Weighing a quantity of material to be analyzed

CHEM 2211Experiment FiveGravimetric Determination of NickelOBJECTIVE:The objective of this experiment is to determine the nickel content in an unknown sample by gravimetry.TEXT REFERENCE:Daniel Harris, Quantitative Chemical Analysis, 8th ed.Chapter 1 (Section 4)Chapter 26 (Sections 1 through 3)INTRODUCTION:Gravimetric analysis is a general and accurate method for determining the amount of a substancethrough measurement of its mass (or mass change). The analyte (substance being analyzed) may beisolated by precipitation, electrodeposition, or by gas evolution. Gravimetric methods utilizingprecipitation involve the following steps:1. Weighing a quantity of material to be analyzed2. Dissolving the weighted sample3. Adding a reagent to selectively form a sparingly soluble (or insoluble) compound with the analyte4. Isolating the resultant precipitate5. Purifying the precipitate6. Drying the precipitate7. Weighing the dried precipitateFrom the final weight obtained, the amount of analyte present in the original sample can be calculated.In this experiment, the amount of nickel present in an unknown sample will be determined byselective precipitation with dimethylgloxime. In ammoniacal solution, nickel is precipitatedquantitatively by DMG as a bright, strawberry-red complex, Bis(dimethylglyoximate)nickel(II)(abbreviated as Ni(DMG)2).Although dimethylglyoxime also forms complexes with other divalent metal ions, under theconditions of this experiment, only the Ni(DMG)2 precipitates.PROCEDURE:1Caution: You Will Be Working With Concentrated Acids And Bases. Wear Gloves and SafetyGlasses Throughout this Experiment.1. Obtain your unknown sample from the TA and record its identification number. Take yourcrucibles out of the oven and allow them to cool to room temperature in your desiccator.2. Deliver 2 mL sample from a volumetric pipet to three, clean 400-mL beakers. Dilute the samplein each beaker with ~200 mL of deionized water. Spillage can be prevented by placing yourunknown in a clamp.3. Slowly add ~25 mL of 2% solution of disodium dimethylglyoxime (Na2DMG). Next, add 1-2 mLof 6M NH4OH. Keep the 6M NH4OH solution covered with Parafilm to reduce the odor ofammonia in the laboratory.4. Place the beakers (covered with watch glasses) on your hotplate and allow the precipitate tocoagulate by warming gently for ~30 min. Add ~10 drops of 10% detergent solution and allow themixture to cool to room temperature before filtering (~1 hour).5. Record the weight of the cooled crucibles. Decant the mixture through the Gooch crucible withminimum disturbance of the precipitate. Be sure the filter is properly sealed by passing a fewmilliliters of a solution containing 1 mL of 6M NH4OH per 100 mL water through the filter whileunder vacuum. Always secure the vacuum flask with a clamp and ringstand.6. Quantitatively transfer the precipitate into the crucible and wash it with small portions of a coldsolution containing 1 mL of 6M NH4OH and a few drops of the detergent solution per 100 mLwater. You may want to use your wash bottle to help transfer the residual precipitate (be sure tofill it with your wash solution).7. Dry the crucible plus precipitate in an oven between 110-130oC for 30 min. Remove the crucibleplus precipitate from the oven and cool to ambient temperature in a desiccator. Determine themass of the dried, cool crucible plus precipitate. It will take at least 30 minutes to cool thecrucibles to room temperature.8. After weighing, put the crucibles back into the oven and re-heat for 15 min, allow them to cool inthe desiccator, and weigh the crucibles again. Repeat this step until a constant mass measurement(± 0.2 mg) is obtained.DATA ANALYSIS:1. Report the percentage of nickel in the sample. The formula weight of the Ni(DMG)2 precipitate is288.9146 g/mol.2. Assess the precision of your results using the standard statistical analysis tools you havedeveloped thus far in the course.QUESTIONS:1. Suppose that the actual percentage of nickel is less than your experimentally determinedpercentage of nickel. What is the likely source(s) of this error and how can it be reduced?2. Suppose that the actual percentage of nickel is more than your experimentally determinedpercentage of nickel. What is the likely source(s) of this error and how can it be reduced?3. What other metals form a precipitate with DMG? If you had analyzed a sample comprised ofthese, what other steps might you have taken to prevent interference (co-precipitation) from these?2