914
INDUSTRIAL AND ENGINEERING CHEMISTRY
11. Colorless plating. Add one drop of nitric acid to plating and if reaction is slow, wait 2 minutes. A. Plating is not attacked. Add one drop of sodium hydroxide to a new spot.
1. Plating
is attacked: Aluminum
2. Plating is not at-
tacked. Add concentrated hydrochloric acid and take up after a few seconds on filter paper.
c. Spot becomes brown-black. Place nitric acid on a new spot, take up on filter paper and add sodium hydroxide. 1. Spot becomes brown-black: Silver 2. Spot remains colorless or white: Lead
Summary A systematic procedure is described for the identification of metals used in the plating of various metallic and nonmetallic articles. The authors have used this method extensively and have found it to be dependable and accurate. Precautions must be carefully heeded to achieve the best results. Very small quantities of reagent and metal surface are used in the identification. A complete test may be run in less than 5 minutes.
Kjeldahl Nitrogen
a. Green solution is formed: Chromium b. No action app a r e n t : Rho.d i m and platinum metals B. P l a t i n g i s a t tack$ by nitric acid, g i v i n g a green solution. Place hydrochloric acid on new spot, take up on filter paper, render ammoniacal, and add dimethylglyoxime. 1. Red c o l o r a t i o n formed on paper:
Nickel
2. No red color ob-
tained: Chromium
u
-
C. Plating is attacked by nitric acid, giving a colorless solution. Perform cacotheline test. P l a c e s m a l l amount of solid cacotheline on a small filter paper and add one drop of w a t e r . Place one drop of h y d r o c h l o r i c acid on t h e m e t a l a n d t o u c h t h e reverse side of the paper holding the cacotheline to the spot of acid. A red-violet color is a positive reaction. 1. R e a c t i o n is positive: Tin 2. Reaction is not positive. Place nitric acid on a new spot, take up on filter paper, render ammoniacal, and add sodium sulfide. a. Spot remains colorless or white: Zinc b. Spot becomes b r i g h t yellow: Cadmium
Vol. 14, No. 11
Determination A Rapid Wet-Digestion Micromethod L. P. PEPKOWITZ AND J. W. SHIVE Agricultural Experiment Station, New Brunswick, N. J.
A
NUMBER of wet-ashing methods published in the past
few years involve the use of perchloric acid. However, they were designed for the determination of the metallic elements ( I I ) , phosphorus ( I ) , and sulfur (5). I n all these methods, nitrogen is invariably lost by oxidation to nitrate or even to free nitrogen ( I I ) , and it is necessary, therefore, to resort to a separate Kjeldahl digestion for the determination of nitrogen. The method here proposed obviates this necessity. The digestion of plant tissues, fertilizer materials, and other organic products for a Kjeldahl determination is a long process even on a micro or semimicro scale. Many methods have been suggested for shortening the Kjeldahl digestion period; a summary of the literature in this connection is given by Gerritz and St. John (4). Some of these methods require the use of relatively large amounts of various salts to hasten the reactions (4, IO). Periods of 1.5 hours or more have been found necessary even where catalysts such as selenium or mercury are added together with potassium sulfate to increase the reaction temperature. The use of a mercury-selenium combination has been found effective in cutting down the digestion time for whole milk (8), but the advantages of mercury are offset by the necessity of precipitating the mercury before distillation. Furthermore, in alkaline solution if any mercuric sulfate is present, the complex NH,-Hg-OH may be formed, which will introduce a negative error. Therefore, on the micro scale, a t least, mercury has many disadvantages which more than counteract the advantages gained by any increase in the rate of digestion with corresponding decrease in the digestion time. I n the use of the wet-ashing method herein reported for the estimation of any particular element other than nitrogen, salts containing the element in question must not be added to the sample during the course of the procedure. Dispensing with such salts or reaction accelerators does not constitute a disadvantage, however, since the digestion time is reduced to a fraction of the previous time; rather, it is an advantage, since a reagent is eliminated, thus simplifying the procedure.
ANALYTICAL EDITION
November 15, 1942
TABLEI. TIMEOF DIGESTIONAND PER CENT RECOVERY OF NITROGEN FROM SOYBEAN LEAFTISSUE Sample N o . 1A 1B
10 14 18 116 20 30
Standard Method Time N
Min.
%
100 100 110 100 80 150 130 130
5.64 5.49 3.40 3.43 3.26 3.42 3.03 3.22
Perchloric Acid Method Time N Min. % 25 5.70 25 5.61 25 3.32 25 3.46 25 3.36 25 3.48 25 3.00 25 3.14
A further advantage of the perchloric acid method is t h a t the final stages of the digestion are carried out at a temperature below the boiling point of the digest. Thus bumping is eliminated and the reaction goes quietly and smoothly. In fact, all the digestions for the determinations reported in this paper were performed in ordinary 20 X 150 mm. rimless Pyrex test tubes and no loss of material was observed at any time. Perchloric acid is an explosive material when used incorrectly, but there is no danger at all in the method described. The cold dilute acid is not dangerous and the very small amount used is further diluted by the sulfuric acid present t o which it is added. In the many digestions performed, the results of which are presented in this report, no evidence of any explosive tendency of the perchloric acid has been observed.
Reagents Required Concentrated sulfuric acid, nitrogen-free. Ranker’s solution, 32 grams of salicylic acid per liter of concentrated sulfuric acid. Selenium oxychloride, 12 grams of selenium oxychloride per liter of concentrated sulfuric acid. Sodium thiosulfate, 33 per cent, 50 grams of sodium thiosulfate monohydrate per 100 ml. of distilled water. Perchloric acid, 35 per cent, diluted from the 70 per cent acid with distilled lvater.
Procedure A suitable sample of the material to be analyzed-for example, 10 to 15 mg. of plant tissue-is weighed out and introduced into the clean dry test tube if nitrogen only is to be determined or 40 to 50 mg. if other elements are to be included as well. If nitrates are to be included in the determination 1 ml. of the salicyclic acid-sulfuric acid solution (9) is added, mixed with the tissue, and allowed to stand in the cold for 30 minutes. Three drops of the 33 per cent sodium thiosulfate to reduce the nitro groups, and 0.5 ml. of the selenium oxychloride solution are now added to the sample. If nitrates are not to be included in the analysis 1 ml. of sulfuric acid and 0.5 ml. of selenium oxychloride solution are added to the sample. The test tube is heated moderately with a microburner (the standard microcombustion stand without modification will usually hold the test tubes) for a minute or two to determine whether the sample will froth excessively. If frothing does not occur the sample is heated vigorously for 10 to 15 minutes. This period depends upon the length of time necessary for the material to go into solution, char, and pass from the black stage, through the muddy brown stage, to the stage in which the solution takes on a clear ruby wine color. Some Sam les may not go through this sequence, but the corresponding finay stage is one in which the solution takes on a clear color, which may be deep yellow or shades of red or green. In this final stage, which is produced in 10 to 15 minutes, a colorless liquid condenses on the walls of the test tube and the evolution of white fumes has decreased considerably. The digest is now allowed to cool so that there is no danger of cracking the test tube, then cooled under the cold water tap for a few minutes more. In order to minimize loss of the perchloric acid by volatilization from the sides of the tube, 2 drops of 35 per cent perchloric acid are now added by allowing the acid to fall directly into the digest. There is no danger of explosion if this procedure is followed. The outside of the test tube is wiped dry and returned to the digestion rack. The burners are turned down to a low flame, since it is important that the temperature of the solution be kept
915
below the boiling point. The mixture rapidly becomes lighter in color, going through shades of yellow and clearing in a few minutes. The digest is kept over the low flame, since after the first clearing, a yellow color reappears in the digested solution, Heating is continued at a temperature below the boiling point until this secondary color disappears and the digest becomes clear and colorless. If there is any iron in the sample a yellow tint may persist, but this usually disappears on cooling and dilution. The total time required for the digestion is usually 10 to 25 minutes. The samples are allowed to cool, diluted with a few milliliters of distilled water, and allowed to cool again. They are now ready for nitrogen determinations or can be diluted to 25 to 50 ml. and aliquots taken for analysis.
Experimental Results Analyses were performed on a relatively large number of samples of materials of diverse character, both by the standard methods now in general use and by the perchloric acid method here described. All the analyses were carried out on a comparative basis, involving in each case the per cent recovery of nitrogen and the time required for the digestion by the two methods. The results of these analyses are presented in the tables. All distillations were performed with the Parnas and Wagner apparatus, using 2 per cent boric acid as the receiver with the methyl red-bromocresol green indicator employed by Ma and Zuazaga (6). Table I presents the results of analyses performed on soybean leaf tissue, which proved difficult to digest. With selenium oxychloride as a catalyst, 20- t o 30-mg. samples required from 100 to 150 minutes to complete the digestions by the standard method. I n addition to this long time for digestion, bumping was vjolent and somewhat difficult to control. The time required t o perform the digestions by the perchloric method averaged only 25 minutes. There was no loss of nitrogen, as is indicated by Table I. Attempts to use perchloric acid in Kjeldahl digestions usually have resulted in a loss of nitrogen. With this in mind, the following analytical tests were made upon samples of ammonium chloride to determine whether the nitrogen could be recovered without loss when subjected to the perchloric acid treatment as here employed in a complete combustion procedure. As the data in Table I1 show, no nitrogen was lost. TABLE11. RECOVERY OF NITROGEN FROM AMMONIUX CHLORIDE AND POTASSIUM h-ITRATE
(After subjecting samples to complete perchloric acid digestion procedure) Nitrogen in Sample Nitrogen Recovered Mg. MQ. % NH4C1 2.46 2.46 100.0 1.57 1.61 102.5 KNOI 0.20 0.1970 98.5 0.40 0.3974 99.4 0.60 0.5912 98.5
With the same object in view similar analytical tests were carried out for the recovery of nitrogen from nitrates. Aliquots of a standard nitrate solution were transferred to test tubes a few drops of saturated calcium hydroxide solution were added to each, and the nitrate solution was evaporated to dryness on a steam bath. When dry, 1 cc. of the salicylic aoidsulfuric acid solution was added, allowed to react for 30 minutes, and reduced with 3 drops of the 33 per cent thiosulfate solution. The resulting solution was then subjected to the digestion process as described and the nitrogen determined. The results are presented in Table 11. Each value given in the table for nitrogen recovery from potassium nitrate represents the average determination of four aliquots. The average per cent values of nitrogen recovered are slightly below the theoretical 100 per cent of the samples. The per cent values of nitrogen recovered from individual aliquots deviated
916
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 CONTENT OF P U R E COMPOUNDS TABLE 111. NITROGEN
(Determined b y perchloric acid method checked against their theoretical per cent values) Deviation from Theoretical Compound Theoretical Experimental Value Asparagine.Hz0 Glutamine Tyrosine Benzidene Diphenylamine Tetramethyldiaminodiphenylmethane
%
%
%
18.66 19.17 7.73 15.20 8.28
18.61 19.14 7.86 15.16 8.16
-0.05 -0.03 +O. 13 -0.04 -0.12
11.01
11.03 $0.02 Av. deviation 0,065
-
TABLEIV. KITROGENCONTENTOF ORGANICMATERIALS
Material Cocoa tankage Processed tankage Beetle scrap dust Chicken manure Milorganite Acid fish scrap Processed tankage Qb Processed tankage 8b 8huey sample 1 2 3
Micro Perchloric Acid Method Digestion time N
Min. 11 13 13 13 11 13 17 14 13 12
14
Nitrogen b y Deviation A. 0. A. C. from A. 0. Official A. C. DeterMethod mination
%
%
2.57 9.68 19.02 2.26 5.60 8.54 6.56 9.44 8.85 8.50 8.65
2.52 9.76 19.02 2.25 5.66 8.54 6.60 9.42 9.04 8.55
%
$0.05 -0.05 0.00 $0.01 -0.06 0.00 -0.04 +0.02 -0.19 -0.05 8.79 -0.14 Av. deviation 0.058
both above and below this theoretical value, but all were well within the limits of experimental error of the procedure, and the average recovery values do not actually represent loss of nitrogen due to the perchloric acid treatment. As a further check upon the effectiveness and accuracy of the method, and also upon the applicability of the procedure to ultimate analysis, a variety of pure organic compounds were analyzed for their nitrogen content and the results were checked against the theoretical nitrogen per cent values of the respective compounds (Table 111). Each experimental value represents the average of duplicate determinations. These values check very closely with the theoretical values. The average digestion time for 10-mg. samples, including the necessary cooling period, was usually less than 10 minutes. A final test of this method was performed by analyzing a variety of organic materials, using a 10-mg. sample of each material and comparing the results with those obtained by the official A. 0. A. C. method (2). Samples of these materials together with the analytical data determined by the official A. 0. A. C. method were kindly provided by the Soils Department of the New Jersey Agriculture Experiment Station. The results of this comparison are presented in Table IV. The average data obtained by the perchloric acid micromethod are in excellent agreement with the corresponding data by the official A. 0. A. C. method. The points to be emphasized in connection with Table IV are the close agreement of the results and the difference in the average time required for the digestion procedure by the perchloric acid micromethod here described and by the official A.O. A. C. method. The average digestion time by the former was approximately 12 minutes; by the latter, it was approximately 2 hours. The perchloric acid micromethod presents a less cumbersome procedure than most other methods, and the great saving in time and a substantial decrease in the cost of reagents are important. The final product of the digestion is a colorless solution suitable for the determination of tissue constituents except sulfur, chloride, and selenium, since no other contaminants than these three are added.
Vol. 14, No. 11
Objections have been offered to the use of selenium as a catalyst on the ground that its use may result in the loss of nitrogen (3,l.Z). This may be due to the fact that either the digestion is not complete with this catalyst even after the digest has cleared or an actual loss of nitrogen occurs if too large a quantity of selenium is employed in the digestion. However, with the perchloric acid micromethod here described, low nitrogen values never resulted from the use of selenium as a catalyst, and further heating after the solution had cleared was unnecessary. This fact is borne out by many trials in which the solution was distilled immediately after clearing with complete recovery of the nitrogen in the sample. The addition of perchloric acid to the sulfuric acid at the start of the digestion as suggested in the macromethod of Mears and Hussey (7) invariably resulted in a loss of nitrogen, as indicated in Table V. This was also often the case if only 2 drops of perchloric acid were used. Furthermore, no significant decrease in the digestion time was obtained by this procedure. In the proposed method, it is important to complete the digestion process a t a temperature below the boiling point of the digest after the perchloric acid has been added, in order to avoid loss of the acid on volatilization and to prevent the perchloric acid from reacting too vigorously, which might result in the possible loss of nitrogen. For some undetermined reason, 2 drops of 35 per cent perchloric acid have been found more effective than 1 drop of 70 per cent perchloric acid in oxidizing the organic matter when used as described. The use of 70 to 72 per cent perchloric acid did not materially hasten the process, but often resulted in a loss of nitrogen. Work is now in progress to develop, if possible, a macroprocedure similar to the micromethod here reported, in the hope that the time required for the digestion of large samples may be proportionately reduced.
TABLE V. Loss IN NITROGENWHENPERCHLORIC ACID Is ADDEDTO SULFURIC AT START OF DIGESTIOK ,
Sample No. 1 2 3 4
Nitrogen Loss 0.25 cc. of perchloric acid and 1.0 cc. of sulfuric acid added a t s t a r t of digestion Standard method
70
%
1.83 1.54 1.90 2.08
2.52 2.72 2.87 3.11
Literature Cited (1) Allen, R. J. L., Biochem. J., 34, 858 (1940). (2) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, p. 26, 1940. (3) Davis, C. F., and Wise, M..Cereal Chem., 10, 488 (1933). ENG. CHEM.,ANAL. (4) Gerritz. H. W., and St. John, J. L., IND. ED., 7, 380 (1935). (5) Kahane, E., and Kahane, M., Bull. SOC. chim., [5], 1, 280 (1934). (6) Ma, T . S., and Zuazaga, G., IND.ENG. CHEM.,ANAL.ED., 14, 280 (1942). (7) Mears, E.,and Hussey, R. E., J. IND.ENG.CHEM.,13, 1054-6 (1921). (8) Meneffee, S. G., and Overman, 0. R., J . Dairy Sei., 23, 1177 (1940). (9) Ranker, E. R., J . Assoc. Oficial Agr. Chem., 10,230 (1927). (10) Robinson, R. J., and Shellenberger, J. A., IND.ENG. CHEM., ANAL.ED., 4, 243 (1932). (11) Smith, F. G., “Mixed Perchloric, Sulfuric and Phosphoric Acids and Their Applications in Analysis”, 1st ed., Columbus, Ohio, G. Frederick Smith Chemical Co., 1935. (12) Snider, R. S., and Coleman, D. A,, Cereal Chem., 11, 414 (1934). JOURNALSeries paper of the New Jersey Agricultural Experiment Station, Department of P l a n t Physiology, Rutgers University.
