740
INDUSTRIAL AND ENGINEERING CHEMISTRY
thickness is not important. A tube of this type, preferably obtained from a broken thermometer, has the advantage of having reference lines conveniently etched, so that the motion of the liquid down the tube can be observed. The tube is cleaned by drawing cleaning solution through with the aid of water suction followed by distilled water. The tubeis dried by aspirating filtered air through the tube. The cleaned dried tube is then dipped into the unknown liquid, allowing a drop to hang from the bottom of the capillary. If this hanging drop is allowed to fall or be brushed off, no result will be obtained for then one has a narrow orifice a t both ends. The liquid rises in the tube. If the tube is tilted, the liquid in the tube moves up and down freely, an indication of the cleanliness of the tube. The tube is attached by a rubber band to the thermometer, so that the upper surface of the liquid in the capillary is level with the bulb of the thermometer. The apparatus is assembled as seen in Figure 2. For re.ading at room temperature, the apparatus may be used as shown in Figure 2. For readings at definite temperatures, a . constant-temperature bath of a tall-form beaker type surrounds the test tube. With a constant-temperature oil bath, a t higher temperatures, the vapor pressure of low-melting solids may be determined. The vacuum pump is turned on, stopcock G is closed. One hand is placed on stopcock E, the top surface of the liquid in the capillary is carefully watched, and a t the first sign of dropping, E iq turned so :ha+ the gage is isolated from the sys-
A
Vol. 17, No. 11
tem. The gage is read a t leisure. The gage was specially prepared so that pressures up to 450 mm. could be read within 0.5 mm. Ordinary commercial types will read only lower pressures. LITERATURE CITED (1)
(2) (3) (4) (5) (6) (7) (8) (9)
(IO) (11) (12) (13) (14) (15) (16) (17) (18)
Barker, 2. phys. Chem., 71, 241 (1910). Davis, IND.ENG.CHEY.,17, 1136-8 (1925). DBjardin, Ann. phys., 11, 253 (1919). Derby and Yngve, J . Am. Chem. SOC.,38, 1439 (1916). Edgar and Swan, I b d . , 44, 570 (1922). Francis, IND.ENG.CHEM.,ANAL.ED.,1 , 3 8 4 (1929). Granovskaya, J . Phys. Chem. (U.S.S.R.), 14, 759 (1940). Herriman, J . Chem. Soc., 103, 628 (1913). Hers and Rathmann, Chem.-Ztg., 36, 1417 (1912). International Critical Tables, Vol. 111, p. 212, New York, MuGraw-Hill Book Co., 1928. Kahlbaum, Z . p h y s . Chem., 13, 34 (1894). Meneies, J. Am. Chem. SOC.,42, 2218 (1920). Meyeren, Z. physiol. Chem., A160, 272-8 (1932). Moore, J. Soc. Chem. Ind., 39, 78-80 (1920). Morton, I b i d . , 38, 363 (1919). Natelson and Pearl, J . Am. Chem. Soc., 57, 1520 (1935). Sameshima, I b i d . , 40, 1488 (1918). Smith and Menzies, Ibid.,32, 1412 (1910).
Selective Spot Test For Copper PHILIP W. WEST
Coates Chemical Laboratories, Louisiana State University, Baton Rouge, La.
A test for copper has been developed based on the reaction between copper and dithiooxamide. By utilizing malonic acid and ethylenediamine to sequester interfering ions, all important interFerences with this familiar reaction have been inhibited; The test is capable of detecting 0.3 microgram of copper at a limiting concentration of 1 to 100,000, N o prior separations are necessary.
A
GREAT many sensitive tests are to be found for the detection of copper. Of particular significance is the fact that ropper readily forms complexes and therefore reacts with a large number of qrganic reagents, especially those where inner-complex salts are formed. Among the better reagents that have been proposed for use in the detection or determination of copper is dithiooxamide (rubeanic acid), This reagent was introduced by R$y and ROy ( E ) , and later studied by a number of other investigators (1, 2, 4-7, 9, 11). Feigl (3) reports a limit of identification of 0.006 microgram of copper at a limiting concentration of 1 part in 2,500,000 for this reagent. I n spite of its great sensitivity, dithiooxamide has not been generally applicable for the spot test detection of copper because of its reactions with other metals generally associated with copper, the most generally known being iron, nickel, and cobalt. Various means have been proposed for eliminating such interferences with the copper reaction. Willard and Diehl (11) were successful in preventing interferenccs due to iron by using citric acid. Feigl(3) suggests using capillary phenomena as a means of separation, so as to permit the distinguishing of the copper test stain when nickel or cobalt is present. The present article describes a method of eliminating interferences which is so generally applicable that it permits the direct identification of copper, even when it is present in mixtures with iron, nickel, cobalt, silver, mercury, etc. EXPERIMENTAL
A preliminwy investigation was made of the copper-dithio-
oxamide reaction t o determine the type and extent of interferences. Interference studies were made according to the general procedure described by West ( I O ) and included the following ions (although it is realized that many of the ions exist as complexes,
only the central atom and its valence are indicated in most cases) : Li+, Na+, K + C u + + Rb+, AgT, Cs+, Au++T, Be++, Mg++, Ca++, Zn++, brT+ d d + - , Ba++, Hg+, Hg++, Boa-, BaOl--, AI+-+. Sc+++. G$+++, I-+--. I n + t + La+-+ re++- TI+
Fe+r,’ Fe++r’ COT.+,’ Co+tS ’ N i + & Ruf$;t Rh++; pd+’+’ Og++t+.t?+T’ I r f r t ~ $ti-+++’ CN-: Fe(Ch)6---: Fe(CS)e---’, CSS-, kcetate, oxhate, malonate, tartrate, citrate, lactate, adipate, succinate, phthalate, gluconate, pyridine, and aniline. These preliminary observations disclosed that without special conditioning of the test drops, the following ions gave colored stains with dithiooxamide: copper, very dark green; cobalt, brown to greenish brown; nickel, violet; ferric, faint orange; silver, yellow turning to greenish black; bismuth, tan; mercurous, brown turning to black; palladium, brown; platinum, rose. The use of capillary separations was considered t o be unsatisfactory where so many possible interferences exist, and the use of’ ordinary separations baaed on precipitation and filtration detracts from the elegance of spot tests. Attention was directed, therefore, toward the application of complex ion formation as a means of sequestering interfering ions. After investigation of the more common complex formers failed to disclose any which could be considered satisfactory, efforts were directed toward the disclosing of new complexes, particularly of the inner-complex salt type. Malonic acid was found to form such complexes, and of special interest was the fact that the copper complex was relatively unstable as compared to the complexes of most interfering metals. A test procedure w&s then devised which consisted of adding malonic acid to a drop of the solution t’o be tested, then adding the reagent. This procedure proved very satisfactory in preventing such interferences as those due to iron, cobalt, and nickel. However, silver remained as a definite and important interference until the procedure was further modified to include the adding of a drop of ethylenediamine.
November, 1945
ANALYTICAL EDITION RECOMMENDED TEST PROCEDURE
Place a drop of malonic acid (20% aqueous solution) on a piece of spot test paper, then add a drop of the solution to be tested. Next. add a dron of ethvlenediamine (10% mueous solution) folldwed by a dcop of t h i reagent (1%dithjooGamide in 95% ethyl alcohol). If copper is present, a green stain is obtained. REMARKS
A study of possible interfering effects due to the presence of other ions was made. One per cent solutions of the various substances listed above were subjected to the test procedure; no positive interferences were found. Mixtures consisting of one drop of 0.005% copper solution and one drop of 1.0% solutions of substances to be studied far interfering effects were next tested. Thiosulfate was found to give a negative interference. Auric gold inhihited the formation of the copper test, but to a lesser extent. The only other effects noted were the formation of yellow flecks when silver was present, the dcvelopment of brawn stains in the presence of bromate and iodate, and the formation of rust and rose colored staim in the presence of palladium and platinum, respectively. The yellow flecks due to silver do ?ot interfere significantly but those due to bromate, iodate, palladium, and platinum do when present in considerable excess over the concentration of copper present. An important consideration is the fact that copper is easily detected in the presence of such highly calmed ions as iridium and permanganate, since the interfering ralars diffuse from the test aone. t is 0.3 microgram a t a
241
limiting concentration of 1 to 100,000. These values are for 0.3-ml. drops diffusing freely into S. and S. spot test paper No. 601. Greater sensitivity can bo obtained using confined spots or Nessler tubes. The pH of the solution t o be tested should be below 7 in order t o prevent possxble interferences. If the pH fdlsbelow a value of 3, the test becomes less sensitive. Dithiaoxamide, ethylenediamine, and malonic acid all give solutions which are stable for indefinite lengths of time. The solutiom required for the test have been used with excellent results in these laboratories after having been prepared for over a year. LITERATURE CITED
(1) Alport, N. L., and Skirmish, G. H., Quart. J . Pkwm. Pharmocol.. 1932). J., and M d n t o s h , R. M., IND.ENQ.CHEW.,ANAL. (1945). pot Tests", pp. 46-9, New York, Nordemsn 20.. 1937. (4) Feigl. F., and Kapulituas, H. J., MikrodLemb, 8, 239 (1930). ( 5 ) Nilsson, G.. Analgst. 64, 501 (1939). (6) RBy. P., Z. anal. Chem., 79, 94 (1929). (7) Riy, P., and Dhar, H. J., J . I d i a n Chem. Soc., 5 , 4 9 7 (1928). (8) Ray, P.. and Ray, R. M., Quart. J . Indian Chem. Soc., 3, 118 (1926). (9) Ringbom. A.. and Sumdrnan, F., F k s k a Kmistsamfundiet Mead., 51,42 (1942). (10) West, P. W., J . Chem. Education, 18, 528 (1941). A n i l v- ~ ~ (11) Willard, H. H., and Di oh1 -..., H .. "Arlvan,-~rlOuant>tative.~ ais", p. 285, New York, D. Van Nostrand Go., 1943.
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cIil-Sealed
v
N ANALY'TICAL PROCEDURES
Spherical Joints for High-V 'acuurn Application
CLARK E. THORP
E
AND
H. L. L A N D A Y . Armour Research Foundation, Chicago 16, 111.
XPANDING re:quirements for eleot.ron chemistry research at Armour R esearcb Foundation have necessitated the " ~ " . + m , r t i n n nf -.age, Ir ~.v..lll.lll.".. highly kinetic vacuum system capable of providing two lzboratories with vacuum facilities in the range of 10-6 to 10-8 mm. of mercury pressure. Previous experience has shou-n that ordinary unsealed, commercially ground glass joints are unsuitable at pressures below 10-4 mm. of meroury. By carefully regrinding the joints by hand, it is possible to use ordinary unsealed joints and attain pressures as low as 10-6 mm. Mercury-sealed joints are provided by many companies for pressure and vacuum applications, but have been found unsuitable for a pressure of less than 10-6 mm. This is to be expected when consideration is given to the fact that mercury has a vapor pressure of 1.2 X 10-8 mm. a t 20"C . Substitution of alow vapor pressureailsuch asApiesonoi1 (vapor pressure a t 20' C.) for the mercury will allow pressures as low as mm. to be obtained u'ith the same type of joint. I n large all-glass systems, provision must be made to prevent rigidity and allow for expansion and contraction of the glass plus ordinary vibration and the st.resses set up by manipulation of large stopcocks on the system. Spherical joints provide a movement of greater than 10' in any direction and are the obvious solut.ion t o the problem. For vacuum systems designed t o operate a t less than 10-6 mm. of mercury pressure, however, the spherical joint must be provided with an oil seal. It is surprising
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P n s s ~ m beiore ~ o the Division of Analytical and Micro Chemistry, Meetingin-Ptint 01 the ANERIOAN Cammu. S O C I E TSeptember. ~. 1945.
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that at present no company manuiacwres a spnencal J O ~ I equipped with an oil or mercury seal. The construction of such a joint may be accomplished, however, by ordinary glassblowing technique. The fallowine descrintion illustrates method used :arch Foundation to construct an oil-sealed spheria t Armonr R,e.% cal joint frc)m I an ordinary commercial 35/20 spherical joint. ~~
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A short le:ogth of 75-mm. Pyrex tubing is drawn to a point at one end, the point is flattened, and a thin-walled bulb is blown in the center of the flat. The bulb is broken and the hole enlarged t o diameter Of 28
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c)imenrionr
and
Conrtructional Details of Oil-Sealed
M a l o Joint
