I am studying the complexation of tetracycline with metals through potentiometric titration of tetracycline with metal solution. The goal is the determination of stability constant:
M + n(TC)= M(TC)n
M is metal, TC is tetracycline and M(TC)n is the complex.
stability constant= [M(TC)n]/[M]*[TC]n
In potentiometry we can find the endpoint of reaction but I want to know how it is possible to calculate the stability constant with knowing this endpoint?
In fact it is confusing, because endpoint means that the TC is finished, so how it is possible to calculate the stability constant with [TC]=0 ?
You must confirm what is the condition of equivalent point for complexmetric titration.
The indicator electrode potential depends on the concentration of metal ion.
ex. Determination of calcium, magnesium, and alkalinity by complexometric titration with potentiometric method.
From the equivalent point, pM can be determination and the stability constant can be calculated.
I would like to add the following points for your question Dear Mehrdad:
1. Complexation ( formation of a coordination complex) is an equilibrium process hence cannot be a 100% reaction in the forward direction due to which in every complexation process there is some amount of free ligand always. Stability constant is nothing other than the equilibrium constant of the complexation process. However depending upon the magnitude of the stability constant or the stability of metal ligand complex or strength of Metal ligand bond the amount of free ligand in a complexation process will vary however it never can be zero due to the equilibrated nature of complexation process hope that clears your doubt of ligand or tetracycline concentration(TC) = 0 in your question.
2. Regarding the calculation of stability constant of metal tetracycline complexation process using potentiometric titration i suggest you follow the Bjerrum method which has been very exhaustively used and described in literature.(ref as pdf). the method is also described in the famous text book synthesis and technique in inorganic chemistry by Angelici.
3. The calculation of stability constant requires the determination of coordination number or number of ligand molecules attached to metal in the complex i.e n.this will depend on your metal its desired coordination geometry and the number of donor sites available from tetracycline ligand under the reaction conditions of your attempted complexation reaction.solving for n is an important task to calculate stability constant attached is a method for calculating n using potentiometry.
4. In case n = 1 or any other value which you can arrive from the ratio in which metal and tetracycline have reacted as per titration data. then a simple calculation will get you stability constant as:
[M] total and [L]total be the concentration of metal and ligand which you have used for complexation and it is known to you say equal to P & Q respectively then we can write equations as:
[M] total = [M] free + [M] complex ..........................(1)
[L] total = [L] free + [L] complex ..........................(2)
from titration you have already solved coordination number "n" and concentration of metal complex . say concentration of metal complex = X
then the free metal concentration as per equation a will be
[M] free = [M] total - X = P - X since both P as well as X is known it means [M] free also becomes known say = Z
Similarly from equation 2 we can obtain free ligand concentration as
[L] free = [L] total - X = Q - X ; since both Q as well as X is known it means [L] free also becomes known say = R . now everything we need to know is known to us as conc of complex = X; conc of free metal = Z, conc of free ligand = R , coordination number = n. we can substitute these in equilibrium constant expression to obtain the stability constant as :
stability constant= [M(TC)n]/[M]*[TC]n = [X]/ [Z] [R]n
I hope i could understand your question and my answer was any helpful. please ask if you want any furtherance however i may be more helpful if you could provide me with more details about your experiment.
Greetings from Kashmir.
I suggest using analytical chemistry such as titration tests.
Thanks Ahmad for your comprehensive reply. Actually the problem is calculating the coordination number with Bjerrum method. I could not find the method in detail. There are only several referring but the derivation and mathematical process are not explained.
Most of the research articles from BJerrum can be downloaded from the link
Bjerrum, Jannik - Acta Chemica Scandinavica
using google webpage.
All classical and early papers of Bjerrum can be read from this webpage. Its chronological as well as title mentioning. i hope it will be good resource for Bjerrum work.
The stability of the complex can be calculated using the molar method, which is a spectral method based on the absorption device. With Regards
If you still require a software for the determination of equilibrium constants from experimental data, I could recommend recently developed KEV (https://k-ev.org/). It is web-interface based and free of charge; though the simple e-mail registration prior to use is required.
Is it possible to determine stability constant for mixed ligand metal complexes in which one ligand with thiosemicarbazone and other with hydroxy group.
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