Abstract: EDTA is a good complexing agent in chemistry. It has six coordination atoms. The complex formed is called chelate. EDTA is often used in coordination titration. It is generally used to determine the content of metal ions. In the application, it is used to eliminate the interference of most excessive metal element ions (such as iron (III), nickel (II), and manganese (II)). When the EDTA method is used to determine the hardness, some water samples interfere seriously, and the hardness is not easy to measure. Using ion chromatography to determine the hardness can avoid such interference. After filtering the water sample through a 0.45μm microporous membrane, one sample is injected, and the content of calcium and magnesium ions in the water sample is measured at the same time, and then converted into hardness expressed by calcium carbonate. The recovery rate of this method is 98.2% -102.0% The relative standard deviation is 0.94% -1.21%. The EDTA titration method is the only method specified in GB5750-85 [1] for determining the hardness in water. Due to the reasons of water pollution, the EDTA method is often difficult to accurately titrate due to many interference factors. The ion chromatography [2] is used to determine the hardness in water, which can effectively avoid the interference of organic matter. And without considering the influence of magnesium ions, it can still be directly measured when the magnesium content is too low. Ion chromatography is to measure the mass concentration of calcium and magnesium ions in water, and then convert it to the hardness expressed by CaCo3. This method has the characteristics of less consumption, simplicity, speed and accuracy. 1 Experimental part 1.1 Instruments and reagents DX-120 ion chromatograph (equipped with PeakNet chromatography workstation), conductivity detector; 98% H2SO4 (superior grade pure), Ca2, Mg2 national standard solution, deionized water 1.2 Chromatographic conditions IonpacCSl2 (4mm) cation separation column and guard column; eluent: c (H2SO4) = 11mmol / L; flow rate: 1mL / min; quantitative tube specification: 25μL; room temperature: 25 ° C. 1.3 Standard working curve Take 5 100mL volumetric flasks, and the mass concentration should be 2.0, 4.0, 6.0, 8.0, 10.0mg / L magnesium and 2.0, 5.0, 10.0, 15.0, 20.0mg / L calcium to make up a standard series, and measure separately, using PeakNet software to collect Test data and compile standard working curve of magnesium and calcium. 1.4 Determination of water samples After filtering the water sample through a 0.45μm microporous membrane, dilute it with deionized water at a certain ratio, and then directly inject the sample for measurement. Substitute the measured mass concentration of calcium and magnesium ions into the following formula to convert to hardness expressed by the mass concentration of calcium carbonate. Ï = (Ï1 / 40.08 Ï2 / 24.31) × 100.1 In the formula: Ï-hardness (calculated by CaCo3), mg / L; Ï1——The mass concentration of calcium mg / L; Ï2——Mass concentration of magnesium, mg / L. 2 Results and discussion 2.1 Linear relationship The test results show that there is a good linear relationship between the concentration (Y) of each sample ion and the peak area response value (X) within the range concentration used in this article, as shown in Table 1: Table 1 Linear relationship of analysis method Element linear regression equation correlation coefficient Calcium Y = 4.315e-6X ten 0.05602R2 = 0.999968 Magnesium Y = 2.721e-6X ten 002360R2 = 0.999938 2.2 Precision According to the above test conditions, one of the water samples was measured 5 times, and the relative standard deviation of the calculation results was 0.94% for calcium and 1.21% for magnesium. It can be seen that the precision is better. 2.3 Recovery rate Select 3 representative water samples, add a certain amount of calcium and magnesium standard solution, shake and measure, the results are shown in Table 2: Table 2 Water sample spike recovery Element mass concentration / (mg · L-1) spiked amount / (mg · L-1) measured value after spiked / (mg · L-1) Recovery rate/% Calcium 5.49 * 5.0010.58101.8 2.175.007.1399.2 7.525.0012.4398.2 Magnesium 7.3594.0011.65101.5 5.814.009.89102.0 3.844.007.8199.5 Note: The sample with "*" is well water with serious interference during EDTA titration. It can be seen from Table 2 that the recovery rate of the water sample spiked by ion chromatography is between 98.2% -102.0%, and the measurement result is accurate *. 2.4 Comparison between ion chromatography and EDTA titration Select interference-free water samples to be measured simultaneously by ion chromatography and EDTA titration. The results are shown in Table 3: Table 3 Control test results Sample Ion Chromatography Ï (CaCo3) / (mg · L-1) EDTA titration method Ï (CaCo3) / (mg · L-1) absolute error 1106.8107.1-0.3 2174.3173.70.6 3379.9378.61.3 4215.2215.8-0.6 5113.4114.5-0.9 6137.3136.80.5 7161.7160.61.1 8203.2202.50.7 9148.2149.7-0.5 10118.5119.30.8 It can be seen from Table 3 that there is no obvious difference between the two methods, which can be regarded as an equivalent method. When the EDTA titration method is used to measure the interference of water samples, the ion chromatography method is more accurate. 2.5 Multi-peak phenomenon The test found that when the mass concentration of magnesium is greater than or equal to 5 mg / L, if the pH value of the water sample is less than or equal to 1.5, magnesium has a double peak (or hump). But as the mass concentration of magnesium decreases, the pH value can be correspondingly lower. For example, when the mass concentration of magnesium is 2mg / L, and the pH value drops to 1, double peaks (or humps) will not appear. The mechanism of this phenomenon needs further study. Under normal circumstances, the water sample does not need to be processed, but if the pH value of the water sample is too low, the pH value should be adjusted when determining the hardness by ion chromatography. 4 burners gas grill, Home Gas Grill, Propane Outdoor Grills, Barbeque Outdoor Grill, Barbecue Grill JIANGMEN XINXIN METAL PRODUCTS CO., LTD. , https://www.bbqoutdoorgrill.com