INDUSTRIAL A N D ENGINEERING CHEMISTRY
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coagulated precipitate forms, depending upon the fineness of the copper sulfate used. It is important, therefore, to sift the hydrated lime into the dilute copper sulfate solution. The results as a whole indicate that a high-grade Bordeaux mixture may be obtained by adding to the required amount of water, copper sulfate and subsequently hydrated lime containing a small amount of any of the organic stabilizers mentioned above It seems preferable to recommend casein as the organic stabilizer, since the natural spreading and adhering properties of this substance have been demonstrated. It is very important that the hydrated lime be as free as possible from carbonates and coarse particles. To obtain best results about 50 per cent should pass through a 200mesh sieve.
Vol. 15, No. 9
RECOMMENDATIONS FOR COMMERCIAL PREPARATIONS
It is advised that cartons be prepared containing the requisite amount of powdered copper sulfate and hydrated lime in separate packages. The packages of copper sulfate should be placed on top in order that it may be removed first when the carton is opened. Printed directions on cartons may read as follows: First, fill the spray tank with 50 gallons (for 4 pounds lime and 4 pounds of copper sulfate) of water. After starting the agitator add contents of Package A (containing copper sulfate). Then sift into the tank Package B (containing the hydrated lime plus 0.01 per cent of casein). The Bordeaux mixture is then ready for spraying.
Determination of Ash, Arsenic, Copper, and Zinc in Gelatin' By Roger M. Mehurin BUREAUO F ANIMALI N D U S T R Y ,
DEPARTMENT O F A G R I C U L T C R E , WASHINGTON,
D. C
T
HE laboratory in-
Undesirable features of published methods for the determination out and combined SO as to of small quantities of arsenic, copper, and zinc are pointed out, and form a coherent method by spection of meats, ' a coherent, rapid, and accurate method for the determination of which accurate results are meat food Products, obtained with a minimum and substances used in conash, arsenic. copper, and zinc i n gelatin is described. nection with their preparaSources of possible error are enumerofed SO that the method will expenditure of time and tion involves the analysis be adaptable f o r inexperienced analysts. labor. This method in subof numerous samples of stantially its present form has been in use in the meat gelatin. I n order to make certain that gelatin of suitable quality is used, samples are inspection laboratories of the Bureau of Animal Industry for collected from each shipment received at any establishment a t several years. which federal meat inspection is maintained, and are subARSENIC mitted to the laboratory for examination. During the last REAGENTS-Dilute hydrochloric acid: Dilute one part arsenicyear, 247 such samples were examined in the Washington free hydrochloric acid (sp. gr. 1.19) with 3 parts distilled water. Potassium iodide solution: 600 grams per liter. Meat Inspection Laboratory. Chief among the objectionStannous chloride solution: Dissolve 50 grams in 50 cc. arsenicable impurities found in gelatin are arsenic, copper, and free hydrochloric acid (sp. gr. 1.19) and dilute to 200 cc. with diszinc. These metals are usually present in amounts not tilled water. Bromine water: Saturated, keep an excess of bromine in the exceeding 1.4, 30, and 100 parts per million, respectively. All the available methods for the determination of arsenic, bottle. Lead acetate cotton: Moisten the cotton with 10 per cent lead copper, and zinc in food have been tried out, but have been acetate solution and dry. found unsatisfactory when applied to the determination of Mercuric bromide paper: Prepared by dipping a small sheet these metals in large numbers of samples of gelatin. The of hard-surfaced drawing paper into a 5 per cent alcoholic soluof mercuric bromide, drying so as to obtain as uniform demethod described in the Methods of Analysis of the Asso- tion posit as possible of the mercuric bromide, and cutting the sheet ciation of Official Agricultural Chemists, involving digestion into strips 2 x 70 mm., preferably in a cutting machine prepared with sulfuric and nitric acids, is entirely unsuitable on ac- for this purpose. METHOD-weigh out 10 grams gelatin into a 150-cc. beaker. count of the excessive frothing, the time required for comAdd 50 cc. dilute hydrochloric acid and place on steam bath. plete oxidation, and the continued exposure of the analyst to As soon as gelatin is dissolved add 20 cc. bromine water. After irritating fumes. The methods employing hydrolysis with digesting for one hour remove from the steam bath and cool. hydrochloric acid have likewise proved unsatisfactory on Make up solution to 100 cc., mix, and filter. To 20 cc. of the account of difficulty in saturating with hydrogen sulfide and filtered solution add 1 cc. potassium iodide solution and 0.3 cc. stannous chloride solution. Heat on steam bath for 5 minutes, the subsequent filtering of large volumes of solutions which are cool, and transfer to a previously prepared Gutzeit apparatus. often of such a character as to filter slowly. The determina- Prepare this apparatus as foIlows: Add to the generator 3 grams tion of copper by titration with sodium thiosulfate is open to 30-mesh, arsenic-free zinc and 20 cc. dilute hydrochloric acid; a considerable error because of the difficulty of obtaining a connect and allow t o run for 5 minutes. Place a small amount of loosely drawn lead acetate cotton in the barrel before connectdefinite end point when small amounts are titrated. The ing the apparatus and a strip of mercuric bromide paper in the employment of Gooch crucibles in filtering and weighing small tube just before adding the solution. Add 0.8 cc. amyl alcohol, amounts of zinc has sometimes been found to be a source of rinse out beaker with 3 to 5 cc. water, and add to generator. The liberated gases pass through the lead acetate cotton and error. over the strip of mercuric bromide paper. Allow generator to The method described below has developed as a result of run for one hour, or longer if necessary, protecting paper from several years' experience in the analysis of gelatin and is free strong light. Compare with standards prepared in the same from any and all of the faults cited. While it utilizes only well- manner from arsenic-free gelatin to which known amounts of known analytical procedures, the details have been worked arsenic have been added. Standards should show the action of 1
Received March 28, 1923.
from 1 to 7 micromilligrams (0.001 to 0.007 mg.) of arsenious acid (Asz08).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1923
ASH Weigh out 20 to 40 grams gelatin in a tared platinum or porcelain dish of approximately 150-cc. capacity and ash a t a temperature of from 500" to 550' C. This is approximately the temperature a t which the furnace presents a barely visible red when viewed through the small vent hole in the door. Samples will usually be satisfactorily ashed in from 4 to 5 hours, but no error is involved in allowing them to remain in the muffle for a longer period. It has been found convenient and economical of time to use an electric muffle furnace, placing the samples in the muffle, previously regulated so as to remain a t the correct temperature, near the close of the day and allowing them to remain over night. After ashing is complete, remove to a desiccator, cool and weigh.
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ture, or the material will boil over the sides of the dishes before being ashed. The heat of the muffle must not reach a bright red, as loss of copper will occur in that case. Hydrochloric acid must be used in dissolving the ash. If sulfuric acid is used, a precipitate of needle-like monoclinic crystals of calcium sulfate will often form before and after the precipitation of zinc sulfide. Contamination of the zinc sulfide by traces of organic iron usually contained in the ammonium hydrate and ammonium chloride used and by traces of nickel which are sometimes found in gelatin, is prevented by the addition of an excess of acetic acid as described.
COPPER REAGENTS-Dihtk hydrochloric acid: Dilute one part (sp. gr. 1.19) with one part distilled water. A m m o n i u m nitrate: 100 grams per liter. Potassium ferrocyanide: 40 grams per liter. Standard copper solulio.ia: 0.3927 gram recrystallized copper sulfate per liter. One cc. equivalent to 0.1 mg. copper. METHOD-Moisten the ash with a small amount of water, add approximately 5 cc. concentrated hydrochloric acid, evaporate to dryness, add 8 cc. dilute hydrochloric acid, heat to boiling, and transfer to a 50-cc. Erlenmeyer flask, using enough wash water t o make the volume approximately 40 cc. Heat nearly to boiling, saturate with hydrogen sulfide, stopper tightly, and allow to stand in a warm place for one-half hour or more. Filter into a 150-cc. Erlenmeyer flask and wash promptly and thoroughly with warm 1:20 hydrochloric acid saturated with hydrogen sulfide. Transfer the paper and precipitate to a 30-cc. porcelain crucible and ignite in a muffle furnace a t a temperature not exceeding that a t which the gelatin was ashed. After ignition, cool, moisten ash with 1 to 2 cc. nitric acid, and evaporate to dryness on steam bath. Take up copper salts with 1 to 2 cc. nitric acid, add approximately 5 cc. water, and warm on steam bath to facilitate solution. Dilute to approximately 30 cc. and make alkaline with ammonia. Heat on steam bath, away from any hydrogen sulfide fumes, until ,all ammonia is expelled, occasionally diluting with water to maintain approximately the original volume. Do not evaporate to dryness. Filter into a 50-cc. graduated flask, wash out crucible with warm water, make up to mark and mix. Measure out 25 cc. into a 50-cc. Nessler tube, add 5 cc. ammonium nitrate solution, and make up to 50 cc. Add 0.2 cc. potassium ferrocyanide solution and mix. Match the color against tubes prepared in the same way from the standard copper solution. Make up standard containing 2, 3, 4, 5, and 6 cc. of standard solution equivalent to 20, 30, 40, 50, and 60 parts per million of copper if a 20-gram sample is used and one-half of solution taken. Solutions giving a stronger reaction than 6 cc. of the standard cannot be accurately compared. If a reaction stronger than that'given by 6 cc. of the standard is obtained, an aliquot smaller than 25 cc. must be taken and the determination repeated.
Pipet for Lactose Determination in Milk' By E. G . Mahin PURDCE UVIVERSITY, LAFXYETTE, IND.
H E polariscopic determination of lactose in milk by the official method2 involves measuring double the lactose normal weight of milk, based upon the Ventzke (or International) scale. The volume of milk to be taken varies according to the specific gravity of the milk under investigation, which means that no ordinary transfer pipet can be 64.25 used for the purpose. Measuring milk samples in burets is always troublesome, espe- 6 4 .oo cially on account of the fact that no buret is designed to deliver its entire contained quan- 63 .75 tity, and flotation of fat globules during the 6 3 slow delivery gives a sample finally measured, not strictly representative of the original sample. A special pipet has been devised for this purpose. This is designed along the general lines specified by the Bureau of Standards3 for transfer pipets. However, instead of bearing a single capacity mark, as is the case with the ordinary transfer pipet, the upper branch of the stem is graduated by intervals of 0.05 cc., from 63.50 to 64.25 cc. From the table given in the official methods the volume to be measured for a milk of predetermined specific gravity is read. This volume is then measured, at one operation, in the pipet and the entire quanZINC tity is delivered into the 102.6-cc. volumetric flask Boil the filtrate and washings from the hydrogen sulfide prein which the proteins are to be precipitated. cipitation of copper until all hydrogen sulfide is removed. Add The special pipet is illustrated in the accompany1 cc. concentrated nitric acid and continue the boiling until the volume is reduced to approximately 25 cc. Add 10 cc. aming figure. The general specifications as to quality monium chloride (200 grams per liter), make definitely alkaline with ammonium hydrate, heat nearly to boiling, and filter into of glass, finish of tip, and other matters of con100-cc. Erlenmeyer flask. Wash with warm, alkaline, ammo- struction, follow those of the Bureau of Standards for transfer pipets. Other specifications are as nium chloride solution, containing 50 grams ammonium chloride and 23 cc. ammonium hydrate (sp. gr. 0.90) per liter. Neutralize follows : filtrate and washings with acetic acid, add 0.5 gram sodium acetate and sufficient glacial acetic acid to make an excess of 2 cc. for each 50 cc. of solution. Warm on steam bath and saturate with hydrogen sulfide. Allow to stand in a warm place for approximately one-half hour. Filter through a small paper and wash thoroughly with warm 1:lo0 acetic acid saturated with hydrogen sulfide. If filtrate is turbid, return to flask, add a few drops of saturated mercuric chloride solution, shake, and filter again. Ignite in a tared platinum crucible a t a dull red heat until completely ashed, then a few minutes a t bright red heat. Weigh as ZnO. Weight of ZnO X 40,000 equals parts per million of zinc if a 20-gram sample has been taken.
PRECAUTIONS NECESSARY Attention is directed to certain sources of error which must be guarded against. The gelatin must not be placed in the muffle until after the latter has reached the proper tempera-
T
Total length of pipet, 50 to 55 cm.; length of upper stem, 20 cm. lengthof graduatedportion of stem, 8 to 8.5 cm. ; smallest graduated interval, 0.05 cc. ; minimum time of delivery for 63.5 cc. of water a t 20" C., 20 seconds, maximum time, 1 minute.
Such pipets may be made by any good glassblower. Of course, all such instruments should be calibrated by the user, after their receipt, as are all volumetric iiistruments for work of precision. 1 To be presented before the Division of Agriculture and Food Chemistry a t the 66th Meeting of the American Chemical Society, Milwaukee, Wis., September 10 t o 15, 1923. 2 Assoc. Official Agr. Chem., Methods, '1919, 226. a Birr. Standards, Circ. 9.
