Analysis of calcium oxide content in limestone

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For the analysis of calcium oxide in limestone, the method we use currently is as follows: Lime stone powder. 1. Method summary: The sample is decomposed using hydrochloric acid, hydrofluoric acid, and perchloric acid; interfering elements such as iron and aluminum are masked with triethanolamine. In a solution with a pH of >12.5, calcium is titrated using an EDTA standard solution, with calcium carboxylate as an indicator. II. Reagents and Solutions 1. Salt acid (1+1) 2. Hydrofluoric acid 3. Hypochlorite* 4. Triethanolamine (1+1) 5. Potassium hydroxide 200 g/L 6. Dextrin at 40 g/L 7. Standard EDTA titration solution: C(EDTA) = 0.02 mol/L 8. Calcium carboxylate indicator: Weigh 1 g of calcium carboxylate and 100 g of sodium chloride dried at 105°C, grind them together, mix well, and store in a stoppered flask. III. Procedure: (According to the national standard method), weigh approximately 0.2 g of the sample, with an accuracy of 0.0001 g, and place it in a 100 ml polytetrafluoroethylene crucible; simultaneously conduct a blank test. Moisten the sample with a little water, cover it with a watch glass, and add salt acid (1+1) drop by drop through the mouth of the beaker. Once the vigorous reaction has ceased, add an additional 1 ml of this solution; then rinse the watch glass and the walls of the beaker. Add 4 ml of hydrofluoric acid and 2 ml of perchloric acid, and place the mixture on an electric heating plate to heat it until it is nearly dry. Remove the beaker, let it cool slightly, rinse the walls of the beaker with a little water, and continue heating until no white smoke remains, at which point let it cool again. Add 3 ml of salt acid, heat the solution until it becomes clear, then cool it to room temperature. Transfer the solution to a 250 ml volumetric flask and dilute it to the mark with water; shake well. For the determination of calcium oxide: Take 50 ml of the prepared solution and place it in a 250 ml beaker. Add 100 ml of water, 10 ml of dextrin, 5 ml of triethanolamine, and 15 ml of potassium hydroxide solution to ensure that the pH of the solution is greater than 12.5. Add a small amount of calcium carboxylate indicator, shake well, and titrate the solution using EDTA standard until the fluorescence disappears (quick method). Weigh approximately 0.2 g of the sample, with precision to 0.0001 g, and place it in a 100 ml beaker; conduct a blank test simultaneously. Moisten the sample with a little water, cover it with a watch glass, and add salt acid (1+1) drop by drop through the mouth of the beaker. Once the vigorous reaction has ceased, add an additional 1 ml of this solution; then rinse the watch glass and the walls of the beaker. Add 10 ml of salt acid (1+1) and heat the mixture on an electric heater until it is almost dry. Remove the beaker, allow it to cool slightly, rinse the walls of the beaker with a little water, and continue heating until no white smoke remains, at which point let it cool again. Add 3 ml of salt acid and heat the solution until it becomes clear. Cool the solution to room temperature, transfer it to a 200 ml volumetric flask, dilute to the mark with water, and mix well.

For the determination of calcium oxide: Take 50 ml of the prepared solution and place it in a 250 ml beaker. Add 100 ml of water, 10 ml of dextrin, 5 ml of triethanolamine, and 15 ml of potassium hydroxide solution to ensure that the pH of the solution is greater than 12.5. Add a small amount of calcium carboxylate indicator, mix well, and then titrate the solution using EDTA standard solution until the fluorescence disappears.

**Calculations:**
- According to the national standard method: CaO% = C * (V – V0) * 0.05608 / m * (50/250) * 100
- According to the rapid method: CaO% = C * (V – V0) * 0.05608 / m * (50/200) * 100

Where:
C = concentration of the EDTA standard solution, in mol/L
V = volume of EDTA standard solution used in the titration, in ml
V0 = volume of EDTA standard solution used in the blank test, in ml
m = mass of the sample, in g

CaCO3% = X1 * 100.08 / 56.08
Where X1 is the percentage content of CaO.

However, when the same sample is analyzed by two different testing laboratories, the results can vary significantly. One laboratory uses sodium hydroxide for dissolution followed by back-titration, while another uses the method specified in GB1574 for determining calcium oxide in ash components. The differences in results are quite large. I would like to know which method is more reliable and why such big differences exist in the data