Experiment 17
Determination of the Content
of Borax
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.
Borax (sodium borate Na2B4O7·10H2O) is soluble in water and
produces Na+ and B4O72- ions. According to Brönsted definition
B4O72- 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.
B4O72- + 7H2O = H3BO3 + 2OHWhen standard hydrochloric acid solution is used to titrate borax
solution, the following reaction takes place:
Na2B4O7·10H2O + 2HCl = 2NaCl + 4H3BO3 + 5H2O
According to the equations above, at stoichiometric
point, we have the following equations:
n(HCl) = n(Na2B4O7·10H2O)
Borax(%) = c(HCl) V (HCl)  M ( Na 2 B4O7 ·10H 2O) 100%
msample  2 1000
1． Preparation of Borax Solution
称量瓶：weighing bottle
硼砂： borax
分析天平：analytical balance
烧杯： beaker
1.9-2.1g of borax
(±0.0001g precisely)
→100mL
搅拌： stirring
玻璃棒： glass rod
刻度线： calibration mark
Adding distilled water
2. Preparation of 0.1 mol/L HCl solution 300 mL
Calculate the volume of concentrated HCl
V(concentrated HCl) =
？
mL?
量筒：measuring cylinder
Adding distilled water
to 300mL
3. Preparation of standard solution of Na2CO3
无水碳酸钠： anhydrous sodium carbonate
刻度线： calibration mark
1.2g～1.4g (±0.0001g precisely)
Na2CO3 →250mL
Adding distilled water
4. Standardization the concentration of HCl
solution with anhydrous sodium carbonate
HCl solution
beforet itration : yellow
The end of titration: orange
20.00mL Na2CO3
standard solution
2 drops of
methyl orange
indicator
5. Determination of the Content of Borax
HCl solution
beforet itration : yellow
the endpoint of the titration : orange
20.00mL borax
solution
2 drops of
methyl red
indicator