Determination’ of Copper Observations on Clarke and Jones Method LORENC. HURDAND JOHNS. CHAMBERS, University of Wisconsin, Madison, Wis.
T
HE importance of minute
directly in the solution. Such The Clarke and Jones method for copper is traces of copper in bioa combination would obviate the suitable only under carefully controlled conditions. logical processes and necessity of ashing and would Because of the many factors contributing to products has been the subject of make the determination suitable color variation forjixed amounts of copper, it is numerous researches during the for field use. Accordingly, a not suitable for general use. It is recommended study of the variables was made past few years. Of the many in an effort to define the limits analytical methods available for that, ;f the method is used, a series of secondary of the method. the detection and estimation of potassium permanganate standards be estabThe concentration of the color small amounts of copper, the lished. Inasmuch as numerous factors appear produced during the course of procedure of Biazzo (9) in one to contribute to the variation in color and color the trial reactions and deterof its many modifications (1, 4, intensity, it is advisable to check the permanganate 6, 8 ) is usually followed. The minations was measured by means of an Eastman Universal method is accurate under most standards against known amounts of copper under Colorimeter (6, 7 ) . “This inconditions and capable, in the the conditions of the analysis. strument oDerates on the subhands of an experienced analyst, tractive principle and any deof yielding concordant results. It is, however, time-consuming and involves an extraction of sired color may be obtained in the comparison field by the suba copper thiocyanate-pyridine complex with subsequent traction of certain parts of the white light used for the illuminaquantitative recovery of all or a known portion of the im- tion of the comparison field. This subtraction is accomplished by the use of dyed gelatin wedges. By using three wedges, miscible solvent. The analytical method proposed by Clarke and Jones (5) each of which absorbs one of the three additive primaries, any appeared to offer distin‘ct possibilities as a micromethod for desired amount of each primary may be subtracted from the copper. The procedure consisted essentially of adding to a white light.” It was found that the pink color produced by very dilute sulfuric acid solution of copper, ammonium per- the Clarke-Jones reaction matched almost exactly that transsulfate, dimethylglyoxime, silver nitrate, and pyridine. In mitted through the green wedge of the instrument. Conthe presence of copper these reagents produce a “permanganate sequently, it was possible to translate the intensity of red in pink” whose intensity varies with the copper concentration. the solutions under examination directly into the reading of The authors advise that comparison with standards be made but one scale. Inasmuch as the scale readings were reprowithout undue delay because of the tendency of the color to ducible, the instrument served as a fixed standard to which all observations and determinations could be referred. fade. Standard copper solutions were prepared by dissolving weighed .amounts of recrystallized copper sulfate in conductivity water and diluting to known volumes. Stock solutions so prepared were checked iodometrically against thiosulfate solutions standardized on pure electrolytic copper. Ammonium persulfate and silver nitrate were of reagent quality and were used without further purification. The pyridine was redistilled. Because ordinary distilled water which had been stored in tin-lined copper tanks gave rise to the characteristic pink color when used in conjunction with the test reagents, care was taken to make all dilutions with freshly prepared “conductivity” water. The dimethylglyoxime was obtained from the Eastman Kodak Company and was used without further purification. A saturated solution of dimethylglyoxime in 95 per cent ethyl alcohol was used throughout the work. The alcohol was redistilled and free from interfering traces of acid. Blank determinations established the purity of the reagents. It was ascertained early that if the concentration of pyridine was varied but slightly for fixed concentrations of the remaining constituents, the intensity of the color varied widely. Figure 1 illustrates the variation in color concentration produced by varying the pyridine concentration. The points I/o/ume of Pyr/idine So/ution were obtained by plotting colorimeter-scale readings against FIGURE1. VARIATION IN COLORCONCENTRATION PROcubic centimeters of 10 per cent pyridine added. To 50 cc. DUCED BY VARYING PYRIDINE CONCENTRATION of solution, prepared by diluting suitable volumes of the copper Inasmuch as the determination is carried out in a slightly standards with conductivity water, were added 1 cc. of 0.4 N acid solution, it appeared to the writers that a wet combustion sulfuric acid, 0.5 gram of ammonium persulfate, 0.5 cc. of could be made upon the organic samples, and after proper dimethylglyoxime solution, and 0.25 to 0.3 cc. of 0.5 per cent adjustment of the acidity the copper could be determined silver nitrate, Following thorough mixing, the pyridine was 236
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April 15, 1932
INDUSTRIAL AND ENGINEERING
added and the Nessler tube inserted in the colorimeter. Five minutes were allowed to elapse between the addition of the pyridine and the final reading. It will be noted that the color intensity reached a maximum when 1.5 cc. of pyridine solution were used, and fell off rapidly with higher concentrations of the base. It was found that if the initial sulfuric acid concentration greatly exceeded 0.008 N the coloration of the solution was very faint and fugitive. The addition of a corresponding amount of pyridine did not stabilize the color.
CHEMISTRY
237
observed that when the copper concentration was less than 307 per 50 cc., the color reached a maximum 5 minutes after the addition of the pyridine and then faded slowly. In higher concentrations, however, the color intensity reached a maximum almost immediately after the addition of the base and faded rapidly. Because of the difficulty of making an accurate color comparison between two solutions, each of which is fading at a different rate, it was deemed advisable to establish a series of secondary standards whose color would closely approximate that produced by known amounts of copper. Fortunately the color of dilute permanganate solutions is almost identical with that produced by the Clarke-Jones reaction. The instrument was calibrated in terms of copper by treating known amounts of copper in the manner described. One cubic centimeter of 10 per cent pyridine solution was used and 5 minutes were allowed to elapse between the mixing of the pyridine and the recording of the color concentration. The instrument was then calibrated in terms of 0.002 N potassium permanganate by adding varying amounts of the permanganate solution to 50 CC. of conductivity water. The relationship between copper and the volume of permanganate required to match the color exactly is shown in Figure 3. This curve has been checked independently by F. Norman Pansch working in this laboratory. It should be mentioned that this relationship holds only under the conditions specified. Tf
n i n e in' Minutes P I W J R E L.
LUMPARISUN U P U N K N O W N
W l l H DI'ANUAKUb
The four curves are illustrative of a large number of data uub~tiueuiui
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per 50 cc. of solution (ly = 0.001 mg.). Pyridine in amounts greater than 3 cc. of 10 per cent solution caused rapid fading. One series of observations indicated that if 5 cc. of the base were used, the scale readings dropped approximately 25 per cent in less than 10 minutes. A significant decrease in the initial concentration of sulfuric acid gave erratic results. An effort was made to find a buffer mixture which would maintain the acidity of the solution a t such a value as to provide maximum color development. All such buffers studied either detracted from the sensitivity of the reaction or caused too rapid fading. Clarke and Jones made reference to the phenomenon of color fading and recommended a rapid comparison of unknowns with standards in order to minimize the error from this source. Inasmuch as the instrument used in the present work was particularly well adapted to a quantitative evaluation of color change, a series of periodic observations was made on a group of samples. The results of some of the observations are shown in Figure 2. The solutions under observation were prepared in the regular manner and the wedge readings recorded for intervals of 1 minute. One cubic centimeter of pyridine was used in each case. It will be noted that the fading in dilute solutions was slight. It was
7
cl Copper FIGURE
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C O P P E R US. V O L U M E OF
cc.
PERMANGANATE
the initial concentration of sulfuric acid exceeds 0.008 N , the colors will be lighter. It is recommended that the permanganate standards be checked under the conditions to which the'determination is being adapted. The permanganate standards may be stabilized by adding to each tube a small amount of potassium iodate and sulfuric acid. Silver nitrate greptly in excess of the quantity recommended by Clarke and Jones tends to promote rapid fading and introduces a yellow coloration which interferes with accurate estimation of the copper present. Significant increases in the initial concentrations of ammonium persulfate causes rapid fading in a short time, owing, no doubt, to the increased acidity resulting from the decomposition of this oxidizing agent. Sulfates and nitrates appear to be without effect on the determination. Sodium sulfate, magnesium sulfate,
238
ANALYTICAL EDITION
calcium sulfate, and potassium nitrate in concentrations as high as 2 mg. per cc. do not appreciably affect the intensity or the stability of the color. Although the turbidity produced by silver chloride may be discharged with an excess of pyridine as recommended by the original authors, the color intensity for a given amount of copper is greatly reduced. If the chloride content exceeds 0.57 per cc., the method is not reliable and should be used with caution. Iron in amounts greater than 27 per cc. introduces serious error. Cobalt in concentrations greater than 0.027 per cc. likewise renders the determination unreliable.
Vol. 4, No. 2
LITERATURECITED (1) Ansbacher, Remington, and Culp, IND.ENG.CHEM.,Anal. Ed., 3, 315 (1931). (2) Birtzzo, Ann. chim. applicata, 16, 2 (1926). (3) Clarke and Jones, Anatgst, 54, 333 (1929). (4) Elvehjem and Lindow, J . Biol. Chem., 81, 436 (1929). (5) Gebhardt and Sommer, IND.ENQ.C E ~ MAnal. . , Ed., 3,24 (1931). (6) Jones, L. A., J . Optical SOC.Am., 4,421 (1920). (7) Jones, L. A., Am. Dyestuf Reptr., 13, 121 (1924). (8) Sohonheimer and Oshima, 2. physiol. Chem., 180, 249 (1929). RECEIVED October 22, 1931. From the senior thesis of John 9. Chambers, University of Wisconsin, 1931.
The Metal Tube in Micro- and Semimicrocombustion Analysis S. AVERY,J. BRACKENBURY, AND W. D. MACLAY, University of Nebraska, Lincoln, Neb.
P
REVIOUS work ( I , 2) showing the advantages of metal tubes for macrocombustion analysis has been confirmed by Klatschin (4). The present communication deals with the use of metal tubes for micro- and semimicrocombustion analysis. Three types of copper combustion tubes provided with water-jacketed ends were used. Each was especially adapted to the analysis of compounds of a certain type. A silver tube, applicable to either micro- or semi-microanalysis, was used for compounds containing halogen. Qualitative tests showed that such a tube, with a filling composed of copper oxide and silver-coated copper oxide, completely retained the halogen. A boat inserter was designed to facilitate the introduction of the boat into the combustion tube. A similar inserter employed as a boat in the nitrogen determinations aided in the introduction of the sample mixed with copper oxide. The writers of the present article have found that the use of metal tubes embodies in micro- and semi-microcombustion analysis the same advantages ( I , 2) over tubes made from other materials, as previously found in macroanalysis. COMBUSTION TRAIN The current of air, oxygen, or nitrogen to be regulated and freed from impurities was passed through a pressure regulator (5),a metal preheater (Z), a bubble counter (see Figure l), and an absorption train made from the same reagents as are used in the weighed absorption train. Since a considerable amount of time was required to sweep out the complete apparatus when changing from oxygen to air, nitrogen to oxygen, etc., a double gas line was set up and the gases were brought together directly in front of the combustion tube. The gases were controlled by pressure regulators and screw clamps on the gas lines in $font of the bubble counters. When one line was in use, the other was shut off with the screw clamp. Diffusion a t the three-way connection of the gas lines was made negligible through the use of capillary tubing. The furnace used in this investigation was the ordinary threedelement electric furnace commonly employed in macro work, except in the case of copper tube I.. I n this instance, a furnace with one 18-cm. element and two 10-em. elements was employed. With an electric furnace and the double gas line, the minimum time necessary for accurate microanalysis of benzoic acid WBB found to be 15 minutes. Pregl’s absorption tubes were used in microdeterminations, and similar tubes of greater capacity were used in semi-microanalyses. By using as counterpoises a duplicate set of absorption tubes filled in the same manner as those of the
absorption train, fluctuations in weight caused by changing external conditions during the time necessary for weighing were found to be negligible.
CARBONAND HYDROGEN DETERMINATIONS IN VARIOUS COMPOUNDS COMPOUNDS WITH OXYGEN AS ONLYADDITIONAL ELEMENT. For strictly microdeterminations (as distinguished from semimicro) in such compounds, the assembly shown in Figure 1 was found most satisfactory. The copper combustion tube (I) had an inner diameter of 5 mm. and a length of 70 cm. It was provided with water-jacketed ends and protected externally by a jacket of nickel. One end of the tube was slightly distended and reamed out in order that a rubber stopper might be inserted. The absorption end of the tube was threaded. Into this was screwed, with a solder seal, a threaded bronze “adapter” with an outer diameter of 3 mm. abd a neck 2.5 cm. long. This eliminated the necessity of a rubber stopper at the absorption end of the tube. It was thus unnecessary to open the absorption end of the tube, the PUIUAM
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FIGURE1. DIAGRAM OF ASSEMBLIES absorption train was more easily attached, and the temperature maintained by the adapter through its conductivity prevented the condensation of water at the entrance of the dehydrite tube. The combustion tube was filled with copper oxide in the usual manner. I n order to facilitate the introduction of the boat into the combustion tube and to avoid spilling the sample or overturning the boat, a boat carrier was designed. This consisted of a copper rod 4 mm. in diameter, to one end of which was attached with silver solder a piece of silver shaped to hold the boat. The copper rod was then silvered for a distance of 5 em.
