neutral solutions) in solutions containing more than 50% of ethanol by volume, and in hot
water,
The precipitate is collected and dried,and then is then weighted.
METHODS :
- 25-mL of nickel (ll) sulfate solution was titrated into each of the two 400-mL beakers.
-
5-mL of dilute hydrochloric acid (2 M) and the solution was dilute to 200mL with distilled water .The solution was heated to 60-80 ° C .
- About 20 mL of dimethylglyoxime solution (1%) was added to the hot solution,followed by dropwise addition,with stirring of ammonium hydroxide until precipitation occurs and the solution is slightly basic (about 2-mL of excess ammonium hydroxide).The completeness of precipitation was tested by adding more dimethylglyoxime solution (1%)
-
The precipitate was digested for 30 minutes . It was cooled to room temperature and was filter through a weighed No.4 sintered glass crucible previously dried at 110°C.
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The precipitate was watched with cool distilled water until it is free from any chloride (was tested with silver nitrate solution) and was dry at 100 -120° C for 1 hour.
- The crucible was cooled in a desiccator and was weight it to constant weight.
- From the weight of the precipitate and a knowledge of the theoritical quantity of nickel in bis (dimethylglyoximato) nickel (ll) ,the concentration (in g L-1) of nickel (ll) ions in the solution.
RESULT AND CALCULATIONS:
The crucible(1) weight without any substance= 22.1713 g
The crucible(1) weight with precipitate= 30.7918 g
The weight of precipitate from crucible (1) =8.6205 g
The crucible(2) weight without any substance = 22.5521 g
The crucible(2) weight with precipitate= 30.9905 g
The weight of precipitate from crucible (2) =8.4384 g
The concentration (in g L-1)of nickel (ll) ions in the solution=
DISCUSSIONS:
1. Nickel is precipitated by the addition of an ethanolic solution of dimethylglyoxime (CH3-C(:NOH)-C(:NOH)-CH3, referred to in what follows as H2DMG} to a hot, faintly acid solution of the nickel salt, and then adding a slight excess of aqueous ammonia solution (free from carbonate). The precipitate is washed with cold water and then weighed as nickel dimethylglyoximate after drying at 110-120 °C. With large precipitates, or in work of high accuracy, a temperature of 150 °C should be used: any reagent that may have been carried down by the precipitate is volatilised.
Ni2+ + 2H2DMG = Ni(HDMG)2 + 2H +
2.For this experiment,there’s a lot of precaution need so that,we can get the best result.
First,the nickel dimethylglyoximate is a precipitate that is very bulky in character. Therefore, the
sample weight used in the analysis must be carefully controlled to allow more convenient
handling of the precipitate during transferral to the filtering crucible.
For precaution for Preparation,advance preparation in this experiment is especially crucial. Plan ahead carefully to avoid wasting your time.
3.clean the crucibles before use ,the6 M HCl is used in this process and that care should be exercised. When finished cleaning your crucibles, also clean the acid out of the filtering apparatus with liberal use of water. As ageneral rule, I should always clean the filtering flask once finished with them.
4.Constant weight means that no more than ± 0.4 mg difference is seen between the last two massing operations.
5. Quantitative transfer and washings are extremely important in the filtration process. (Be
certain, when it is desired to save the filtrate, that the receiving flask has been scrupulously cleaned.)
6. Possible sources of error may have arisen through:
∙ Inaccurate standardization of solutions – this was minimised by great care taken in
making solutions to the desired volumes in volumetric flasks.
∙ Concentration gradient in nickel solution – the solution was agitated to minimise any
possible gradient of aqueous species concentration. Although not a terribly reliable
indicator of homogeneity, the nickel(II) solution was observed to be uniformly coloured.
∙ Transfer losses – Gooch filtration of the solution was preformed for 3 different solution
preparations. Washing of glassware involved was thorough and transfer was carried out
with great care to minimise chances of spills, however as with all filtrations some losses
are almost assured. Losses through precipitate passing through sintered glass, however
are absolutely negligible, accounting for their use in gravimetric analysis.
∙ Precipitation of other species, such as iron and chromium hydroxides. This possibility
was minimised by the design of the experiment itself. Citric/tartaric acid was added in the
interest of creating soluble iron, chromium and trace transition metal complexes that
would pass through the crucible. The precipitate was also washed thoroughly under
vacuum to prevent the deposition of soluble species in the insoluble precipitate.
∙ Mass fluctuations in Gooch crucibles, minimised by handling with tongs where
appropriate, to preclude fingerprints. The cleanliness of the top pan balances and
microwave trays employed cannot, however, be guaranteed.
QUESTIONS:
- Draw a structural formula of bis (dimethylglyoximato)nickel (ll) and describe the type of hybridization involved.
The structural formula for the complex is as illustrated. Arrowed bonds indicate a coordination
bond. Note the removal of two protons from the original ligands and the potential for hydrogen
bonding between the strongly electronegative coordinated oxygen atoms and nearby hydroxyl
groups. This complex is square planar as is often observed in d8 complexes, i.e. bonding
interactions take place in the dx2 − y 2 orbitals of the parent Ni(II).
bis(dimethylglyoximato)nickel(II)
-
Explain why the temperature of the solution must be mantained at 60-80°C during the precipitation process.
CONCLUSIONS :
1.The weight of precipitate from crucible (1) =8.6205 g
2. The weight of precipitate from crucible (2) =8.4384 g
3. The concentration (in g L-1)of nickel (ll) ions in the solution=
References :
1.Inorganic laboratory manual (1st year)-Department of Chemistry ,UM
2. http://users.tpg.com.au/terrett/chemgravnick.htm
