2-(0-Hydroxyphenyl)-benzoxazole as a Reagent for Gravimetric Determination of Cadmium JOSEPH L. WALTER AND HENRY FREISER Department of Chemistry, Ciiicersity of Pittsburgh, Pittsburgh, Pa.
This work is an outgrowth of one of the findings of the organic reagents program at the University of
may be determined in the presence of copper by prior precipitation of the latter at a lower pH with this reagent. Nickel and cobalt interfere, if present in amounts in excess of 20 mg. The reagent appears to be more selective for cadmium than any other previously used for the determination of macro quantities. The method described is fairly sensitive, the procedure is simple, and the precipitate formed has a comparatively small gravimetric factor. The precipitate is thermally stable and rather insoluble in water and most organic solvents.
Pittsburgh, sponsored by the Atomic Energy Commission. Since qualitative tests with the reagent, 2-(o-hydroxyphenyl)-benzoxazole, indicated the possibility of its application to estimation of cadniiuni, this was attempted. I t was found possible to determine gravimetrically from 1 to 80 mg. of cadmium to an average accuracy of 0.3 mg. By carrying out the precipitation at a pH of 10.5 in a tartrate buffer, virtually all interferences are eliminated. Cadmium
at about 90" C. and evolved ammonia and water. After 1 hour the temperature was raised to 200" C. and maintained there until the ammonia evolution ceased. The total heating period was about 4 hours. The melted miyture was transferred into a distilling flask and distilled under atmospheric pressure. At 338340' C. the 2-(o-hydroxyphenyl)-benzoxazole distilled over to give a red colored mass, which nas recrystallized from ethyl alcohol three times. The pure compound melted at 124" C. The yield was about 48%. The solid compound fluoresces strongly in the ultraviolet with a green color Its alcoholic solution has a strong blue fluorescence. Standard Cadmium Solution. Exactly 1.6848 gram of cadmium metal (obtained from the Anaconda Copper Mining Co. and assayed a t 99.927, purity by W. E. Wallace and J. AI. Singer) was dissolved in dilute nitric acid and the volume brought to 1000 ml. in a calibrated volumetiic flask. Aliquots of this solution were taken as needed. Reagent Solution. A 1% weight/volume solution of the reagent in 957, ethyl alcohol was used. Met@ Ion Solution. The reagent grade nitrates of barium, magnesium, zinc, mercury, aluminum, chromium, iron, manganese, silver, copper, cobalt, and nickel were dissolved in distilled water to make a solution that contains ap roximately 50 mg. of metal per nil., for use in testing the specgcity of the reagent. Apparatus. All pH measurements were carried out using a Model G Beckman pH meter.
T
HE reagent, 2-( o-hydroxyphenyl)-benzoxazole, here applied to the determination of m c r o amounts of cadmium seems to be more selective than any other reagent mentioned in the literature ( 2 ,5 ) . The most popular reagents now in use for the determination of cadmium are 8-quinolinol, quinaldic acid, and anthranilic acid. Although these reagents may be used with success, no other heavy metals may be present in solution (save a few of the alkaline earths in the case of anthranilic acid). a-(o-Hydroxyphenyl)-benzoxazole may be used for the determination of cadmium in the presence of most of the common metallic ions with a minimum of interference. These few interferencesnamely, copper, nickel, and cobalt-my be completely eliminated by following a simple separation procedure. Copper may be precipitated with the reagent in acid solution, and nickel and cobalt, if present in excess of 20 mg., m y be separated by the use of dimethylglyoxime and 1-nitroso-2-naphthol. In quantities less than 20 mg., they do not interfere. When an alcoholic solution of the reagent is added to an alkaline solution containing cadmium ions, a precipitate is formed which when dried at 130' C. corresponds to the formula Cd(CI3H&S)?. The following equation best describes the reaction involved:
QUALITATIVE REACTIONS OF 2-(o-HYDROXYPHE\YL)BENZOXAZOLE
Qualitative tests were performed upon several of the common ions as listed below ( I ) . In acetic acid-sodium acetate buffered solution, 2-(o-hq-drosyphenyl)-benzoxazole precipitates copper, cobalt, and nickel. I t does not form a precip2H+ itate with calcium, barium, magnesium, zinc, cadmium, mercury, aluminum, lead, arsenic, bismuth, chromium, manganese, iron(III), sodium, and potassium. In an alkaline sodium tartrate solution only copper. cadmium, cobalt, and nickel are precipitated.
+ Cd2
PROPERTIES OF CADMIUM PRECIPITATE OF Z-(o-HYDROXYPHENYL)-BENZOXAZOLE
Thermal Stability. A quantity of the dry yellow cadmium precipitate was introduced into a capillary melting point tube and heated slo\vly. ? i o change was noted until 275OC., a t which temperature a darkening of the precipitate was noted. S o sign of melting was noted even a t 300' C. It wm ooncluded that bet\veen 130' and 140' C. was a safe and sufficient d p i n g teniperature for the precipitate. At this temperatue the precipitate could be heated to constant weight in 2 hours.
REAGENTS
2-(o-Hydroxyphenyl)-benzoxazole. The method of Siegfried and lloser for the preparation of this conipound was followed n ith pome modification (4). One mole of o-aminophenol and 1 mole of salicylamide were mixed in a mortar until finely powdered, then heated in a roundbottomed flask immersed in an oil bath. The mixture melted 984
V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2 Composition. A microanalysis was performed a t the uriivcrsity nlicroamlytical laboratory to determine the amount of nitrogen present in the precipitate. The reported percentage of nitrogen wm 5.217,. The percentage of nitrogen calculated for Cd(C,,H,GN), is 5.25%. Further proof of the validity of this structural formula is found in the fact that 0.2109, the factor ior cadnliurn in Cd(C1sHsOJ-T)?,was used in the cz xlations for all the cadmium determinations with good results. UI ganic
985 are p~esent. The effect of pII upon the precipitation of c.adniium in the presence of tartrate was also investigated. The procedure described above was followed, except that 2.0 grams of sodium tartrate were added before t,he solution was heated to 60' C., and 0.5 .V sodium hydroxide was used to adjust the pH, rather than ammonium hydroxide. The per cent preripitation is plotted against pH in Figure 1 (curve B ) . The qualitative tests with the reagent indicated the possibility of separating cadmium from copper, cobalt, and nickel. I t n-as found possible to separate cadmium from copper by precipitation of copper in acid solution with 2-(~-hydroxyphenyl)-l)enzoxazole as copper is qwiititatively precipitated a t a pH of about 3.5 to 4.0, and catfniiuni does not begin to precipitate until pH 6.5 in the presence of tartrate. Cadmium is then determiiieti in the filtrate a t a pH of 11 to 12. When a great exes5 of ammonium tartmte is prescmt in solut,ion, no metallic ion, escrpt cobalt and nickel in excess of 20 mg., will interfere with thc p1.ecipitation of cadmium.
Table I. Precipitation of Cadmium Cadmium Taken, Gram 0.0421 0.0421 0.0421 0.0421 0.0421 0.0421 0.0421 0.0421 0.0421 0.0421 0,0425 0.0425 0.0161 0,00842 0 0016
Precipitate, Grain 0.2007 0,2001 0.1993 0.1994 0.2024 0.2001 0.2012 0.2002 0.2006 0.2020 0.2013 0.2017 0.0760 0.0401 0.0081
Cadmium Found, Gram 0.0423 0.0422 0.0420 0.0420 0.0426 0,0422 0.0424 0,0422 0,0423 0.0426 0.0424 0.0425 0.0160 0.00846 0.0017
-~
Error, Grnlll
+o. 0002
t0.0001 -0.0001 -0.0001 - 0 0005 -0.0001 -0.0003 TO.0001 T O . 0002 -0.0003 - 0 0001 0 0000 - 0 0001 - 0 00004 -0 0001 ~
~
__
pH of Preciptation
Figure 1. EEect of pH on Precipitation of 2-(o-Hydroxyphenyl)-benzoxazole Complex of Cadmium A.
R.
W i t h o u t tartrate W i t h tartrate
Solubility. The solubility of the cadniiuni chc1:ttes in mrious organic solvents was determined in a quantitative mai~ner. The ~)recipitatewas found to be insoluble or only very slightly soluble i l l t.tiiyl alcohol, methanol, hexane, acetone, chloroform, carbon trti,achloride, and benzene. It was veq- soluble in glacial acetic acitl, giving a solution that had an orange tinge which fluoresced with a bright blue color under ultraviolet light. The ultraviolet epwtra curves of both the reagent itself and the cadmium chelate Iwirig superiniposable, it was concluded that acet,ic acid decompo..t.(I the chelate rather than dissolving it. EFFECT OF pH ON PRECIPIT.ATION OF CADAIIUAI
I n order to find the optimum pII range of precipitation of cn~liiiiuni,the following series of esycrinients was carried out.
.I tlefinitc amount (25.00 ml.) of the standard cadmium solution (containing 0.0422 gram of cadmium) was diluted to 100 nil. v i t h distilled water and heated to 60" C. and 25 ml. of the alcoholic reagent solution (containing 0.180 gram of reagent) representing about 5 to 10% excess was added. The pH was then adjusted to the desired value by adding dilute ammonium hydroxide. The precipitate was digested a t 60' C. for 15 minutes, allowed to cool to room temperature, filtered through a mediumporosity sintered-glass crucible, and dried a t 130" to 1400 C. to constant weight (at least 2 hours). The per cent prwil)it:rtr m x p plotted against pH as shown in Figure 1 (curve A ) . 9 s the pH of complete precipitation is so high, the precipitation must be carried out in the presence of some coniplesing n ~ t e r i a lsuch &s tartrate in order to prevent th(b torniation of h>-dromoxides of such metals as aluminum a n d iron \\.hen theso
It is advantageous to iwdi the precipitate thorouglill- with 50:; alcohol to make sui'(?that the precipitate is free of all excess reagent. This alcoholic solut.ion should be made bmic with a little sodium hydroside solution, for a solution made wi%hordinary 05% ethyl alcohol and tlistilled Lvater gave a p H reading of G.4. This solution, being slightl!. acid, dissolved a small amount or the precipitate. RECOM.UENDED PROCEDURE
Procedure for Cadmium. To a solution containing from 1 to 80 mg. of cadmium, and which may contain large amounts of most metals, up to 100 mg. of copper, and a maximum of 20 mg. of nickel and cobalt, add 3 grams of ammonium tartrate. Heat the solution to about 60" C.; if calcium is present, a cryst:tlline precipitate of calcium tartrate will be formed. Filter off this precipitate and add to the filtrate 1 gram of ammonium tartrate. Add 3 iV acetic acid until t,he pH is about 3.5 and heat the solution again to 60" C. .Idd it fresh!y prepared alcoholic solution of the reagent containing a slight excess of the amount needed for the complet,e precipitation of t'he copper, nickel, anti cobalt. present in solution. Adjust the pH to 4.0 with 3 N sodium acetate solution. Then digest the precipitate of the copper salt a t 60' C. for 15 minutes, sllom it to cool to r m m temperature, filter through a medium-porosity sint'ered-glass crucible, and wash several times wibh water. This procedure will ensure complete separation of copper and calcium from the solution. Kickel and cobalt, if present, in large amounts, must be removed before the cadmium is precipitated (dimethylglyoxime and 1nitroso-2-napht'hol may be used, 3 ) . Make the filtrate, which contains cadmium and the tartrate complexes of such ions as barium, magnesium, zinc, mercury, aluminum, bismuth, chromium, iron(III), and manganese, basic with a 1 N solution of sodium hydroxide, or until the pH is approximately 9. Heat the solution to 60' C., and add sufficient reagent solution to cont,ain in slight excess the amount necessary to precipitate completely the cadmium present in solution. I n the procedure given, only a small excess of reagent was used ( 5 to 10%); however, up to 75% may be completely washed free of the precipitate with :t 50% alcohol solution. T h r n rake
986
ANALYTICAL CHEMISTRY
the pH to 11 with 1N sodium hydroxide solution. Digest the precipitate a t 60’ C. for 15 minutes and allow it to cool to room temperature. Filter the precipitate through a weighed mediumporosity sintered-glass crucible, wash several times with ammoniacal 50% alcohol solution, and dry a t 130” t o 140’ C. to constant weight (at least 2 hours). Calculate the actual weight of cadmium present in the sample analyzed by multiplying the weight of precipitate by the gravimetric factor 0.2109. Results of the precipitation of cadmium with this reagent in both the absence and presence of interfering ions can be found in Tables I and 11. CONCLUSIONS
The reagent is very easily prepared in good yields. The method described is fairly sensitive, the procedure is simple, and the precipitate formed has a comparatively small gravimetric factor. The precipitates are thermally stable and very insoluble in water and most organic solvents. The reagent is highly selective for cadmium and gives results that are accurate to somewhat better than +0.0002 gram in samples containing from 5 to 50 mg. of cadmium and in the presence of most metals except cobalt and nickel. These can be tolerated only in very small quantities, giving resulte which are accurate to 10.0004 gram. ACKNOWLEDGMENT
The authors gratefully acknowledge the support of this work by the iltomic Energy Commission.
Table 11. Precipitation of Cadmium
+
Metal Ion in ion G. Ba 0,100 0.100 0.100
2
2 Pb
Cadmium Taken, G. 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0.0424 0,0424 0.0424 0.0420 0.0424 0.0424
0.100 0.100 0.100 0.100
Cr
0.100 0.100
A8
Bi Mn 0.100 Fe(II1) 0.100 Ca 0.100 cu 0,050 co 0.050 Ni 0.0~0 Ni Co 0 . 0 ~ 0each
+
Cadmium Found, G. 0.0426 0.0425 0,0422 0.0422 0.0432 0.0433 0.0420 0.0421 0.0422 0.0420 0.0415 0.0420 0.0428 0.0476 0,0447 0.0493
Error,
G. +0.0002 +0.0001 -0.0002 -0.0002 +o. 0008
+o. 0009 - 0 0004 +o. 0002
-0.0002 -0.0004 -0.0009 - 0 0004 +O 0004 +O 0052 +O 0023 4-0.0069
LITERATURE CITED
(1) Charles, R. G., and Freiser, H., unpublished research.
(2) Flagg, J. F., “Organic Reagents,” New York, Interscience Pub-
lishers, 1948. (3) Hillebrand, FV. F., and Lundell, 0. E. F., “Applied Inorganic Analysis,” Ken- York, John Wiley & Sons, 1929. (4) Siegfried, S., and bfoser, bl., Be?., 5 5 , 1089 (1922). ( 5 ) Welcher, F. J., “Organic Analytical Reagents,” Kew York, D. Van Nostrand Co., 1947. RECEIVED for review January 4 , 1952. Accepted March 17, 1952. tribution 847, Department of Chemistry, Cniverslty of Pittsburgh,
Con-
Primary Coulometric Determination of Iron(ll) and Arsenic( Ill) New Method f o r Current Summation WILLI.431 M. MAcNEVIN
AND
BERTSIL B. BAKER
Department of Chemistry, T h e Ohio State University, Columbus, Ohio This work had tw-oprincipal objectives: to carry out the primary coulometric oxidation of Fe” and As”‘ in acid solution at a platinum anode, and to investigate the potentialities of a new method of estimating the total number of coulombs of electricity used in a primary coulometric analysis. Iron and arsenic were successfully determined using a hydrogen-oxygen coulometer, with a relative accuracy of about 1%. It w-as shown that a plot of log current DS. time for theoxidation of iron gives a straight line and that from the values of the slope and intercept the number of coulombs used, and thus the amount of iron present, can be calculated with a relative accuracy of about 2%. The area of application of primary coulometric analysis has been extended to iron and arsenic. A new method for current summation obviates the necessity of running the electrolysis to completion and thus allows calculation of results after only 10 to 15 minutes instead of the 60 minutes usually required.
T
HE work reported here had two principal objectives. The first was to develop suitable procedures for the primary coulometric oxidation of iron(I1) and arsenic(II1). The second was to investigate the potentialities of a new method of estimating the total current (coulombs) used in a primary coulometric analysis. The determinations of iron(I1) and arsenic(II1) are of interest for several reasons. Neither element has been determined in a primary coulometric determination. Both reactions involve changes between higher valence states, and because the deposition of a solid phase is not involved, these reactions could not be used in classical electrogravimetric analysis. As the electrolytic oxidation of iron proceeds smoothly and rapidly, it was expected
to provide an ideal case for testing the new method of current integration. The electrolytic oxidation of arsenic(II1) in strong acid solution had been previously suggested (6) as a possible coulometric reaction. I n the early part of this work the oxidation of iron(I1) and arsenic(II1) was studied using a hydrogen-oxygen coulometer. I n the latter part the total current used was estimated from the slope and intercept of the curve of the log current us. time for the electrolysis. The expression used is mathematically derived from the following equation, developed by Lingane ( 2 ) for the current in an electrolysis in which a single reaction is proceeding a t 100% efficiency:
