RESEARCH· ·1970·

B178
terranes where the search for nickel is most likely to
be concentrated.
REAGENTS
The assumption is made that readers interested in
this method are familiar with the philosophy underlying
the design and use of geochemical methods as
well as the basic equipment and techniques necessary
for the operation of a geochemical laboratory. Those
individuals who need additional information may refer
to Ward, Lakin, Canney, and others (1963).
The quantities suggested below are sufficient for 100
analyses. Reagent-grade chemicals and demineralized
water should be used.
Buffer solution: Dissolve 50 g of ammonium citrate [ (NH,)ll
HC6HG07] in 550 ml of water. Add 85 ml of cone NH40H, and
mix. (Ammonium hydroxide is added before sodium thiosul·
·fate to prevent the precipitation of sulfur.) Dissolve 200 g of
sodium thiosulfate (Na2S203·5H20) in the solution, and then
add additional cone NH10H to give a pH of 10.0. Add 4N
NaOH until the pH is 10.8-11.0. Finally, dilute the solution
to !liter with water.
a-Furildioxime solution, 0.2 percent: Dissolve 0.4 g of
a-furildioxime in 30 ml of 95-percent ethanol, and dilute this
solution to 200 ml with benzene.
Hydrogen peroxide, 30 percent. (See section, "Interfering
elements.")
Nitric acid, 4N: Dilute 25 ml of cone HN03 to 100 ml with
water.
Sodium hydroxide solution, 4N: Dissolve 40 g of sodium hydroxide
pellets in enough water to make 250 ml of solution.
Standard nickel solution (100 pg per ml) : Dissolve 0.2 g of clear
unetHoresced crystals of nickel cllloride (NiCl2·6H20) in 500
ml of O.lN HCI. From this solution prepare more dilute standard
solutions (10 ,ug/ml and 1 ,ug/ml) prior to preparing the
series of nickel standards.
PROCEDURE
The following procedure is given in terms of a single
sample. When batch operation is used, the length of time
of contact ·between sample and either acid or buffer is
critical and must be closely controlled; therefore, steps
. 2-5 should be performed on no more than five samples
at a time. Once the nickel complex has been extracted,
large batches of sample solutions can be accumulated
before the estimation step is carried out.
1. Place a 0.2-g (weighed) or 0.2-cc (scooped) quantity
of sample in a 25 X 100-mm test tube.
2. Add 1 ml ·of 4N HN0 3 to the tube.
3. Slosh the sample and acid for 30 seconds.
4. Add 10 ml of buffer solution and mix.
5. Add 2 ml of a-furildioxime solution, stopper the
tube, and shake it vigorously for 30 seconds.
6. Compare . the color in the floating benzene layer
visually against the standard series. Centrifuging
the tube aids in breaking up any emulsion that
ANALYTICAL METHODS
may form and is strongly recommended for
samples that contain much organic matter.
7. Calculate the content of readily extractable nickel
according to the following formula :
Parts per million (ppm) =
micrograms (p.g) of nickel found
sample weight (g) or sample volume (cc)
Results are expressed as ppm weight/weight
if a weighed sample is used, and as ppm weight/
volume if a scooped sample is used. The density of
sieved soil or sediment samples is often about 1,
except for organic-rich samples, so ppm (w/w)~
ppm (w/v).
PREPARATION OF STANDARDS
1. To a series of 25 X 100-mm test tubes add 1 inl of 4N
nitric acid.
2. From the appropriate nickel standard solutions
transfer, respectively, 0, 0.1, 0.2, 0.4, 0.8, 1.5, 3.0,
6.0, and 12.0 p.g of nickel to the above series.
3. Add 10 ml of buffer to each tube and mix.
4. To each tube add 2 ml of a-furildioxime solution,
stopPer the tube, and shake it vigorously for 30
seconds.
These standards are normally stable for several days
and must be kept tightly capped to prevent solvent
evaporation. Screwcap test tubes are suggested.
DISCUSSION
Data produced by cold methods of this type are useful
in a relative sense only; there are no "correct"
results as this term is conventionally used. Therefore,
every sample must be treated as nearly alike as possible
to a void varying degrees of bias in a data set. V ariability
in the time of extraction is a well-known major
source of bias in cold-extractable field tests; consequently
the extraction time should be closely controlled.
Stability of :reagents
The buffer solution loses ammonia very readily, with
consequent drop in pH, and thus the buffer must be
kept in a closed container when not in use and the pH
must be checked as frequently as conditions of temperature
and use demand.
The a-furildioxime solution is reasonably stable and
does not deteriorate appreciably over a period of a few
days even when kept at room temperature and exposed
to light. When first prepared, the reagent solution
usually is slightly yellow. A solution that is allowed to
stand for a period of several weeks will first become
darker yellow and then will gradually turn to orange.