Section
Introduction
The purpose of this lab was to find the amount and percentage of nickel present in an unknown compound using the "mole method." The unknown nickel compound was dissolved in water and treated with dimethylglyoxime and ammonia solutions to obtain the precipitate of known chemical composition (bis(dimethylglyoximo)nickel(II)), which then was weighed and using this data the amount and percentage of nickel in the initial compound were found. The insoluble Ni(DMG)2 complex is formed in two steps. First, the dimethylglyoxime reacts with hydroxide ion formed through the ammonia hydrolysis to produce dimethylglyoximate anion:
C4H8N2O2(aq) + OH-(aq) → C4H7N2O2-(aq)+ H2O
Then, the two DMG- anions react with nickel ion to form insoluble brick red Ni(DMG)2 :
2 C4H7N2O2-(aq) + Ni2+(aq) → Ni(C4H7N2O2)2(s)
The overall equation for the chemical reactions looks as follows:
Ni2+(aq) + 2 C4H7N2O2-(aq) + 2 OH-(aq) → Ni(C4H7N2O2)2(s) + 2 H2O
The analytical method used in this lab is called gravimetric analysis. This is the simplest method used in quantitative analytical chemistry. It consists in isolating the substance and measuring the weight or change in weight of the compounds. In this lab the compounds containing nickel were weighed – unknown nickel compound and insoluble Ni(DMG)2. The gravimetric analysis is the most basic form of chemical analysis and is used to analyze ores for specific compounds, sewage waters for the amount of suspended solids, flour for its moisture content, and many others (Harvey, 2016). Although analysts mostly use more elaborate methods of analysis in practice, the gravimetry due to its basic nature remains the constituent of many others methods of analysis.
Procedure
First, approximately 0.2 g of the unknown nickel compound were weighed and put into a beaker. Deionized water (20-30 mL) was added to the sample to dissolve it and then, the solution was stirred until the complete dissolution of nickel compound. Under a fume hood, 5 drops of concentrated ammonium hydroxide were added to the solution. Then the solution of DMG (about 25 mL) was added to the analyzed solution while stirring. The Ni(DMG)2 precipitate appeared as a pink solid. The precipitate was then isolated via vacuum filtration using the Buchner funnel. For this, the filtration apparatus was assembled, and the masses of the filter paper and boat were found and recorded. The solution with precipitate was poured into a Buchner funnel and, thus the liquid was removed. The beaker and stirring rod were rinsed with deionized water into the funnel. Then, the precipitate was dried by washing it with acetone. It was added to the funnel to cover the solid and allowed to flow down; the rinsing was repeated several times. After the washing, the filtration flask was emptied, and the precipitate was allowed to dry in the Buchner funnel with aspiration turned on until the scent of acetone has not disappeared. The funnel top was then removed from the filtration apparatus, wiped with a paper towel and weighed. The whole experiment was repeated three times. As nickel is a suspected carcinogen, and concentrated ammonium hydroxide is a corrosive, safety glasses and plastic gloves should be used when conducting this experiment to ensure safety. Concentrated ammonium hydroxide also gives off harmful fumes and should be operated only under the fume hood.
Discussion
In the first trial, the percent error in nickel content comprised 9.9%, in the second trial, the error was lower – only 2.9%, while in the third trial the value of percent error comprised 7.9%. The mean percent error from three trials was 6.9%. The value of error was relatively low, which is reasonable because the procedure of the experiment is relatively simple and is in accordance with the chemical background. However, there were possible sources of error connected to the procedure itself in this experiment. First of all, the accuracy of this analysis is based on the assumption that the Ni(DMG)2 is completely insoluble in both water and acetone. This is not completely true as it is still soluble to some very small extent. While the quantities of the substances in this experiment were relatively small and the amounts of solvents reasonably substantial, this tiny solubility might have played its role in the accuracy of the analysis.
What was weird about results obtained, was that the calculated percentages of nickel in the unknown compound in all of the three trials were larger than the actual value. It would be more reasonable if the tiny losses due to dissolution and human error yielded a result which was lower than the actual value. Such results led to the conclusion that either unwanted chemical processes interfered with the experiment (the incorporation of water into the chemical structure of the precipitate) or the human error in treating funnel top (not having wiped all the liquid or not having dried the precipitate completely). The other source of such errors in data could be the procedure of drying. If not all acetone evaporated from the precipitate when there would already be no sharp smell, then the results would be reasonable. Another possible source of the data obtained could be the contamination of unknown nickel compound with other metals which also form insoluble compounds with either DMG or ammonium hydroxide. Still, the average percent error of the experiment was relatively low, so the data was accurate and consistent.
Conclusion
In this lab, the amount and percentage of nickel in its unknown soluble compound was found using quantitative gravimetric analysis. The purpose of the experiment was achieved because the percent errors in the obtained data were relatively low - ranging from 2.9% to 9.9% with an average of 6.9%. The lab may be regarded as successful, however, the direction of the deviations differed from the anticipated, and the identified sources of such errors are to be tested experimentally.
Citations
Harvey, D. (2016). Analytical Chemistry 2.0 (Ch. 8) http://www.asdlib.org/onlineArticles/ecourseware/Analytical%20Chemistry%202.0/Text_Files_files/Chapter8.pdf.
