A chromograph set complete with case.

Figure 1.


solutions are mixed by squeezing the rubber bulb, ACKNOWLEDGMENTS
which forces air to circulate the reagents through

the sample solution. The mixed solution is fil- Mr. H. E, Hawkes and Mr. T. S. Lovering,
tered and the confined spot that remains is washed geologists of the U. S. Geological Survey, col-
with acid and dried, It is then compared with a lected samples that were used in determining the
standard series of spots. cobalt content by the two methods.

REFERENCES CITED

Boyland, E., 1946, Separation of the cobalt complex of 2-nitroso-1-naphthol from other colored
metallic compounds: Analyst, vol. 71, pp. 230-231.

Deijs, W. B., and Feldmeyer, J. H., 1949, Microdetermination of cobalt in plant material: Plant
and soil, vol. 1, pp. 359-371.

Feigl, Fritz, 1946, Qualitative analysis by spot tests: 3d ed. New York, Elsevier Publishing Co.
pp. 111-113,

Hillebrand, W. F., and Lundell, G. E. F., 1929, Applied inorganic analysis: New York, John Wiley
& Sons, Inc., pp. 322-327.

Stevens, R. E., and Lakin, H. W., 1949, The chromograph, a new analytical tool for laboratory
and field use: U. S. Geological Survey Circ. 63.

INT.-DUP, SEC., WASH., D.C. ygou


Sodium hydroxide, N. Dissolve 40 g NaOH in
water and dilute to | liter.

Potassium bisulfate. Pulverize in a mullite
mortar.

Sodium citrate. Dissolve 50 g NasC¢H507.53H20
Jn water and dilute to 100 ml. Litmus paper, red.

Borate buffer. pH about 6.5. Dissolve 19 g
Na 2B,0 7. 10H, in 1 liter of water. Add 17
ml-constant boiling HCl.

Sulfuric acid. N. Add 28.5 ml conc H2SO,4 to
about 500 ml water and dilute to 1 liter.

Cobalt standard solution. 0.01 percent. Dissolve
0.40 g CoCl2, 6H 0 in water, add 1 ml of 1:1
HCl and dilute to 1 liter. Prepare more dilute
solution from this stock solution.

Water. Water passed through the resin de-
mineralizers now on the market is of satisfac -
tory purity. The Bantam Demineralizer sold by
the Barstead Still and Sterilizer Co. was used
in this work.

EQUIPMENT

Gasoline stove. A Coleman "GI" pocket stove
available in sports shops is satisfactory.

Fusion test tube.rack. Constructed of two disks

of sheet steel welded to a central supporting rod.
Each plate has eight holes for test tubes; the holes
in the bottom plate are small enough that the tubes
will not slip through but large enough to permit
the ready flow of heat.

Digestion rack. Similar in construction to that
described for the fusion rack, but modified to fit
into the utility cup of a‘Coleman "GI" pocket stove.

Filter sticks. Fuse a fritted pyrex disk of medium
porosity on one end of a pyrex glass tube, 7 mm
inside diameter and 130 mm long. (Developed by
R. E. Stevens, U. S. Geological Survey)

Test tubes, 16 x 150 mm (marked at 10 ml),
pyrex.

Dropping bottles. 6, pyrex.

Reagent bottles. 12, 250 ml, pyrex.

Balance. A torsion balance with a 0.01 g sensi-
tivity is satisfactory.

Ear syringes. 6,

Pipettes. 2-0.5 ml, serological.
2-2 ml, volumetric.
2-5 ml, volumetric.

Filter paper. Whatman no, 50 cut ingin. strips.

Mullite mortar and pestle. Outside diameter of
mortar 3 in.

PROCEDURE

Mix 0. 1 g of finely ground soil or rock sample
with 0.5 g pulverized potassium bisulfate in a
pyrex test tube and fuse for 5 min. Cool,
rotating the test tube so that the melt solidi-
fies in a thin layer on the sides of the test tube.

Add 5 ml of the sodium citrate solution and
digest in a boiling water bath until the lumps
of flux are decomposed.

Add 1 small piece of litmus paper, then N
sodium hydroxide dropwise until the litmus
turns blue.

Add 2 ml of borate buffer, followed by N sul-
furic acid to make the solution just acid. Di-
lute to 10 ml with water.

Place filter stick in test tube and apply suc-
tion to upper end of filter stick with an ear
syringe.

Insert a strip of filter paper in the chromo-
graph and adjust the water level to the base
of the connector tube (see fig. 2).

7. Transfer 0.2 ml of the filtrate in the filter
stick (from operation 5) to the filter head of
the chromograph and add 0.1 ml of the 2-
nitroso-1-naphthol reagent.

8. Open the stopcock to the rubber bulb and
squeeze the bulb until the water column is at
the top timing mark.

9. Turn the stopcock into position so that drain-
age takes place through the slow capillary.

10. After filtration is complete, add 1 drop of N
sulfuric acid, and drain again.

11. Write the sample number adjacent to the spot

on the paper.

Table 1.--Effect of pH on precipitation of elements with 2-nitroso-1-naphthol (citrate present).

Ha Quantity of precipitate from solutions of indicated percent concentration
Ni(0.1) Fe(5) Cu(O.1)* Co(0.01)
absent maximum absent absent
trace moderate absent trace
trace absent absent moderate
trace absent absent maximum
trace absent absent trace

No copper precipitation with 2-nitroso-l-naphthol in presence of citrate.

2


panies, protector
plug i

Capillary

Clean-out.
plug

Filter head

Filter base

=: |

Mg

—

Connector tube

3 way stopcock
Adjustor bulb tube

Adjustor
bulb

Figure 2. The chromograph assembled showing on capillary.

4


GEOLOGICAL SURVEY CIRCULAR 125

October 1951

A SEMIMICRO METHOD FOR THE
DETERMINATION OF COBALT IN SOILS
AND ROCKS: A FIELD TEST
USING THE CHROMOGRAPH

By

Hy Almond and Harold Bloom


12, Remove the strip of paper and allow the spot
to dry.

13. Wash and dry the chromograph head and parts
that come in contact with the filter paper to
remove traces of acid.

14, Compare the spots with spots of the standard
series.

15, Repeat the test on those samples containing
more than 400 ppm by diluting the sample
solution.

Standard "spot" series: Prepare a series of spots
ranging from 10 to 400 ppm as follows: To a
series of 0.1 g cobalt-free soil samples, add 1,
3, 5, 8, 10, 20, and 40 micrograms of cobalt
from standard cobalt solutions respectively.
Evaporate to dryness over a steam bath. Add the
flux, mix and prepare cobalt spots as described
in operations 2 to 12.

DISCUSSION

The direct fusion of soil or rock with potassium
bisulfate is based on the Hillebrand - Lundell
(1929) treatment of cobalt minerals in which
cobalt minerals are broken up by a preliminary
acid attack, followed by a fusion of the insoluble
residue. To simplify the decomposition proce-
dure, the acid attack was omitted.

Feigl (1946)describes spot test techniques for
the detection of traces of cobalt with 2-nitroso-
1-naphthol in the presence of iron and copper. He
coprecipitates iron and cobalt as the phosphate
and reduces copper (II) to copper (I). These steps
reduce the sensitivity of the.test about tenfold. In
the chromographic technique the interferences of
copper and iron are eliminated by using a citrate
solution at a controlled pH, and the test remains

Table 2.--A comparison of cobalt content of soils
determined by spectrophotometric and chromographic

procedures.
Cobalt (ppm)
Sample No. | Spectrophotometric | Chromographic

1 15 5
2 20 30
3 25 30
4 35 50
5 50 30
6 50 70
7 55 50
8 70 60
9 85 100
10 100 90
n 170 220
12 180 170
13 240 300
14 375 300
5 400 350

highly sensitive. ‘The reddish-brown precipitate,
cobalt nitroso-naphtholate, forms between pH
3.5 to pH 8.5. After filtration is complete, ex-
cess reagent is removed by washing with N
sulfuric acid.

Color differences on spots containing from
0.02 to 0.8 micrograms of cobalt can be deter-
mined by the test described here. When a solu-
tion contains more than 0.8 micrograms of
cobalt in the 0.2 ml aliquot, a spot of maximum
intensity is formed, and a visual comparison
with standard spots is not feasible; however, the
concentration of cobalt in such a solution can be
determined by dilution. Since the original sample
weighed 0.1 g, 20.2 ml aliquot from a 10 ml
sample solution corresponds to 1/500 of a 1 g
sample. The lowest detectable spot contains 0.02
micrograms of cobalt which corresponds to 10
ppm in the original soil; the highest comparable
spot contains 0.8 micrograms af cobalt which
corresponds to 400 ppm.

The effect of pH on precipitation of several
elements is shown in table 1. The quantity of
precipitate from a 0.1 percent nickel solution
was small at any pH. A 5 percent iron solution
gave strong dark colored precipitates below pH
of 3, but above pH 3 no precipitate was formed.
A 0.1 percent copper solution gave no precipitate
in the presence of citrate.

RESULTS

A comparison of the results obtained by two
methods of analyses is given in table 2. In the
laboratory (spectrophotometric) method, the
sample was decomposed with perchloric acid.
Copper was removed from an acid solution (pH
2.5 to 3.0) with dithizone. Cobalt was separated
from iron by extraction with dithizone at pH 8.
Cobalt dithizonate was destroyed with nitric acid,
and the solution evaporated to dryness. The
oxides were dissolved in acid and the cobalt in
the solution estimated with nitroso R salt. The
soils were high in copper and all contained large
amounts of iron. Neither of these elements inter-
fered. Except for sample no. 1, all results fall
within 40 percent of each other. The greater
number of steps required by the spectrophoto-
metric method may lead to greater errors than
those inherent in the simpler, chromographic
procedure. For the purpose of a quick test the
chromographic technique is accurate enough to
compare the cobalt contents of soil samples in a
given area. It is fast; one operator can do 25 to
30 determinations daily. Filtration is slow
enough to allow time for the cobalt to react with
the 2-nitroso-1-naphthol.

Copper and iron also react with 2-nitroso-1-
naphthol, but the cobalt determination described
here is successfully carried out directly even in
the presence of large amounts of these elements.
Citrate is used to prevent the precipitation of the
copper salt of 2-nitroso-1-naphthol and the reac-
tion with iron is prevented by carefully control-
ling the pH of the test solution (see table 1).

Traces of cobalt are determined by adding a
test solution to a solution of 2-nitroso-1-naphthol
in the head of the chromograph (see fig. 1). The


UNITED STATES DEPARTMENT OF THE INTERIOR
Oscar L, Chapman, Secretary

GEOLOGICAL SURVEY
W. E. Wrather, Director

Washington, D. C.

Free on application to the Geological Survey, Washington 25, D. O.


A SEMIMICRO METHOD FOR THE DETERMINATION OF COBALT
IN SOILS AND ROCKS: A FIELD TEST USING THE
CHROMOGRAPH

By Hy Almond and Harold Bloom

CONTENTS
Page Page
Abstract.........05 1 2
Introduction........ 1 2
Acknowledgments... 6 Discussion . 7 5
Reagents ... 1 References cited. 6
Results... . 5

ILLUSTRATIONS
Figure 1. A chromograph set complete with case ...... eee
2. The chromograph assembled showing one capillary..........eeeee eee aan
TABLES
Table 1. Effect of pH on precipitation of elements with 2-nitro-1-naphthol (citrate present).... 2
2, A comparison of cobalt content of soils determined by spectrophotometric and
chromographic procedures..........cccccceeeeeeeeeeeeeeeeeeeee ane elgmiviere vice amie 25

ABSTRACT

A chromographic method for the quantitative
estimation of cobalt in soils and rocks is described,
A 0.1 g sample is fused with potassium bisulfate
and the fusion product extracted with 5 ml of cit-
rate solution. A 0.2 ml aliquot of the sample solu-
tion is mixed with 0. 1 ml of 2-nitroso-1-naphthol
solution in the chromograph and the reddish brown
cobalt nitroso-naphtholate that precipitates on the
filter paper is compared with that of a standard
series. Cobalt contents from 10 to 400 ppm in soil
or rock can be determined to within+40 percent of
the figure obtained spectrophotometrically in the
laboratory.

INTRODUCTION

In the course of the U. S. Geological Survey
project to develop quick tests which can be car-
ried out under field conditions, a satisfactory
procedure has been found for determining cobalt,
using 2-nitroso-1-naphthol as a confined spot
test reagent.

Sensitive cobalt reagents are nonspecific. In
order to determine the cobalt content of natural
materials, a preliminary separation of the in-
terfering elements is generally required. Before
determining the cobalt content of a solution of
plant material by the nitroso R salt method,
Deijs and Feldmeyer (1949) separated the cobalt

from iron and copper with dithizone. Boyland
(1946) suggested that all the metal complexes of
2-nitroso-]-naphthol be extracted into an organic
solvent such as amyl alcohol or carbon tetra-
chloride. The immiscible organic phase can be
separated from the aqueous phase and then in
turn treated with concentrated hydrochloric acid
or other acids which destroy the complexes of
all the metals except cobalt, In the chromographic
spot test described here, 2-nitroso-1-naphthol
is used to precipitate cobalt from complex solu-
tions and the estimation is made without pre-
liminary separation.

The chromograph, a device for making con-
fined spots on reagent paper, was developed by
Stevens and Lakin (1949) of the U. S. Geological
Survey. The device is used "to confine areas of
definite size on a strip of reagent paper fed
through the apparatus, and to control automati-
cally the rate of flow of test solution through the
spot". The chromograph is used in this cobalt
test to confine the precipitate resulting from the
reaction of a cobalt solution with a solution of
2-nitroso-1-naphthol in a fixed area.

REAGENTS

All reagents should be analytical grade.
2-Nitroso-1-naphthol. 0.01 percent. Dissolve
Sor g Ponitroso- Tom ‘aphthol in 100 ml of water

to which 4 drops of N NaOH has been added.