Seek improvements to the method for determining calcium and magnesium using EDTA complexometric titration

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For the pretreatment of calcium and magnesium, we use EDTA complexometric titration; the values are between 100-200 mg/L. However, it’s not easy to determine the endpoint color. Below are our operating procedures. Dear teachers, could you please help us figure out how to make the endpoint color more visible? 1.3 Analysis of calcium and magnesium contents This method is suitable for determining the contents of calcium and magnesium ions in the saltwater used in the pretreater. 1.3.1 Principles Calcium ion analysis: Under alkaline conditions with pH=12, a calcium reagent is used as an indicator, and the sample is titrated with an EDTA standard solution. The calcium reagent first forms a red complex with calcium ions, which has poor stability; when EDTA is used for titration, it displaces the calcium ions from this complex, releasing the calcium reagent anion. The solution changes color from purplish-red to blue at this point, indicating the end of the titration. Magnesium ion analysis: The pH of the sample solution is adjusted to around 10 using a buffer solution, and chromium black T is used as an indicator. The sample is then titrated with an EDTA standard solution. The total amount of calcium and magnesium ions is determined, and the calcium ion content is subtracted from this total to obtain the magnesium ion content. 1.3.2 Instruments and reagents: a) General laboratory instruments; b) Hydrochloric acid solution: 0.5 mol/L; c) Hydroxylamine hydrochloride solution: 10 g/L; d) Triethanolamine solution: 30%; e) Sodium hydroxide solution: 2 mol/L; 113 1.3.4 Calculation of results: C·V = 40.08; Ca2+ (mg/L) = 1×10^3; Mg2+ (mg/L) = C·(V–V0)/24.31×10^3; Q/BH J JY 73-2022; f) Ammonium chloride buffer solution: pH=10; g) Calcium indicator: 1:100; h) Eriochrome Black T indicator solution: 5 g/L; i) EDTA standard solution: C(EDTA)=0.005 mol/L. 1.3.3 Analysis steps a) Calcium ion analysis: Using a pipette, transfer 50 mL of the sample into a 250 mL conical flask. First, adjust the pH value to around 2 using hydrochloric acid to acidify the sample. Then, add 5 mL of hydroxylamine hydrochloride solution and 5 mL of triethanolamine sequentially, stir well. Adjust the pH of the solution to 12 using sodium hydroxide, then add 0.1 g of calcium indicator and stir again. Titrate the solution with EDTA standard until it changes from red to bright blue; this marks the endpoint. Record the volume V1 of EDTA used. b) Magnesium ion analysis: Using a pipette, transfer 50 mL of the sample into a 250 mL conical flask. First, adjust the pH value to around 2 using hydrochloric acid to acidify the sample. Then, add 5 mL of hydroxylamine hydrochloride solution and 5 mL of triethanolamine sequentially, stir well, add 10 mL of buffer solution, and then add 2–5 drops of chrome black T indicator. Titrate with an EDTA standard solution until the color of the solution changes from red to bright blue; this marks the endpoint. Record the volume V2 of EDTA used in this titration. In equation 50: C is the molar concentration of the EDTA standard solution, in mol/L; V1 is the volume of EDTA standard solution consumed when calcium indicator is used as an indicator, in mL; V2 is the volume of EDTA standard solution consumed when Eriochrome Black T is used as an indicator, in mL; 40.08 is the molar mass of calcium, in g/mol; 24.31 is the molar mass of magnesium, in g/mol. 1.3.5 Precautions: This method is a complexometric titration; the titration rate should not be too fast. Near the endpoint, it is necessary to stir thoroughly after adding each drop