Spectrophotometric Determination of Copper(1) and Cobalt(l1) with Ferrozine S. K. Kundra, Mohan Katyal,' and R.
P. Singh
Chemistry Department, Delh! Unwersity, Delh, 7 70007, and St Stephen's College, Delhi 7 70007, lndia
Based on the selective reactivity of the compounds containing the ferroin grouping 1
~~
Table I. Simultaneous Determination of Copper and Iron in Synthetic Mixtures
:
.-N.=C-C=Nwith iron(II), copper(I), and cobalt(I1). such compounds have been used in determination of these metals ( I , 2). More recently, Stookey ( I ) introduced ferrozine (registered trade mark of Hach Chemical Co.)-the disodium salt of 3-(2-pyridyl)-5;6-bis(4-phenyl sulfonic acid)-1,2,4triazine-for the spectrophotometric determination of iron(11). Later on, this reagent was used for the indirect determination of sulfur dioxide (3). Based on its formation of colored complexes with copper(1) and cobalt(II), its use is now extended for the spectrophotometric determination of these two metals. Also, a method is developed for the simultaneous determination of copper and iron when present together. In selectivity and sensitivity, the present compound is comparable to its precursors. The main advantages of its use are: the solubility of the reagent and its complexes in water (extraction not required) and its comparative low cost.
Copper taken, mg,l.
Iron taken, rngll.
Copper found,
rng/La
Iron found. rng/l."
2.62 2.62 2.62 5.24 10.48
1.68 0.56 1.12 0.56 0.56
2.60 2.61 2.62 5.30 10.50
1.70 0.56 1.12 0.58 0.56
Average of 8 samples.
Table 11. Determination of Copper and Iron in Water Samples
Source
Present address, St. Stephen's College. Delhi 110007. India. Stookey. Ana/. Chem.. 42, 779 (1970). Sandell, 'Colorimetric Determination of Traces of Metals," 3rd ed.. Interscience, New York, N . Y . , 1959, pp 429 and 451 (3) A Attar1 and B Jaselskis.An,s/ Chem.. 44, 1515 ( 1 9 7 2 ) . (1) L L (2) E B
Copper found, mg 1 "
Iron found, mg/l."
0 50 1 00 1 50 2 00
0 25 0 03 0 52 1 03 1 53 2 02
1.59 0.34 0.34 0.34 0.34 0.34
Well water, N a r e l a
Delhi University Tap water
EXPERIMENTAL Apparatus. A Unicarn SI' 600 spectrophotometer and a Metrohm pH meter, type E-350 were used. Reagents. Ferrozine was procured from Hach Chemical Co., U.S.A. It was grossly impure and needed recrystallization from water at least three times before use. The stock solution (10-2M) of the recrystallized sample was prepared by dissolving an appropriate amount 15.140 grams per liter) in doubly distilled water. Acid Reagc'nt Solution. I t was prepared in accordance with Stookey's method ( 1 ) ; 5,140 g of ferrozine and 100 g of hydroxylamine hydrochloride were dissolved in water, 500 ml of concentrated hydrochloric acic added, and the solution was finally made up to 1 liter with doubly distilled water. Copper Solution. This was 249.68 mg of copper sulfate pentahydrate (Analar, British Drug Houses) per liter for 10-3M. Cobalt Solution. Pure metal powder was dissolved in hydrochloric acid, the solution evaporated to dryness, and the residue extracted with doubly distilled water (1.1780 grams per liter for 2 X 10- 2Mstock solution). Iron Solution. Hydrated ferric oxide was precipitated from ferrous ammonium sulfate. and the former was dissolved in appropriate concentration of hydrochloric acid. The iron content was checked gravimetrically Hydroxylamine H3,drochloride. Ten grams of' the compound (Analar, British Drug Houses) per liter gives a 1%solution. Huffrxs. Buffer solutions were prepared by mixing appropriate amounts of potassium dihydrogen phosphate ( 2 x 10-2N) with sodium hydroxide ( 2 x 10 ~ 2 A 7 . Procedure. Determination of Copper. To a suitable aliquot of copper Solution, add 1.0 ml of 1% hydroxylamine hydrochloride solution. boil for 5- 10 minutes. cool to room temperature, add 1.0 ml of ferrozine (10-2M) and 5.0 ml of buffer (to adjust pH around 7.0). hlake up the volume and note the absorbance at 470 nm us. rhe reagent blank prepared under similar conditions.
Copper added, mg 1
''
Average of 8 samples
Determination o,f Cobalt. To a suitable aliquot of cobalt solution, add 5.0 ml of ferrozine (10-2A.I) and 5.0 ml of buffer (to adjust pH around 6.5). Make up the volume and note the absorbance a t 500 nm us. the reagent blank. Simultaneous Determination of Copper and Iron. To a suitable aliquot of solution containing copper and iron, add 1.0 ml of Acid Reagent Solution, heat to boiling for 5-10 minutes. cool to room temperature, add 5.0 ml of buffer (to adjust pH around 7.0) and allow 5 minutes for full color development. Measure the absorbance a t 470 and 562 nm u s . the corresponding reagent blank. The results for the determination of copper and iron in synthetic mixtures and water samples are given in Tables I and 11.
RESULTS AND DISCUSSION Copper Complex. Copper(1) forms a water soluble complex with ferrozine. For accomplishing reduction of 5 x 10-5M of copper(I1) to copper(I), the presence of 1.0 ml of 0.1% hydroxylamine hydrochloride or 0.5 ml of 0.170 ascorbic acid was found necessary. However, the excess amount did not interfere. The characteristics of the complex are given in Table 111. Interferences. The criterion for an interference was an absorbance varying &5% from the expected value. At least u p to 1000 times the molar excess of each of fluoride, chloride, bromide, iodide, nitrate, perchlorate, iodate, sulfite, sulfate, borate, oxalate, citrate, phthalate, EDTA, thiourea, and phosphate: 100 times of each of alkaline earth metals, zinc, cadmium, mercury, aluminum, and manganese; and 10 times of each of tin, lead, and bismuth did not interfere in the determination of copper. Serious interference was caused by iron, cobalt, and nickel.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
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T a b l e 111. Characteristics of t h e Complexes Characteristic
Color
Brown 470 nm 4320 4-times
X”,, emax
Co(I1) complex
Cu(1) complex
W ine-red 450-520 nm 4643 80-times
Ferrozine needed for full color development (titration method) pH range for maximum 6.5-8.8 5.5-7.0 and constant absorbance Not ascertained Molar composition 1:2 (metal: ligand) by continuous variations method 0-20 pg/ml 0-6 pg/ml Beer’s law range of cobalt of copper 0.5-19.6 pg/ml 0.6-5.8 pg/ml Accurate range of of cobalt of copper determination 0.013 pg/cm2 Sandell’s sensitivity (4) 0,015 pg/cm2 0.002 0,002 Standard deviation (calculated from 8 samples
the species contains cobalt and the ligand in 1:3 ratio. The characteristics of the complex are summarized in Table 111. Interferences. In the determination of cobalt, at least up to lo00 times the molar excess of each of fluoride, chloride, bromide, iodide, nitrite, nitrate, thiocyanate, perchlorate, acetate, sulfite, sulfate, borate, oxalate, citrate, phthalate, thiourea, and phosphate; 300 times tartrate; 100 times of each of alkaline earth metals, zinc, cadmium, mercury, tin, lead, bismuth, and manganese did not interfere. However, the presence of copper, iron, and nickel caused interference. Simultaneous Determination. The magenta-colored ferrous complex of ferrozine absorbs maximum at 562 nm with molar absorptivity 27,900 ( I ) . On the other hand, A,, of the cuprous complex lies a t a much lower wavelength (470 nm). The two components were determined using the principle of additive absorbances and simultaneously solving Equations 1 and 2. K470CU C C U
+ K470Fe CFe = E1 (Total absorbance at 470 nm) (1)
K562CU CCU4- K56ZFe CFe
= E2
(Total absorbance a t 562 nm) Cobalt Complex. The cobalt complex exhibits a broad maximum a t 450-520 nm. The composition of the complex could not be accurately deduced by the application of the method of continuous variations. The limb on the excessligand side of the Job’s curve did not show significant decrease in absorbance after 1:3 (coba1t:ferrozine) composition was attained in the solution. However, it seems that ( 4 ) E B Sandell, ‘Colorimetric Determination of Traces of Metals ed Interscience N e w Y o r k N Y , 1959 p 8 3
3rd
(2)
where K470Cu, K562CU, K470Fe, and K 5 6 z F e , the molar absorptivities of copper and iron complexes a t the given wavelengths are 4320, 3300, 9760, and 27900, 1. mo1e-I cm - respectively.
RECEIVED for
review July 16, 1973. Accepted March 20, 1974. Financial assistance from the University Grants Commission under the program “Water Pollution” is gratefully acknowledged.
Perchloric Acid Procedure for Wet-Ashing Organics for the Determination of Mercury (and Other Metals) Cyrus Feldman Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
T o be useful for the determination of mercury, a wetashing procedure must meet the following requirements: 1) I t must prevent the loss of mercury (and/or other elements of interest). 2) I t must involve minimum risk of contamination. 3) I t must be applicable to large (55-gram) samples of a wide range of materials, including animal tissue (fatty and lean), vegetation, soils, and up to 2 grams of coal and petroleum products. 4) It must be rapid. 5) I t must require the minimum amount and the simplest types of apparatus. 6) It must permit processing many samples simultaneously, with minimum attention. 7 ) I t must be safe. Experience and a survey of the literature indicated that the G. Frederick Smith HC104-HN03 procedure ( I ) came closest to meeting these requirements, but that the procedure would have to be modified in order to fully satisfy requirements 4, 5, 6, and 7 if used on samples of the size and nature specified. The essential features of the present procedure are: careful control of refluxing and evaporation by the use of tem-
perature programming and an insulated air condenser, rather than a Bethge still ( I ) ; the use of certain minimum amounts of reagents per gram of sample; and use of the same vessel for digestion and storage. Three general types of behavior were observed in the wet-ashing of various samples, and three variants of the basic procedure were developed to accommodate them. The category of treatment for each new type of material was established by preliminary tests on small amounts of material.
( 1 ) G. Frederick Smith, Anal. Chim. Acta, 17, 175 (1957)
Calif.).
1606
EXPERIMENTAL Apparatus. Equipment used includes 250-ml borosilicate glass volumetric flasks with wide, flat bottoms ( e g . Kimax);supplemental air condensers made from ST 19/38 ground glass joints, with jackets of woven asbestos tubing held in place by wrapping with Teflon sealing tape (See Figure 1);variable temperature hotplates with specific power of at least 1.7 watts/cm* ( e . g . , Thermolyne Model 9425 or Type 2200) and sufficient area for the work load expected; surface temperature thermometers ( e . g . , PTC Model 314 C, manufactured by Pacific Transducer Company, Los Angeles,
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
