1. The student will use the complexometric
titration for analysis.
2. A buffer will be used.
3. The student will use a 10-mL buret.
PROCEDURE Good planning says to do the digestion at the same time that the gravimetric samples are done. (See Gravimetric Nickel Determination)
Weigh a 2 gram sample of dry unknown into a 400 ml. beaker. Cover and add 35 ml. of 15 M nitric acid. Warm gently then boil until red fumes no longer come off (UNDER THE HOOD), all black particles should be dissolved and the volume should be about 5 ml. If not, add 5 ml. of 12 M HCl and heat for a few minutes under the hood.
Cool and add 20 ml. 12 M HCL. Take to dryness, add 10 ml. hydrochloric acid and again take to dryness. Bake at 110o for one hour. Cool, add 10 ml. HCl, heat easily a few seconds, transfer to a 200 ml. volumetric flask and dilute with hot deionized water, cool to room temperature and adjust volume to mark. Stopper the flask and mix the solution thoroughly.
Throughout this experiment, it is essential that all glassware and associated equipment be clean and be rinsed thoroughly with deionized water. Trace amounts of certain cations found in tap water (and sometimes even in distilled water), such as copper, cobalt, and iron can prevent proper indicator action.
Rinse a 25.00 ml. pipet with three small portions of the unknown nickel nitrate solution, and then pipet a 25.00 ml. aliquot of this nickel ion solution into each of four clean 250 ml. Erlenmeyer flasks. Add to each flask 10 ml. of an ammonia-ammonium chloride buffer solution and dilute each nickel solution to approximately 100 ml. with distilled or deionized water.
Next add one tablet of solid murexide indicator to the unknown nickel solutions.
Rinse a 10 ml. buret (a microburet) with two or three small portions of standard 0.05 M EDTA solution, allowing the rinsings to run out the buret tip. Fill the buret with the standard EDTA solution and record the initial position of the meniscus. Titrate the nickel solution with standard EDTA until the yellow color of the nickel-murexide complex starts to change to the purple color of the free indicator. Thereafter, add the titrant in drops and in fractions of drops until the color change from yellow to purple is permanent. Wait 30 seconds for drainage from the inner wall of the buret, and then record the final positions of the meniscus.
Repeat the titration procedure with three other unknown nickel solutions, adding a tablet of the indicator at the start of each titration. From the experimental data, calculate and report the average nickel ion concentration in the unknown solution. Replicate results should agree to within a few parts per thousand.
NOTE: The student should standardize the EDTA with a known solution of copper wire, or calcium carbonate. Nickel salts (nickel nitrate, nickel sulfate, and nickel chloride are all hydrates – nickel hydrates tend to pick up moisture and do not make good primary standards. Use the procedure under copper or calcium to standardize the EDTA. Nickel metal takes TIME to dissolve.
If in doubt check with Dr. Sherren.
EDTA Preparation: Prepare 200 ml. of standard 0.05 M EDTA (disodium ethylenediaminetetraacetate) solution. Use the EDTA as it is in the bottle. (GMW for EDTA = 372.24, or check the bottle). Weigh the required amount on the analytical balance and place in a 200 ml. vol. flask and dilute with di. Solution is slow so shake patiently. The solution should be standardized with either Copper wire or Calcium carbonate. If the solution is not to be used within 48 hours store in plastic container.
Preparation of pH 10 ammonia-ammonium chloride buffer: Dissolving 70 gm of solid NH4Cl in 600 ml. of concentrated, reagent-grade (15 M) ammonia and then diluting the mixture to 1 liter with distilled water. Check, this buffer may be prepared for you!
CALCULATION: Calculate percent nickel in sample. Include a computer print-out for the calculation.
CAUTION: Remember to treat all chemicals with respect.
Return to the Table of Contents