EDTA & EDDS are 2 chelating agents used for personal care products. Would you please let me know any method through which I can determine the stability constant value or binding capacity at pH 6 for my product (Shampoo) ?.
I am making a 500 ppm solution of Ca2+ & adding tetra sodium EDTA. I want to know how much calcium ions are remaining?
Dear Aakash,
For knowing the Ca ion concentration remaining, you should do the following:
1-You should calculate the moles number of Ca ion in 500 ppm (500 part per million); for that you need to know which calcium salt you have and its molecular weigh.
2-The next step you will should use the following equation:
N1V1 = N2 V2; where N1 and N2 are the normality of calcium salt and tetrasodium EDTA, respectively and V1 and V2 are the volumes of calcium salt and tetrasodium EDTA, respectively.
Please note that Na4EDTA is very strong chelating agent and it can chelate all the ca ions exist in the solution.
N = MW/valency.
Hoping this will be helpful,
Rafik
The chelating affinity of these agents is indeed very powerful, although it becomes less so as pH is reduced because H+ competes with Ca2+ for binding. If you want to estimate the concentration of free (that is, unbound) Ca2+ in your solutions, you could use a program like MaxChelator - see the "Older Web Maxc versions Standard" towards the bottom of the page in the link below. You can use this to calculate the free and bound concentrations of a mixture of metal ions and chelators at any given pH; the results also show the apparent Kd at that pH. Of course, this is not as good as direct measurement of Ca2+ concentration, but most methods to measure free Ca2+ directly struggle when the concentration of Ca2+ is below about 10-8 M.
http://maxchelator.stanford.edu
Dear Aakash,
Yes. There is IC method to determine Ca+2 concentrations. For details, please use the following link:
http://serc.carleton.edu/microbelife/research_methods/biogeochemical/ic.html
Hoping this will be helpful,
Rafik
Dear Aakash,
The constants (log form) in your table are the absolute stability (affinity) constants, for example for Ca2+ binding to the EDTA4- ion. These constants do not vary with pH. However, because the EDTA4- ion also binds H+, as pH is lowered the fraction of EDTA present as EDTA4- in a solution will decrease (since more H+ binds to the EDTA4- to form H.EDTA3-, H2EDTA2-, etc.), so the apparent affinity of EDTA for Ca2+ will fall as pH is lowered. In effect H+ is displacing Ca2+ from EDTA. To calculate free Ca2+ concentration in a solution at any pH, you can do either of 2 things: (1) Use the absolute Ca-EDTA4- constant in combination with the absolute stability constants for the sequential binding of H+ to EDTA4- (these proton constants are not shown in your table but are in the Martell book). This is the method used by Maxchelator program, which then automatically corrects for any change in pH. (2) Find or calculate the apparent affinity constant for Ca2+ binding to EDTA at a defined pH (since this apparent constant varies with pH). You can then calculate Ca2+ concentration by somehow rearranging the equation: Kapparent = [Ca.EDTA] / ([free Ca2+] [free EDTA]); sorry, I don't know how to rearrange this equation! This apparent constant is shown as one of the results in the Maxchelator calculation. I use the first method.
- Badges
- Science topic
- Similar topics
- Microbiology
- Assays