EDTA Procedure for using EDTA to determine Ca2+ and Mg2+ in Natural Waters Presented by Jeff Kovalesky

Step 1- Preparation of Titrant Dry EDTA for 1 hour at 80 degrees Celsius to remove H2O from the EDTA. After cooling, dissolve approximately 0.6 grams in 400 mL of distilled, with heat, in a 500 mL volumetric flask. Dilute to mark after EDTA is dissolved completely.

Step 2 – Preparation of Unknown for Titration Combine 1.00 mL of water unknown with 50.0 mL of distilled water. Make a total of 4 unknown samples in 125 mL Erlenmeyer Flasks. Add 3.00 mL of pH 10 buffer, and 6 drops of Eriochrome Black Indicator to each sample. Titrate the first one quickly to determine the end point and to practice seeing the Wine Red to Blue color change of your indicator. Repeat by adding more water to practice seeing the end point

Step 2 - Continued Accurately titrate your 3 remaining samples to determine the [Ca2+] and [Mg2+] in your unknown. Perform a blank titration on 50 mL distilled water and subtract that value from the value of your 3 unknowns.

Step 3 – Determination of [Ca2+] Prepare 4 more samples of your unknown by combining 1.00 mL of unknown with 50.0 mL distilled water. This time, add 30 drops of 50 wt% NaOH to precipitate Mg(OH)2 to each sample. Add approximately 0.1 grams of solid hydroxynapthol blue indicator to each sample. Because we are preparing solutions with higher pH levels, we must use this indicator because it remains blue at higher pH levels.

Step 4 – Titrating to determine [Ca2+] Titrate one sample quickly to determine the approximate volume of your end point as well as practicing to see your color change of Red to Blue. Carefully titrate 3 remaining samples. After reaching your end point, allow each sample to sit for 5 minutes with occasional swirling so that any Ca(OH)2 precipitate may redissolve. If solution turns back to red, titrate back to blue end point. Perform a blank on 50 mL of water and subtract from values of 3 unknown samples.

Question??? Why was EDTA chosen over other chemicals as the titrant for this lab? It works best to sequester di- and trivalent metal ions.