Dry the solid sample at 140oC for 3 hours. Weigh three samples of
1.0 to 1.2g into 250 mL beakers. Add 20 mL of concentrated
nitric acid (in the hood) carefully down the sides of
the beakers, using a measuring pipet and a rubber bulb.
Boil the solutions in a hood until reddish brown nitrogen
dioxide gas is no longer evolved. About half of the
nitric acid should have evaporated at this point. Cool
to room temperature and in the hood add 15 mL of concentrated
hydrochloric acid down the sides of the beakers. Bake
to dryness on a hot plate to dehydrate any silicic acid
to silica (Be careful that the contents do not pop.
Use glass hooks and a cover glass to prevent unwanted
loss of the nickel sample.). (Evaporation with hydrochloric
acid destroys excess nitric acid which might oxidize
the dimethylglyoxime reagent to be added.) Cool, again
add 10mL of concentrated hydrochloric acid, and bake
to dryness. Cool, add 0.2 mL of concentrated hydrochloric
acid, and dilute each sample with 15 to 20 mL of water.
Heat gently until all salts are dissolved. If any residue
is present (probably silica), filter immediately through
a medium porosity filter paper (Whatman No. 40) and
wash the precipitate with water. Catch the filtrate
and washings in 400mL beakers. Dilute each sample to
150 mL with water, and add 60 mL of 20% tartaric acid
solution. Tartaric acid forms a stable complex with
iron (III), which prevents the precipitation of oxide
when ammonia is added. Citric acid or ammonium citrate
may be used for the same purpose. Heat the solution
nearly to boiling and add 1:1 ammonium hydroxide (1
part ammonium hydroxide, 1 part water) slowly until
the solution is alkaline (perform this in the hood).
The final solution should smell distinctly of ammonia
after blowing away the vapors from above the solution.
If during the addition of ammonia any percipitate of
ferric hydroxide appears, dissolve it by adding hydrochloric
acid, add more tartaric acid solution, and again neutralize
with ammonia. Any white or gray precipitate is probably
silica, and should be removed by filtration.
Do not start the following steps unless adequate
time remains in the laboratory period to complete the
filtration of the precipitate (approximately two hours
will be required).
will now be followed in the treatment of the samples.
Adjust one of the samples, to pH 7 with hydrochloric
acid, heat to about 60o and slowly add 15 mL of 1% dimethylglyoxime
reagent. Digest the precipitate on the hot plate for
an hour; cool to room temperature and let stand for
one hour. (If a white precipitate occurs at this point,
it may be dimethylglyoxime. Dissolve by adding 20 mL
of 1-propanol and heating to 60¡É.) Filter
through a previously weighed crucible that has been
brought to constant weight. The precipitate has a great
tendency to creep in the liquid film. Hence, during
filtration and washing, do not fill the entire crucible
with liquid - keep the upper 1 cm of the filter crucible
dry if possible.
Make each of
he remaining two samples slightly acidic with hydrochloric acid.
Add 15 mL of 1% dimethylglyoxime and 4-5 grams
of urea to each sample. Cover each beaker with a watch
glass and heat for an hour at 80-85¡É. At this
time check the pH with pH paper to see whether or not
it is above 7. If it is not, add a drop of ammonium
hydroxide and check again. Cool to room temperature
during the laboratory period (use cold water to speed
the cooling in this case). Filter through a previously
weighed crucible that has been brought to constant weight.
Test the filtrate
on all samples for completeness of precipitation by
adding a little more dimethylglyoxime solution. Wash
the precipitates with cold water to which a few drops
of ammonia have been added until the washings are free
from chloride ion (test with acidic AgNO3). Dry the precipitate at 130oC
to constant weight. Calculate your results and
report them as % nickel in the sample. Report
relative average deviation (%).
Clean the crucibles
by dissolving the precipitate with concentrated hydrochloric
acid. Rinse thoroughly with distilled water.