CHE 226 EXPERIMENT 3

Complexometric Titration of Zn(II) with EDTA

Reference: D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical Chemistry: An Introduction, 7th ed. Chapter 15, pp. 345-381.

Unknown Solution

Submit a clean 250-mL volumetric flask to the instructor so that an unknown zinc solution may be issued. Your name, section number, and your locker number should be written legibly on this flask. Note that the flask must be turned in at least 1 lab period before you plan to do the experiment, so that the Teaching Assistants will have time to prepare the unknown.

Preparation of Solutions

USE ONLY DEIONIZED WATER (NOT DISTILLED WATER!) IN THIS EXPERIMENT.

1. EDTA, 0.01 M. (Prepare at least one day ahead of time to make sure that the solute is dissolved.) Dissolve about 3.8 g of the dihydrate of the disodium salt (Na₂H₂Y•2H₂O) and 0.1 g MgCl₂ in 1 liter of water. Store in a plastic bottle. A small amount of sodium hydroxide may be added if there is any difficulty in dissolving the EDTA. Try not to exceed 3.8 g of Na₂H₂Y•2H₂O because much more than this dissolves only with difficulty. The EDTA solution should be filtered using suction filtration. See TA for the apparatus. NOTE: Break the suction before you turn off the water flow on the vacuum aspirator.

2. Buffer, pH 10. Each titration will require the addition of pH 10 ammonia buffer. The stock buffer solution has been prepared for you, and the appropriate quantity is dispensed directly into your titration flask from the "Repipet" repetitive dispenser located in hood #7. The buffer should be added immediately before you titrate a sample.

3. Calcium Standard Solution. A Ca²⁺ solution is prepared as the primary standard. Obtain from the TA approximately 0.7 g of predried analytical-reagent-grade CaCO₃. Accurately weigh a 0.25-g sample by difference into a 100-mL beaker. Add about 25 mL water and then add dilute HCl dropwise until the sample dissolves, then add 2 drops more. Mild heating will speed dissolution if necessary. Transfer quantitatively to a 250-mL volumetric flask (rinse well with water) and carefully dilute to the mark. (eye dropper!) Mix thoroughly. Because this Ca²⁺ standard solution is used to standardize the EDTA titrant, it must be prepared very carefully so you know its exact molarity, and therefore the exact (to ± 0.1 mg) mass of Ca₂CO₃ weighed out.

Standardization of EDTA Solution

1. Pipet 25-mL aliquots of the standard Ca²⁺ solution into three or four 250-mL Erlenmeyer flasks. Remember that each aliquot contains one-tenth of the CaCO₃ weighed out to prepare the standard solution.

2. Take each sample to completion before starting next sample. Add 7-8 mL of pH 10 buffer, 15 mL of water, and 3 drops of Eriochrome Black T and titrate immediately with EDTA until the light red solution turns a LIGHT SKY BLUE.

Titrations must be performed swiftly (but carefully) because ammonia will evaporate and thus the pH of the solution will change. In general, the faster the titrations are performed the better the results will be (as long as the endpoint is not overshot due to the speed). It is advantageous to perform a trial titration to locate the approximate endpoint and to observe the color change. In succeeding titrations, titrate very rapidly to within about 1 or 2 mL of the endpoint, then titrate very carefully (dropwise) to the endpoint. (See Note 1 at the end of the report.)

3. Calculate the molarity of the EDTA from the volume of EDTA used in the titration of each aliquot.

The values (M_EDTA and titration volumes) should all agree very closely. If not, titrate additional aliquots until agreement is reached, and any spurious values can be rejected with confidence.

Determination of Zinc

1. Dilute your unknown sample in the 250-mL volumetric flask to the mark with water. Mix thoroughly.

2. Pipet 25.00-mL aliquots into 250-mL erlenmeyer flasks. Add 15 mL of water, 9-10 mL of pH 10 buffer, and 2 drops of Eriochrome Black T immediately prior to titrating a sample.

3. Titrate with standardized EDTA until the red solution turns blue. (See note at the end of the report.)

4. Calculate the number of milligrams of zinc in the total sample. Remember that each aliquot represents one-tenth of the total sample volume.

= M_EDTA x (V, mL EDTA) x (molar mass Zn) x 10

Hazardous Waste Disposal

1. Empty all titrated Ca and Zn solutions into the proper Hazardous Waste Bottle for this experiment.

2. When completely done with the experiment mix any remaining EDTA titrant and any remaining Ca stock solution together in a large beaker. Pour down the drain with copious amounts of cold tap water. The two solutions are slightly basic and slightly acidic, respectively; when mixed, they will be near neutral. There are also no toxic chemicals present, so disposal directly down the drain is allowable and safe.

Notes:

1. Eriochrome Black T Indicator. The color change of Eriochrome black T at the endpoint is rather subtle. It is not an abrupt change from bright red to a dark blue; but rather it is from a light red (or pink) to a pale blue. At least one trial titration is recommended. (You can always discard a "bad" value when you know there is a definite reason for its being bad.)

If you have trouble distinguishing the endpoint, a "before" and an "after" flask are recommended. Prepare two 250-mL flasks exactly the same as for the samples, except use distilled water instead of a sample; add additional distilled water to approximately equal the volume of EDTA titrant that would be titrated into the flask for the sample. Add the indicator. To one flask (the "after" the endpoint flask) add a small amount of EDTA to just past the color change at the endpoint. Stopper the flasks and keep them nearby for comparison of the colors. Titrate against a white background for better discrimination of colors. (Extra flasks can be checked out from the stockroom.)

Sometimes the Eriochrome black T solution goes bad because of air oxidation. If the endpoints seem very indistinct to you, try a fresh bottle of indicator. Alternatively, try adding a small amount of solid Eriochrome black T mixture (1 g indicator ground with 100 g NaCl); a small amount on the end of a spatula is sufficient.

2. pH 10 Ammonia Buffer: Dissolve 64.0 g of ammonium chloride in 600 mL of concentrated ammonia. Slowly and carefully add 400 mL deionized water. This should be sufficient for over 120 titrations.

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