Experiment 4

 Gravimetric Determination of Nickel

 

   

   Nickel(II) is precipitated quantitatively by the organic compound dimethylgyloxime, C4H6(NOH)2, in the pH range 5 to 9.  The nickel ion is chelated by two molecules of dimethylgyloxime :

 

 

Ni2+

+

2

Ni[C4H6(NOH)(NO-)]2(s)

+

2H+

 

   Although nickel displaces a proton from one oxime group (NOH) on each dimethylglyloxime molecule, it is chelated by electron pairs on the four nitrogens, not by electrons on the oxygens.  An ammonia or acetate buffer is added so that the pH of the solution does not drop below 5 and displace the equilibrium in favor of the soluble nickel (II) ion.  Precipitation of iron and the other metals as hydroxides is prevented by addition of tartrate. Iron should be present in the ferric state, since ferrous iron tends to be coprecipitated.  Cobalt and a number of other metals form soluble complexes with the reagent and a greater excess must be added in their presence.

   Dimethylgyloxime is soluble in alcohol (and in alkaline aqueous solutions in consequence of its acidic character) but only to a limited extent in water (0.063 g in 100mL at 25oC).  Therefore, too large an excess of the reagent must not be added for fear that it may crystallize out.  Moreover, the nickel complex is soluble to some extent in alcoholic solutions.  The error from this source is very small in cold solutions containing a small excess of reagent.

   Nickel dimethylglyoximate is a bulky precipitate which is difficult to transfer to a filtering crucible.  It has been found that more compact, easily handled crystals are formed by precipitating nickel from homogeneous solution. This is accomplished by adjusting the pH to 2 or 3, where little precipitation occurs, and then homogeneously generating ammonia by the hydrolysis of urea:

 

 

NH2CONH2

+

H2O

2NH3

+

CO2

 

The slow increase in the concentration of ammonia gradually raises the pH, causing the precipitation of nickel to occur.

  

Procedure

 

 1.

 

 

 

 

 

 

 

 

 

Sintered-glass crucibles of medium (M) and fine (F)  porosity are recommended for filtering many precipitates if drying of the precipitate can be accomplished at temperatures below approximately 250oC. To clean the crucible, first remove any visible dirt with detergent solution by brushing; rinse. Assemble the filter flask, filter holder, and crucible.  Connect the filter flask with rubber suction tubing to the aspirator.

   Fill the crucible about halfway with concentrated hydrochloric acid and, using gentle suction, draw the acid slowly through the crucible.  Wash the crucible several times with distilled water.

   Dry the crucibles for 1 hour in an oven at 130oC using a beaker, glass hooks, and a cover glass. (While the crucibles are drying, proceed with step 2.)  Remove the crucibles to the desiccator. Cool for 30 minutes and reweigh.  Repeat this last process until consecutive weighings agree within ±0.4 mg (i.e., bring to constant weight).  Always store the crucibles in the desiccator until they are to be used.

 

 2.

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).

   Separate procedures 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.