Library / Solution chemistry, equilibria and kinetics / SCH-33
Solution chemistry, equilibria and kinetics
Conditional EDTA formation constant
Effective formation constant of a metal–EDTA complex at a fixed pH, from the absolute constant Kf and the fraction αY of free EDTA in the fully deprotonated form Y⁴⁻.
- Ref
- SCH-33
- Rev
- v1
- Field
- Solution chemistry, equilibria and kinetics
- Result
- Kf' [L/mol]
EDTA complexes depend on pH because H⁺ competes for the Y⁴⁻ form. The conditional formation constant is Kf′ = αY Kf, where αY is the fraction of uncomplexed EDTA present as Y⁴⁻ at that pH. It is the constant that applies in a buffered solution and shows whether a titration will have a sharp endpoint: log Kf′ above about 8 is often needed. αY is tiny at low pH and approaches 1 above pH 12.
Effective formation constant of a metal–EDTA complex at a fixed pH, from the absolute constant Kf and the fraction αY of free EDTA in the fully deprotonated form Y⁴⁻. It drops steeply at low pH, which limits titrations to suitable pH ranges.
Calculator
- log₁₀ Kf′8.301
- log₁₀ Kf (absolute)10
- Margin over log₁₀ Kf′ = 80.301
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- At this pH the effective constant is 2×108 L/mol (log 8.301), 50 times lower than the absolute value.
- This is above the usual log Kf′ ≥ 8 guideline, so a sharp endpoint is likely.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (L/mol) | Local step | Change in result |
|---|---|---|---|---|---|---|
| Kf Formation constant of the complex | 1,000 to 1e25 L/mol | 20 | 2e23 | 2e23 | +10% | +10 % |
| αY Fraction of EDTA as Y⁴⁻ at the pH | 1e-8 to 1 | 100 | 1e10 | 1e10 | +10% | +10 % |
Sweep of Fraction of EDTA as Y⁴⁻ at the pH
| Fraction of EDTA as Y⁴⁻ at the pH | Conditional formation constant (L/mol) |
|---|---|
| 1e-8 | 100 |
| 1e-7 | 1,000 |
| 1e-6 | 10,000 |
| 1e-5 | 100,000 |
| 1e-4 | 1,000,000 |
| 0.001 | 1e7 |
| 0.01 | 1e8 |
| 0.1 | 1e9 |
| 1 | 1e10 |
Sweep one variable
Variables and how each one contributes
Conditional formation constant
What the equation gives you. Everything else feeds it.
Fraction of EDTA as Y⁴⁻ at the pH
The fraction of free EDTA as Y⁴⁻ at the working pH. Raises the conditional constant in direct proportion. Raising it by 10 % (to 0.022) moves the result by +10 %, from 2×108 to 2.2×108 L/mol.
Formation constant of the complex
The absolute formation constant of the metal–EDTA complex, in L/mol. Raises the conditional constant in direct proportion. Raising it by 10 % (to 1.1×1010) moves the result by +10 %, from 2×108 to 2.2×108 L/mol.
Worked example with the values above
- Fraction: αY = 0.02.
- Multiply by Kf = 1×1010.
- Kf′ = 2×108 L/mol.
Assumptions and limits
- A 1:1 complex, or a single neutral species partitioning, with constants in concentration units.
- Equilibrium reached in each step, and no side reactions unless the conditional constant includes them.
Common mistakes
- Using the total ligand concentration for the free ligand when binding depletes it.
- Using the absolute EDTA constant at a pH where much of the EDTA is protonated.
- Mixing the units of the two phase volumes.
Statistics and uncertainty
Formation constants depend on the medium and the temperature, and a quoted constant for a different ionic strength can be off by a factor of two or more. Extraction results are sensitive to pH near the pKa.
Where it is used
- Designing complexometric titrations and metal-binding experiments.
- Planning solvent extractions and pH-controlled separations.
Maths you need
- Formation constant: Kf = [ML]/([M][L])
- Partition coefficient: the ratio of a species in two phases
- Mass balance: total equals free plus bound