rendering it ineffettiv in preventing the formation of zinc dithizone complexes. Lead recoveries in the presence of zinc normally in +up anti recoveries of known amounts of lead added t o standard sirups are qhown in Table 11. TABLE11. RECOVERT OF Sample YO.
8A 8B
Weigh 15 grams of sirup into a clean 100- to 125-cc. (4-ounce) oil sample bottle or other tube suitable for centrifuging. add 15 cc. of acid reagent 1, and mix n-ell. Add 15 t o 25 cc. of x a t e r , 15 cc. of ammonia-cyanide-cit reagent, and exactly 15 cc. of strong dithizone solution ( times) and vhirl in a centrif exactly 11 cc. of the dithizone separator?. funnel containing 11 cc. of the acid reagciit. To remove the chloroform-dithizone mixture, place the finger on the pipet before imniersion in the liquid arid allow the tip to come in contact TT-ith the bottom of the tube before removing the finger. D r a x the mixture above the 11-cc. mark, rcniove the tube, and wipe the pipet tip x i t h a clean cloth before adjusting t o t Shake the funnel vigorously (200 times), rrleasing the several times, alloiy the layers to separate cleanly, and the chloroform-dithizone mixture from ivhich the chloro be reclaimed later. Pipet 10 cc. of the aqueous acid layer into a Sesder tube, add 10 cc. each of ammonia-cyanide-citrate weak dithizone (3b) solutions, stopper, and shake vigoroL A4110i~the layers to separate and compare the dithizone colors I the standards in a comparator block. When the sample contains more than 0.027 grain per pound of lead, use a 5-cc. aliquot a-ith 5 cc. of blank acid and multiply the result by 2 . If more than 0.050 grain per pound ip prrsent, it is advisable to use 30 cc. of strong dithizone as in the initial extraction and a 5-cc. aliquot as above and multiply the results by 4. For amounts of more than 0.100 grain per pound usc a smaller init,ial sample. Make a blank detprmination for the reagents in the a h o w manner, substituting 1.5 cc. of x a t c r for the sirup. Make color comparisons n-ith a tube of clear chloroform h c k ing the sample tube and titbcs of chloroform xhich have h e n shaken n-ith blank acid anti 3mnionia-c!-anide-citrate solutions backing two standard tuhes on both iidri of the sample tube. Tse a comparator blc~cka l h i - i n g the rnininirun spacc. lii.t\nxn tubes v i t h a uniforni light source. The use of artificial light conjunction with a Corning groiind-glas- daylight filtw is lati factory. I-iciv the color. a t right angles t o the tiibc length Since the standards are made up in interval. corre.sponding to 0.003 grain per pound, it is necessary to interpolate Tvhm reading the sample tube. This can he c o n ~ e n i m t l ydone to within 0,001 grain per pound (grain per pound X 143 = p. p. n1.j. '
135
.iDDED
LEADFROU
Pb Present Grain/lb. 0 003
Zn Present P. p . I n . 30.4
0,025 0.003 0 003
S5,4 30.4 30 4
0,003 0,003 0.003 0 02s
30.4
8.1-1
8.4-2 8.A-3 8.4-4
0.003
r-1
8.4-5 F5044
STANDARD SIRUP
Pb Added Grain/!b.
... 0: io3 0,009 0 013 0,022 0 . no6 0 027
30.4 30'4 300 0
Total Pb Recovered Grain/lb. 0.003 0.026 0.006 0.01'' 0.017 0.025 0.010
0.029 0 029
Tn enty-taiiclarcl siiiip were analyzed for the11 leal content. by the iecent dithizone electrolytic nietliotl ( 2 ) and by the colorinietric method. The iesults are compai et1 in Table 111. T ~ B L111. E C O V P l R I s O S O F DITHIZOSE-ELECTROLYTIC ASD DITHIZOSE-COLORIMETRIC XIETHODS FOR LE.ID IS hIAPLE S I R U P Sample
so.
8- 4
8-B
I
F3173 F5493 F7959 F11220 F8305 Fl549 F3Ol1 F.5044 F5045 F5046
F50i3
Lead Found Dithizone- Dithizoneelectrolytic colorimetric Grain per pound 0.003 0.003 0.023 0.0% 0.006 0 003 0.009 0.003 0.004 0.002 0 042 0 002 0.006 0.028 0.013 0 003 0 003
0.009 0.004 0.003 0.001 0.044 0.002 0.005 0.02'3 0.013 (1.003 0.003
Sample So.
F.5498 Fi503 F7883 Fa000 F8304 Fa306 F9l51 F9152 FB160 F11225 B9ll B411 By19
Lead Found Ditiiizone- Dithizoneelectrolytic colorimetric Grain p e r p o u n d 0.003 0.003 0,006 0.006 0.009 0.009 0.009 0,009 0.0:38 0.039 0.0.51 0 052 0.Oi9
0.064 0.011 0 . 003
0.015
0.019 0 020
0.079 0.063 0.011 0,003 0,015 0,019 0 021
Glawwwe whicli will riot add lead to the solutions should be ,selected. Flint or lead glass should not he iised. A good grade of soda-lime or horosilicate glass lvhicli has been cleaned with hot qtrong acid and ainmoiiiacal cyanide solution has been fc~undsuitable. d n oil sample bottle submitted by Ace Glass Incorporated, Tineland, S.J., was found satisfactory. Summar?
Discussion and Sotes
S o interferences from metal? ordinarily present in maple product.. were encountered in this method. Tin and zinc contamination from the tin-plated and galvanized mapleproducing equipment is expected. Observations indicate that tin contamination from the tin-plated equipment, if present, offers little or no interference. Stannous tin \vi11 form a dithizone complex under conditions of the method, while stannic tin nil1 not. Any till if present is probably in the stannic form. Sap ~ d i e nboiled down to sirup in the presence of a large excess of pure granulated tin tended to lose an appreciable amount of the lead present by electrochemical deposition, but gave no abnormal results upon analysis by
A rapid colorimetric method for the determination of lead in maple sirup is suggested. Single samples can be analyzed in about 15 minutes and about' 50 samples can be esaniined daily. Literature Cited 11) F o o d D r u g . I d m i n . , U. S . D e p t . X g r . , " A p p l i c a t i o n of D i p h e n y l t h i o c a r b a z o n e t o R a p i d C o l o r i m e t r i c E s t i m a t i o n of L e a d on AlJples a n d P e a r s , " m i m e o g r a p h e d m e t h o d ( S e p t e m b e r 17,
1933). ( 2 ) P e r l m a n , J. L., J . Assoc. Oficial r l g r . Chem., 20, 622 (1937). (3) P e r l m a n , J. L., a n d M e n s c h i n g , J. E., Ibid.. 20, 627 (1937).
RECEIVED January
18, 1938
