RESEARCH· ·1970·

GEOLOGICAL SURVEY RESEARCH 1970
DETERMINATION OF COBALT IN GEOLOGIC MATERIALS BY
SOLVENT EXTRACTION AND ATOMIC ABSORPTION SPECTROMETRY
By WAYNE MOUNTJOY, Denver, Colo.
Abstract.-Cobalt content in geologic materials can be accurately
and rapidly determined by use of a new solvent extraction-atomic
absorption analytical method. In this method
the cobalt is concentrated and separated from interfering elements
by extracting the 2-nitroso-1-naphthol chelate of cobalt
into methyl isobutyl ketone. A lower limit of 2 ppm in a 1-g
sample is readily attained. Analyses of seven reference samples
confirmed the accuracy of the method.
In the atomic absorption determination of cobalt in
aqueous solutions, significant interferences from Ca,
Mg, Na, K and Fe have been noted in an air-propane
flame (Angino and Billings, 1967). In addition, the
sensitivity of that method is inadequate for the determination
of cobalt in many geologic materials without
preconcentration. Extraction of an element into an
organic solvent is often used in atomic absorption work
to separate interfering elements, to concentrate the
element, and to enhance the sensitivity of the determination
(Allan, 1961). Mansell and Emmel ( 1965)
extracted the ammonium pyrrolidine dithiocarbamate
chelates of Co, Cr, V, Mo, Mn, and Ni from brine into
chloroform or methyl isobutyl ketone prior to their
determination by atomic absorption. This method is
not applicable to many types of rocks because iron is
also extracted with the cobalt and it interferes with
the determination of cobalt content.
This paper presents a solvent extraction-atomic absorption
method for the determination of cobalt in the
parts-per-million range in rocks. Interferences, such
as iron and calcium, are removed by a modification of
a method described by Sandell ( 1959) in which the
2-nitroso-1-naphthol complex of cobalt is extracted
into chloroform prior to its colorimetric determination.
His method has been modified for atomic absorption
and simplified by substituting the lighter-thanwater
solvent MIBK (methyl isobutyl ketone) for
chloroform~ Experiments using Co 60 tracer show that
approximately 98 percent of the cobalt complex is extracted
with a single extraction using 10 ml of MIBK.
The cobalt content of seven U.S: Geological Survey
reference samples, ranging in concentration from 5 to
140 ppm, has been determined and compared with results
obtained in other laboratories. The results are in
good agreement.
Reagents
EXPE.RIMENTAL METHOD
Standard cobalt solutions.-Transfer 0.5000 g of pure
cobalt metal to a liter volumetric flask. Add 25 ml of
concentrated hydrochloric acid and heat gently to dissolve.
Cool and dilute to volume with water. This stock
solution contains 500 pg/ml (micrograms per milliliter)
Co. Dilute 10 ml of this stock solution to 500 ml with
2.5 percent v/v hydrochloric acid (1 ml=10 pg Co).
Sodium citrate solution.-40 percent w /v in water.
~-nitroso-1-naphthol.-Dissolve 2 g of 2-nitroso-1-
naphthol in 100 ml of glacial acetic acid, add 0.5 g of
activated carbon, shake, and allow to settle before using.
This solution is stable for at least 6 months.
Instrument parameters and settings
A Perkin-Elmer Model 303 atomic absorption spectrophotometer
was used with the following operating
conditions :
Wavelength___________________ 2407 A.
Slit__________________________ 0.3 mm.
Hollow cathode current_ ______ _ 16 rna.
Acetylene flow setting _________ _ 3.5.
Airflow setting _______________ _ 5.0.
Burner ______________________ _
Standard head.
Scale expansion ______________ _ 2, or as needed.
Flame _______________________ _
Oxidizing.
B174
U.S. GEOL. SURVEY PROF. PAPER 706-B, PAGES B174-B176