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 K56zFe, 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,
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 11, SEPTEMBER 1974
Before use, thoroughly wash and hake all flasks overnight a t 450 OC. When the flasks have cooled, seal their mouths with cellulose tape to prevent contamination during storage. If, after use, the inner surface of a flask appears t o he coated with dehydrated silica, remove the coating by rinsing the flask with 2% H F solution. Scrub and rinse the supplemental air condensers before use. Reagents. Analyze all available hatches of reagent grade HC104 and select the batch lowest in mercury and/or other contaminants of interest. If the samples to he analyzed are expected to he very low in mercury, use HCIO4 containing less than 0.5 ng/ml Hg. Select the nitric acid in a similar way; if necessary, purify this acid by distillation. The permissible limit of contamination for "03 should he similar to that for HC104. The water used should contain no more than 0.02 ng/ml Hg; demineralized distilled water can usually meet this requirement. The 20-mg quantities of K2Cr207 used in the procedure did not contribute significant amounts of Hg. Preliminary Test Procedure. Before wet-ashing 1-5 gram quantities of any new type of material, test small (
