Titration of HCO3- with OH-, followed by titration of total
CO32- with Ba2+
HCO− +OH− ↔CO2− +HO 332
CO32− +Ba2+ ↔BaCO3 ↓
1. Standard 1mol/L sodium hydroxide
2. Standard 1mol/L barium acetate (Ba(OAc)2)
3. A.R. potassium hydrogen phthalate (for standardization of NaOH titrant)
4. A.R. anhydrous sodium carbonate (for standardization of Ba(OAc)2)
Basic Experimental Parameters:
Titrant delivery rate (mL/min.) 5
Data smoothing factor NaOH titration 70
Data smoothing factor Ba(OAc)2 titration 55
Stirring rates: NaOH titration 11
Ba(OAc)2 titration 12
Procedure: The titration is setup so that the NaOH titration of HCO3- is performed first. The succeeding Ba(OAc)2 titration for the total carbonate content is “daisy-chained” to the first titration, so that it starts automatically following completion of the HCO3- titration. It is only necessary to click “Start” for the HCO3- titration for the entire sequence to be completed automatically.
The example illustrated here is a solution of sodium hydrogen carbonate and sodium carbonate, with a total carbonate concentration of approximately 10g/L Na2CO3. A 50mL aliquot is pipetted into a titration vessel, and the titration sequence commenced without further sample preparation.
Standardization of titrants:
1. NaOH titrant is standardized against potassium hydrogen phthalate (KHPhth). Quantitities of KHPhth corresponding to approximately 1,2, 3, 4 and 5 mmole are weighed accurately into titration vessels and allowed to dissolve in 30mL of DI water before titrating at 5mL/min against the NaOH titrant. The results are plotted, mmole KHPhth on the x-axis, mL NaOH titrant consumed on the y-axis. The molarity of the titrant is calculated from the reciprocal of the gradient, as computed by regression analysis.
2. Ba(OAc)2 titrant is standardized against a 0.1mol/L Na2CO3 solution prepared from anhydrous A.R. Na2CO3. Aliquots are prepared according to the following table:
| mL 0.1mol/L Na2CO3 | mL DI water |
mmole Na2CO3 |
| 50 | 0 | 5 |
| 40 | 10 | 4 |
| 30 | 20 | 3 |
| 20 | 30 | 2 |
| 10 | 40 | 1 |
Solutions are titrated at 5mL/min, the results being plottedwith mmole Na2CO3 on the x-axis, mL Ba(OAc)2 titrant consumed on the y-a xis. The molarity of the titrant is calculated from the reciprocal of the gradient, as computed by regression analysis.
Solution comprising NaHCO3 and Na2CO3 with mole ratio of ~2:1, and with total carbonate concentration ~0.1mol/L as Na2CO3
NaHCO3 =5.38±0.014 g/L, n=10
Na2CO3 = 7.42±0.015 g/L, n=10
Total carbonate (as Na2CO3) = 10.81±0.012, n=10
NaHCO3 g/L = ((mL NaOH−mL blank)×84.01×MNaOH) mL sample
Total Carbonate g/L = (mL Ba(OAc)2 −mL blank)×105.99×MBa(OAc)2 mL sample
Na2CO3 g/L = Total Carbonate g/L − NaHCO3 g/L×105.99 2 × 84.01
Legend:
Red = solution temperature curve
Black = second derivative curve
