EDTA Titration of Calcium in Brine: Masking and pH Control
A 50.00 mL brine sample contains approximately 0.020 M Ca2+, 0.010 M Mg2+, 0.0010 M Fe3+, and 0.0015 M Al3+. The analyst titrates with 0.0100 M EDTA at pH 10 using Eriochrome Black T (EBT). The endpoint is diffuse, and the calculated Ca2+ concentration is 0.031 M, higher than the expected 0.020 M. The analyst suspects interference. At pH 10, EBT binds Ca2+ and Mg2+; Fe3+ and Al3+ form stable hydroxy complexes and can block the indicator. The lab has triethanolamine (TEA), KCN, ascorbic acid, and NaOH. The analyst must design a procedure to determine Ca2+ alone and total hardness, and explain the overestimate.
1. Explain why the direct EDTA titration at pH 10 overestimates Ca2+. Identify which ions interfere and how.
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At pH 10, EBT forms a red complex with both Ca2+ and Mg2+, and the endpoint is the conversion of the red indicator complex to the blue free indicator. The titration therefore measures Ca2+ + Mg2+, not Ca2+ alone. The expected total hardness is 0.020 + 0.010 = 0.030 M, close to the reported 0.031 M, so Mg2+ accounts for most of the overestimate. Fe3+ and Al3+ also interfere: at pH 10 they precipitate as hydroxides, which can adsorb EBT or consume EDTA, blurring the endpoint. Their concentrations are small (0.0010 and 0.0015 M) but they can block the indicator and cause a premature or diffuse endpoint. The diffuse endpoint and high result are therefore consistent with Mg2+ plus Fe3+/Al3+ interference.
2. Propose a masking strategy using the available reagents. Specify the order of addition and pH, and explain the role of each reagent.
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For total hardness (Ca2+ + Mg2+): add triethanolamine (TEA) to mask Fe3+ and Al3+ by forming stable soluble complexes; add a small amount of KCN to mask trace heavy metals such as Cu2+, Ni2+, and Zn2+ (use only in a fume hood); adjust to pH 10 with NH3/NH4Cl buffer; add EBT and titrate with EDTA. For Ca2+ alone: add TEA and KCN as above, then add NaOH to pH 12–13. Mg2+ precipitates as Mg(OH)2, and Fe/Al remain as hydroxides; use a Ca-specific indicator such as calcon or murexide (EBT is unsuitable because Mg2+ is absent). Titrate with EDTA. The difference between the total-hardness volume and the Ca2+ volume gives Mg2+. Ascorbic acid can reduce Fe3+ to Fe2+, but Fe2+ still interferes, so TEA is the better masking agent.
3. Calculate the expected volume of 0.0100 M EDTA for Ca2+ alone and for total hardness in 50.00 mL. Compare with the analyst's result.
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Moles Ca2+ = 0.020 mol L^-1 × 0.05000 L = 1.00 × 10^-3 mol. Volume EDTA = 1.00 × 10^-3 mol / 0.0100 mol L^-1 = 0.1000 L = 100.0 mL. Moles total hardness = (0.020 + 0.010) × 0.05000 = 1.50 × 10^-3 mol; volume = 0.1500 L = 150.0 mL. The analyst's apparent Ca2+ of 0.031 M corresponds to 0.031 × 0.05000 / 0.0100 = 0.155 L = 155 mL. This is close to the total-hardness volume of 150 mL plus a small Fe/Al contribution. Therefore the direct titration is measuring total hardness, not Ca2+ alone. After masking Fe/Al and precipitating Mg2+, the Ca2+ titration should require 100.0 mL; the Mg2+ contribution is 50.0 mL.