Determination of Traces of Cobalt in Soils A Field Method HY ALMOND
U. S. Geological Survey, Department of the Interior, Washington 25, D . C. The growing use of geochemical prospectingmethods in the search for ore deposits has led to the development of a field method for the determination of cobalt in soils. The determination is based on the fact that oohalt reacts with 2-nitmso-1-naphthol to yield a pink compound that is soluble in carbon tetrachloride. The carbon tetrachloride extract is shaken with dilute cyanide to complex interfering elements and to remoye excess reagent. The cobalt content is estimated by comparing the pink color in the carbon tetrachloride with a standard series prepared from standard solutions. The cobalt 2nitroso-1-naphtholate system i n carbon tetrachloride follows Beer's law. As little a s 1 p.p.m. can be determined in a 0.1-gram sample. The method is simple and fast and requires only simple equipment. More then 40 samples can be analyzed per man-day with an accuracy within 30% or better.
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H E U. S. Geological Survey is investigating the amounts of trace elements in rocks, soils, plants, and water as a means of locating hidden ore bodies. To facilitate these studies, methods of quantitative analysis suitable for field use are being developed. The field method for the determination of cobalt in soils described herein was developed as part of this program. Laboratory colorimetric methods (4, 5) for the determination of traces of cobalt in soils or in other complex material such as rocks or vegetation are lengthy, requiring vigorous acid digestions of the sample, separation of cobalt from interfering elements in the sample solution, reaction with a n organic reagent, and then colorimetric estimation of the cobalt compound extracted in a n organic solvent. A chromographie method (1, 6 ) has been used successfully in the field, but the care required to adjust the pH makes this technique somewhat difficult under field conditions. Boyland (e) demonstrated that the cobalt salt of 2-nitroso-1naphthol can be extracted by carbon tetrachloride in acid solution and that the copper, iron, and nickel nitrosonapbtbolates that accompany cobalt can be removed by shaking the carbon tetrachloride extract with concentrated mineral acid. At pH greater than 4 iron does not react with Znitroso-1-naphthol ( l ) , while cobalt reacts st pH between 4 and 8.5 to form an insoluble compound. I n slightly acid solution excess reagent precipitates. To prevent the reaction with iron and the precipitation of reagent the solution is adjusted to pH about 6.5 before the slightly alkaline reagent solution is added. Under these conditions, copper and nickel also react with the reagent and follair cobalt Znitroso-1-naphtholate a t least partially into the carbon tetrachloride. However, the 2-nitroso-1-naphthol complexes of nickel and copper are unstable in the presence of cyanide, and these elements along with any exce~sreagent are removed from the carbon tetrachloride extract by shaking with dilute cyanide solution. Only the pink cobalt 2-nitroso-1-naphtholate remains in the organic solvent. Beer's law was found to hold a t 500 mp over the optimum concentration range of 1 to 10 microgram of cobalt extracted as the 2-nitroso-1-naDhtholate in 5 ml. of carbon tetrachloride. The size of samde collected for the eeochemical investimtion of residual soil depends upon the particular problem and usually
a preliminary study is made to determine the advantage of collecting large amounts of sample material rather than small amounts. Generally about 10 grams of residual soil is submitted far rapid analysis. The presence of a geochemical anomaly is indicated by the statiiitieal data accumulated from the analysis of a large number of small samplcs rather than from one large sample representing a large area. REAGENTS A N D APP4RATUS
Potassium bisulfate, analytical reagent grade, ground to a fine powder. 2-Nitroso-1-naphthol, 0.01%. Add 2 drops of 1 N sodium hydroxide to 0.01 gram of 2-nitroso-1-naphthol in a 250-ml. beaker, add enough water to wet the reagent, stir, then add about 2 ml. of water and stir until the reagent is in solution. Dilute to 100 ml. with water. Ammonium citrate. 10%. . - Dissolve 10 crams in water and dilute to 100 ml. Borate buffer. Dissolve 19 grams of hydrated sodium tetraborate in about 800 ml. of water, add 10 ml. of concentrated ammonium hydroxide, and dilute to 1 liter. Hydrochloric acid, approximately 1 N . Dilute 88 ml. of concentrated acid to 1 liter with water. Phenol red indicator, 0.02%. Add 0.1 gram of phenol red indicator (phenolsulfonphthalein) dry powder to 0.3 ml. of 1 N sodium hvdroxide. Dilute with water to 500 ml. Carbon tetrachloride, analytical grade. Potassium cyanide, 10%. Dissolve 10 grams in water and dilute t o 100 mi. Water. Pass tap water through a resin demineralizer to remove heavy metals. Standard cobalt solution, 0.01%. Dissolve 0.04 gram of I
I
I
.
golutidns a s neehid. Culture tubes, 16 X 150 mm. Calibrate a t 5 and 10 ml Separatory funnels, Squibb type, 60-ml. capacity. Graduated pipets. Capacity 1, 5, and 10 ml. Buret, 50-ml., or dropping bottle, 50-ml.
Figure 1.
Sample Being Fused over Gasoline Stove
~
Digestion and fusirm rack holds the culture tvhes
166
167
V O L U M E 2 5 , N O . 1, J A N U A R Y 1 9 5 3 Tabll I.
Determination of Cobalt in Soils by Laboratory and Field Methods
Sa inpi e so.
Sitroso R Salt (Laboratory), P . P 31
1
10
a
2 3 4
10 20 20 30 30 40
20 30
5 6
7 8 9 11 12 13 14 15 17 18
19
2n ~.
21 22 23 24
2; 2fi
Table 11.
100 100 180 180 180 200 200 240 300 400 400 600 800
9 12 19 24 26
(8. Cobalt in the soil sample was dissolved by digesting the sample with perchloric acid as described by Holmes ( 5 ) . Copper was removed as the dithizonate a t a pH of about 2.5. Cobalt was separated from the aqueous solution as the dithizonate at a pH of about 8.5; then the dithizonate and excess dithizone were decomposed with oxidizing acids. Cobalt was determined by the nitroso R salt method.
350 600 800
Repeat Determinations of Cobalt by Field Procedures
Saiiipli, 1
40 40 20 50 60 120 90 80 80 90 100 230 180 160 150 200 180 250 300
100
16
COMPARATIVE RESULTS
A imomparison of results by field and nitroso R salt methods of analysis is made in Table I. Results in the column labeled nitroso R salt were obtained by a method similar to one used for the analysis of cobalt in vegetation by Deijs and Feldmeyer
40
60 70 80 90 100
10
NO.
2-Sitroso-1-naphthol (Field), P P.M.
Cobalt Found. P.P.31.
LOW 6 50
6,6,6, 8, 8 , 8
60, 50, 65, 6 5 . 5 5 , 75 90, 90, 100, 8 5 , 100, 100 1.50, 150, 150. 170, 130, 120 4.50. 450. 250, 400, 400, 450 800. 800, 1000. goo, 800, 700
85
120 250 700
High
-8
:48
450
1000
Av, 7 62 94 145 400 833
0.8, 1.0, 2.0, and 4.0 micrograms of cobalt taken from standard solutions. Add 2 ml. of 1 N hydrochloric acid to each and proceed as above. Let the diluted cyanide solution remain over the carbon tetrachloride extract and stopper the culture tubes.
Laboratory Results, P.P.M. 10
70 100 200 400 800
Torsioii balance, with a sensitivity ol' 2 mg. Polyethylene wash bottle, 250-ml. capacity. Test tube rack to hold 40 test tubes. Separatory funnel rack to hold 8 separatory funnels. Glass stirring rod. .%utomatic pipet, calibrated GO deliver 1 ml. Digestion and fusion rack (see Figure 1). This piece of apparatus is used to support 8 culture tubes over a gasoline stove. The rack consists of t.wo disks of sheet steel welded to a central supporting rod. Each disk has 8 holes for test, tubes; the holes in the bottom are smaller, so that the culture tubes cannot dip through, but large enough t,o permit the ready flow of heat. hIedicine dropper. Gasoline stove. -4pocket stove is satisfactory. PROCEDURE
Place 0.1 gram of a soil sample and 0.5 gram of potassium bisu!fate in a culture tube and mix. Place the culture tube in a digestion and fusion rack and fuse over a gasoiine stove for about 5 minutes (see Figure 1). Xeanwhile prepare a hot-water bath by filling the 1 5-pint unit of the two-piece aluminum interlocking case of the gasoline stove with water and warm over another stove. Remove the culture tube from the rack and rotate so that the melt solidifies in a thin layer on the wall of the tube. Place in a test tube rack, let stand until cool, then add 1 S hvdrochloric acid to the 5-ml. mark. Again place the culture tube in the digestion and fusion rack, and set in the water bath. \\-arm over the gasoline stove until the melt has broken up. Remove the culture tube from the water bath, dilute to the 10ml. mark with water, anti mi\. .411on to stand for about 0.3 hour. Transfer 2 ml. of the supernatant liquid to a separatory funnel, add 1 ml. of ammonium citrate so!ution and 2 drops of phpnol red indicator, and titrate with borate buffer from a buret (or a dropping bottle) to a faint pink (pH about 6.5). Add 3 nil. of 2-nitroso-1-naphthol solution, then 1 ml. of carbon tetrachloride measured from an automatic pipet, and shake for 60 seconds. Drain the carbon tetrachloride into a clean culture tube containing 10 ml. of water and 1 drop of the potassium cyanide solution. Shake for 10 seconds. Compare the pink color in the carbon tetrachloride with standard. Once a week, prepare standards containing 0, 0.2, 0.4, 0.6,
I n general the values repoitcd by the field method (2-nitroso-lnaphthol) closely parallel thc values obtained by the laboratory (nitroso R salt) method. The deviation is greatest among those samples having low cobalt content. The dependability of the field procrdure \\as further studied to show that results obtained with it would be hufficiently accurate to indicate geochrmical trends. Although field results are often close to laboratory results, IThich are assumed to represent absolute values, such agreement is not so essential as that the field method give reliable and consistent relative values in the comparison of samples. For this purpose six samples were selected from those recorded in T a b k I, with lsbcratory values that difTer by amounts that should he indicated by the field analyses. Each sample \vas analyzed six times by the field method and the cobalt found is listed in Table 11, column 2. The average values, column 5 , arc within 30y0 of laboratory values in column 6. The result of any single determination is sufficirntly accurate to place the sample in its proper group. For field work visual comparison of the colors has several advantages over spectrophotometric measurement. The expense of an instrument and the extra careful handling entailed are obvious undesirable features of the spectrophotometric method. Field analyses often omit operations that are necessary if the color is to be measured with an instrument. For example, in the field method for cobalt, the carbon tetrachloride layer may be turbid because the test solution is not filtered. A visual comparison will be only slightly in error, hut the same cloudy solution placed in an instrument nil1 greatly affect final results by absorption of light. The 2-nitroso-1-naphthol cobalt compound is rather stable in carbon tetrachloride solution; consequently, new standards nerd br prepared only once a week. Evaporation of the carbon tetrach!oride 15 prrvented by leaving the aqueous cyanide solution over the organic solvent. When the method is used for field work, proper precautions must be taken in packing, shipping, and handling cyanide solutions usrd in the determination. Sonprofrssional personnel should he fully advised of the dangers in u4ng rvanide. LITERATURE CITED
(1) .-llmond, Hy, and Bloom, Harold, U. S. Geological Survey, Circ. 125 (1951). (2) Boylsnd, E., A n a l y s t , 71,230-1 (1946). (3) Deijs, W. G., and Feldmeyer, J. H., Plant and Soil, 1, 35971 (1949). (4) Ellis, G. H., and Thompson, J. F., IND. ENG.CHEM.,A.v.4~. ED., 17, 254 (1945). ( 5 ) Holmes, R. S., S o d Sei., 59,77-84 (1946).
(6) Stevens, R. E., and Lakin, H. W.,V. S. Geological Survey, Circ. 63 (1949). RECEIVED for review May 8, 1952.
Accepted September 5 , 1952.
