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.
943
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.
