1. Experiment 17 Determination of the Content of Borax

2. 1. To know the application of acid-base titration. 2. To master the technology of titration and how to determine the end point of the titration.

3. Borax (sodium borate Na 2 B 4 O 7 ·10H 2 O) is soluble in water and produces Na + and B 4 O 7 2- ions. According to Brönsted definition B 4 O 7 2- is a proton acceptor. It takes protons from water molecules and thus is a base. Therefore, the quantity of borax can be determined through acid-base titration. B 4 O 7 2- + 7H 2 O = H 3 BO 3 + 2OH - When standard hydrochloric acid solution is used to titrate borax solution, the following reaction takes place: Na 2 B 4 O 7 ·10H 2 O + 2HCl = 2NaCl + 4H 3 BO 3 + 5H 2 O

4. According to the equations above, at stoichiometric point, we have the following equations: n(HCl) = n(Na 2 B 4 O 7 ·10H 2 O) Borax(%) =

5. 1 ． Preparation of Borax Solution 称量瓶： weighing bottle 硼砂： borax 分析天平： analytical balance 烧杯： beaker 搅拌： stirring 玻璃棒： glass rod 刻度线： calibration mark 1.9-2.1g of borax (±0.0001g precisely) →100mL Adding distilled water

6. Calculate the volume of concentrated HCl V(concentrated HCl) = ？ mL? 量筒： measuring cylinder 2. Preparation of 0.1 mol/L HCl solution 300 mL Adding distilled water to 300mL

7. 无水碳酸钠： anhydrous sodium carbonate 刻度线： calibration mark 3. Preparation of standard solution of Na 2 CO 3 1.2g ～ 1.4g (±0.0001g precisely) Na 2 CO 3 →250mL Adding distilled water

8. 4. Standardization the concentration of HCl solution with anhydrous sodium carbonate 20.00mL Na 2 CO 3 standard solution 2 drops of methyl orange indicator HCl solution beforet itration : yellow The end of titration: orange

9. 5. Determination of the Content of Borax 20.00mL borax solution 2 drops of methyl red indicator HCl solution beforet itration : yellow the endpoint of the titration : orange