38
ANALYTICAL EDITION
Vol. 5 , No. 1
periods of t,ime will ordinarily be negligible. Moreover, it is possible to readjust the concentration a t any time by slightly varying the rate of flow through the saturating bottles.
Entomology, for the loan of several pieces of laboratory equipment.
represented in Figure 1, and t o C . I. Bliss, of the Bureau of
R~~~~~~~~~~~~t3, 1932.
LITERATURE CITED
A Simplified Karns Technic for the Micro-Estimation of Iodine HARRY VON KOLNITZ AND ROE E. REMINGTON South Carolina Food Research Commission and Department of Nutrition, Medical College of the State of South Carolina, Charleston, S. C.
I
N T H E estimation of the minute quantities of iodine which normally occur in animal and vegetable matter, the step of greatest difficulty has been the destruction, without loss of iodine, of the organic matter present. Earlier workers used fusion with alkali, with or without nitrate, while in recent years combustion in a current of oxygen in a silica or Pyrex tube, i.gnition in a muffle a t a controlled low temperature, or digestion with hydrogen peroxide and sulfuric acid has been employed with varying success in different laboratories. Alkaline fusion is applicable only to small samples, and introduces large quantities of reagents, which in themselves constitute a danger of contamination. Low-temperature ignition in a m d e (450' C.) was proposed by Remington (6),and used on a, lar e number of samples of ve etables. The authors were able to &ow good recoveries when &ied thyroid gland was added to dried potatoes, but Reith (8) reported large losses when iodine compounds were added to cereal products or dried milk. Karns (a), on the other hand, found good recoveries of added potassium iodide, but that plant and animal tissues alone gave consistent1 lower values than by other methods. Di estion wit{ hydrogen peroxide and sulfuric acid, proposed by Pfeiffer (T),requires costly and fragile apparatus, and possessea objections which include the distillation of unoxidized fatty acids and other volatile substances, inability to handle large samples, and time required for the oxidation. The silica tube furnace devised by McClendon (4) and adopted with various modifications by Reith, von Fellenberg ( I ) , McHargue ( 6 ) , and others, insures, with proper operation, rapid and complete oxidation in a semi-enclosed system;all evolved gases being drawn through an absorption train. The capacity of the absorption s stem has to be large, so that while the actual combustion oPthe sample may take half an hour or less for 100 grams, the subsequent evaporation and manipulation of the large volume of absorption liquid and washin s will require several days. The intense heat developed in t i e McClendon furnace, in an atmosphere enriched with oxygen, creates conditions favorable to fixation of atmospheric nitrogen. Nitrite, thus formed, is hard to get rid of, and unless eliminated may oxidize and drive out of solution all the iodine present. ComFIete failures to recover any iodine by this method are annoyingly requent. Recently Karns (2) has proposed ignition in oxygen in a specially constructed flask or bulb, claiming that the oxidation could be made slower and could be more easily controlled, and that hence it is possible to work with a much smaller absorption train than with the tube furnace. Karns also introduced the idea of condensing the evolved iodine by freezing, instead of absorbing it in solutions. The method of Karns seemed to be a distinct advance, particularly for those laboratories where iodine assays must be made frequently, and the authors accordingly undertook to simplify the apparatus and manipulation. The essential
part of the simplified set-up is the torch (Figure 1) which contains combined in one piece of apparatus the feed device for the sample, jets for supplying oxygen, exit tube for products of combustion, and support for the flask or combustion chamber. It is made of brass, the cup being of such size as to accommodate and provide water seal for a 500CC. wide-mouth Erlenmeyer flask of Pyrex glass. The oxygen feed tubes are so set as to throw the jets of oxygen against the tip of the advancing candle, and are offset slightly so as to give the gaseous stream a whirling motion, For dry milk and some other substances two oxygen jets are sufficient, but for samples more difficult to burn cleanly torches with four jets have been constructed. Oxygen consumption approximates 1 to 1.5 liters per minute. The Erlenmeyer type of flask was chosen after many trials as giving the maximum efficiency with the least volume. An essential part of the operation is the manner of feeding the sample. This is packed into Visking sausage casing1 (0.94 inch, 2.35 cm.) to form a cartridge which fits very freely in the feed tube. The feed plate has two pins projecting from its upper surface, which engage the cartridge and cause it to rotate with the feed screw, thus enabling the oxygen jets to exert a cutting action as the cartridge advances. With this device it has not been found necessary to use carbon dioxide, but the feed tube is provided with a hole near the lower end, to permit a small amount of air to be drawn in and guard against the water seal being broken by sudden changes of pressure. The exit tube leading to the absorption train is of glass rather than metal, since the products of combustion are apt to be corrosive, and all brass surfaces exposed in the combustion chamber and water-seal cup are given a coating of nitrocellulose lacquer, it having been found that copper in the washings may interfere in the analysis. Samples of dry milk or dried vegetable matter weighing 25 to 50 grams can be burned readily in one operation. The combustion can be interrupted to introduce new cartridges of material, or to change the flask, should it become so coated with sublimed material as to make proper observation of the operation impossible. Good vision of the tip of the burn1 The Visking 8ausage casing may be objected to as a possible source of iodine, but we have used it continuously for several years when burning samples in the silica tube. Some of the diets employed to produce goiter in the r&t oontain so little iodine (16y per kilo) that a kilogram must be burned in one sample, and any appreciable amount of iodine in the sausage crteing would become evident.
January 15, 1933
I N D U S T R I A L A N D E N G I N E E R I N G C I3 E M I S T R Y
39
ing candle is important, for if the sample is fed too fast combustion will be incomplete and tarry products formed. This is shown by the color and appearance of the flame. With regard to the absorption train, Karns has shown that absorbing liquids can be dispensed with and gaseous iodine compounds frozen out, using solid carbon dioxide and acetone as the refrigerant. Since the flask and absorption tubes must afterward be rinsed with water containing some alkali, an efficient liquid absorption system, provided the volume of liquid and amount of alkali could be kept
the partition was 6.1, 83.3, and 10.2 per cent, respectively, the difference being due to the larger orifice and more rapid gas flow in the latter case. Ordinarily the second wash bottle serves only as a trap to collect splashings from the first. After the combustion, the water from the cup is used to rinse the flask, combined with the solutions from the absorbers, and the whole apparatus is rinsed with a minimal amount of water. After testing to insure alkalinity, the combined solutions are concentrated to small volume in a beaker, centrifuged free from insoluble matter, and evaporated to dryness in a platinum dish. An additional pellet of sodium hydroxide is added to the dish and the contents just fused, holding the dish with tongs and keeping it moving over the flame. This quick fusion has been found necessary to destroy any organic dust particles that may have found IOcm their way into the solution. If the iodine is to be determined colorimetrically, extraction with alcohol can frequently be dispensed with, the contents of the dish being dissolved in a small volume of water, a pinch of sodium azide added to decompose nitrites, followed by a little sodium sulfite to reduce iodate, this in turn by - E phosphoric acid to acidity, then boiling to expel sulfur dioxide transferring to the small separatory funnel. A minute crystal Deta i \ of sodium nitrite will now liberate the iodine, which is taken up with 1 cc. of carbon tetrachloride and compared with a standard in a microcolorirneter. If iodine is to be estimated by titration, the reduced residue in the platinum dish should be extracted several times with alcohol, the alcohol removed by evaporation, iodide c oxidized by bromine, and the balance of the determination carried out in the usual way. In this laboratory results by titration are invariably somewhat higher than by colorimetry, and since the possibility of the ,presence of traces of reducible substances other than iodate cannot be rigorously excluded, the colorimetric method is preferred. U The results given in the following table indicate that reFIGURE1. ESSENTIAL PARTOF SIMPLIFIED SET-UP. DETAIL coveries are quite as good with this apparatus as with more SHOWSMANNER IN WHICHOXYGENTUBESARE OFFSET complicated trains.
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A . Oxygen feed tubes 5 mm. outside diameter. Standard copper tubing B. Air vent 3 mm. di6meter (important) C. Glass odtlet tube, 7 mm. outside diameter, connected with Milligan absorbers &; D. Rubber tubing gasket for outlet tubea E. Hole for outlet tube, 9 mm.
small, should possess no disadvantages over the refrigeration method, and would be simpler in operation. The electrical precipitator, introduced first by McClendon to remove solid particles from the gas stream, has proved so inconvenient in operation and of so little value in the analysis that it was discarded by McClendon as well as by the authors several years ago. With the slow burning possible in the Karns type of apparatus and efficient wash bottles, no more trouble is experienced from dust, most of it being deposited in the flask and the balance in the absorption apparatus. The authors use only two Friedrich’s or Milligan wash bottles in series, the two bottles being identical as to type and giving equally satisfactory results, but the Friedrich’s bottle requiring much less liquid. Rhodes and Rakestraw (9) report this type most efficient of seven kinds tested. Two pellets of sodium hydroxide are added to the water in the first bottle, and one in the second. Practically all the iodine will be found in the washings of the flask and the contents of the first absorber, as noted by Karns (2). The relative distribution will depend on the amount of iodine in the sample and rate of burning. For example, burning 25 grams of a sample of milk powder which contained the enormous amount of 2000 y of iodine, 9.9 per cent was found in the flask, 87.2 per cent in the first Milligan wash bottle, and 2.2 per cent in the second. With Friedrich’s bottles
SAMPL~ DRY IODIXE TOTAL IODINE MILK IN IODINE IODINE RECOVMETHOD TAKENSAMPLEADDED PRESENTERED RECOVERY Grams y Y 7 Y % Silica-tube furnace 50 27.7 ... ... ... ... 50 26.8 ... ... ... ... 50 27.8 ... ... ... .
I
.
26 26 25
14.7 15.1 14.5
... ... ...
... ...
...
... ... ...
Flask; iodine added as NaI 25 25 25
14.9 14.9 14.9
14.,0 14.0 25.0
28.9 28.9 39.9
27.1 30.0 38.5
93:7 103.8 96.6
Flask. iodine added asdyroidpowder 25 25 25
14.9 14.9 14.9
25.0 26.0 100.0
39.9 39.9 114.9
40.1 38.0 116.0
100.5 95.2 100.9
Flask
t
... I . .
...
LITERATURE CITED (1) Fellenberg, T. von, Biochem. Z., 224, 170 (1930). (2) Karns, G . M., IXD.ENG.CHEM.,Anal. Ed., 4, 299 (1932). (3) Ibid., 4,375 (1932). (4) McClendon, J. F., and associates. J . Am. Chem. Soc., 50, 1093 (1928); 51, 394 (1929); 52, 541, 980 (1930); 53, 1245 (1931). ( 5 ) McClendon, J. F., and Remington, R . E., Ibid., 51, 394 (1929). (6) MoHargue, J. S., Young, D. W., a n d Roy, W. R., IND.ENQ. CHEM.,Anal. E d . , 4, 214 (1932). (7) Pfeiffer, G., Biochem. Z., 228, 146 (1930). (8) Reith, J. F., Ihid., 224, 223 (1930). (9) Rhodes, F. H., and Rakestraw, D. R . , IXD.EXQ.CHEX.,Anal. Ed., 3, 143 (1931). RECEIVXUD August 30, 1932
