Dimethylglyxomine(DMG) is an chemical compound which normally is used to analysis palladium or nickel by forming complex with the particular ion which will precipitate in the solution. It can be prepared by butanone reacted with hydroxylamine monosulfonate.
Ni(dmgH)2 is an complex which show bright red in colour. It is slightly soluble in water, so it can be filtered in the experiment.
Apparatus and Material
- Nickel ammonium sulphate (NiSO4(NH4)2SO4·H2O)
- Concentrated hydrochloric acid (HCl)
- Dimethylglyoxime reagent(dmg)
- Filter paper
- Dilute ammonia solution
- Beaker (250cm³)
- Thermometer
- Glass rod
- Electrical heater
- Graduated cylinder
Procedure
- Two 0.3 – 0.4 g samples of nickel ammonoium salt was measured and placed into 250 cm³ beaker respectively.
- The salt was dissolved into hot water and 2.5 cm³ of concentrated HCl was added into the beaker.
- The water level in the beaker was topped until 150 cm³.
- The beaker and its content were heated 70 – 80 °C on the electrical heater.
- 10 cm³ of dimethylglyoxime reagent was added into the beaker.
- Concentrated ammonia solution was added dropwise until the red precipitate was seen to form. Excess of ammonia solution was added into the beaker to complete precipitation.
- The beaker was digested on a steam bath for 20 minutes and was tested for complete precipitation.
- The beaker was left on the table for 1 hour to cool.
- The filter paper was measured and recorded.
- The solution was filtered using suction machine.
- The filtrate was placed in oven for around 45 minutes, and then the filtrate was placed in desiccators.
- The weight of filtrate was measured and recorded.
Data & Result
Table 1: Weight Measurement of Samples
Average weight of filtrate = (0.4299g + 0.1826g)/ 2
= 0.3063 g
Calculation & Analysis
Molecular weight of nickel ammonium sulphate salt (NiSO4(NH4)2SO4·6H2O) = 394.69g/mol
Molecular weight of dmG = 116.11g/mol
Molecular weight of Ni(dmgH)2 = 288.69g/mol
Molecular weight of Nickel = 58.69 g/mol
Gravimetric factor of nickel in precipitate = 58.69 g/mol ÷ 288.69 g/mol
= 0.2032
The reaction of the formation of chelate is given by the chemical equation below,
Ni2+ + 2CH3(CNOH)2 CH3 +NH3/H2O → Ni(dmgH)2
From the equation, 1 mole of nickel salt should produce 1 mole of Ni(dmgH)2
Mole of Nickel salt (theoretical) = 0.3471g ÷ 394.69g/mol = 0.000879mol
Theoretically, 1 mole of salt can produce 1 mole of Ni(dmgH)2
So, that means 0.000879mol of salt should produce = 0.000879mol of Ni(dmgH)2
But,
Mole of Ni(dmgH)2 (Practical) = 0.3063g ÷ 288.69g/mol = 1.06 x 10-3mol
The data shows that the practical value is more than theoretical value.
Weight of Ni(dmgH)2 (Practical)= 1.06 x 10-3mol x 288.69g/mol = 0.3063g
Weight of nickel from salt = 0.000879mol x 58.69g/mol
= 0.051g
Per cent of nickel in precipitate = (0.2032 x 0.3063g X 100) / 0.3471
= 17.93
Percentage error = (difference in practical value and theoretical value ) ÷ Theoretical value x 100%
= 1.81x10-4 ÷ 8.79x10-4 x100%
= 0.2059 x 100 %
= 20.59%
Discussion:
Nickel(II) forms a precipitate with the organic compound dimethylglyoxime, C4H6(NOH)2. The formation of the red chelate occurs quantitatively in a solution in which the pH is buffered in the range of 5 to 9.
An alcoholic solution of dimethyglyoxime (DMG) is used as the precipitating reagent during the experiment because DMG is only slightly soluble in water (0.063 g in 100 mL at 25°C). Therefore, addition of too large an excess of the reagent is avoided because it may crystallize out with the chelate. It is also important to know that the complex itself is slightly soluble to some extent in alcoholic solutions. By keeping the volume added of the chelating reagent small, the errors from these sources are minimized.
The nickel ammonium and hydrochloric acid mixture was heated 70-80 °C to remove the ethanol from the solution and to ensure that the solution does not contain ethanol during precipitation.
The nickel dimethylglyoximate is a precipitate that is very bulky in character. Therefore, the sample weight used in the analysis is carefully controlled to allow more convenient handling of the precipitate during transfer to the suction filter. To improve the compactness of the precipitate, homogeneous precipitation is often performed in the analytical scheme. This is accomplished by the adjustment of the pH to 3 or 4, followed by the addition of urea. The solution is heated to cause the generation of ammonia by the hydrolysis of the added urea, as indicated by the following reaction.
NH2CONH2 + H2O = 2 NH3 + CO2
A slow increase in the concentration of ammonia(during the addition of ammonium) in the solution causes the pH to rise slowly and results in the gradual precipitation of the complex. This results in the formation of a more dense, easily handled precipitate. Once the filtrate has been collected and dried, the nickel content of the solution is calculated stoichiometrically from the weight of the precipitate.
In this experiment, we successfully collected Ni(dmgH)2 in a precipitate form by using the Gravimetric analysis method. The average amount of precipitate we obtained in this experiment is 0.3063g.
The per cent of nickel in precipitate that we acquired in sample A is 17.93, this is counted as near to the theoretical value that given in the lab manual which is 20.32. However, the reason for the practical yield to be lesser than theoretical yield may be caused by improper handling of the various techniques used in the gravimetric analysis.
The advantage of gravimetric analysis is that if the procedures are followed carefully, it provides for exceedingly precise and accurate analysis. It provides very little possibilities for instrumental error and does not require a series of standards for calculation of an unknown. This method does not require expensive equipment.
However, it has few disadvantages. One of the disadvantages is that this method is time consuming. We also found that each step in Gravimetric analysis method must be done cautiously because a small mistake during the process can affect the end result.
Precaution steps:
- This experiment is very time consuming. Therefore, the experiment should be planned ahead carefully to avoid wasting time. In that way, time could be saved and the experiment can be conducted systematically.
- Since highly concentrated HCl is used in this experiment, care should be exercised.
- The filtrate should not be stored wet in the dessicator. The filtrate should only be placed in the dessicator after the filtrate is dried in the oven for at least 30-40 minutes.
Conclusion
We successfully obtained Ni(dmgH)2 by using the Gravimetric analysis method. The amount of Ni(dmgH)2 we obtained from the experiment is 0.427g and 0.1793g from sample A and sample B respectively.The weight per cent of nickel in precipitate is 25.17 and 10.69 for sample A and sample B respectively.
References
http://en.wikipedia.org/wiki/Dimethylglyoxime --- [at 22/3/2008 18.30 ] for information of the dimethylglyxomine.
ttp://en.wikipedia.org/wiki/Gravimetric_analysis --- [at 22/3/2008 18.30 ] for information of the Gravimetric analysis
Question
- Calculate the weight per cent of nickel in the salt and compare your result with the theoretical weight per cent.
weight per cent of nickel in salt = 58.69g/mol ÷ 394.69g/mol x 100%
= 0.14
There are no value given for nickel salt in lab manual, there are only weight percent of precipitate given.
-
Draw and explain the structure of Ni(DMGH)2.
This is the structure of Ni(DMGH)2. A pair of dmgH- ligands are joined through hydrogen bonds to give a . Ni(dmgH)2 is formed by treatment of Ni(II) sources with dmgH2. This planar complex is very poorly soluble and so from solution.
-
Why Ni(DMGH)2 complex is not soluble in water. Explain with probable structure.
-
How Ni2+ form complex with DMG. Explain.