[lourdes_cat]

Hi everyone,

I have some problems in writing my chem lab report.
I am doing a EDTA titration to determine the concentration of calcium and Magnesium ions in water using Eriochrome Black T as indicator.
What is the characteristics of Eriochrome Black T which justify itself as the indicator for the complexmetric titration with EDTA ?
Besides , what is the alternative method to determine the concentration of calcium ions by emf measurements and how does it work ? What is the merit of the two approachs(complexmetric titration and emf measurements)?
 
Thx a lot~

[enahs]

What is the characteristics of Eriochrome Black T which justify itself as the indicator for the complexmetric titration with EDTA ?

What is a metal-ion indicator and what is special about EDTA?

Besides , what is the alternative method to determine the concentration of calcium ions by emf measurements and how does it work ?

You would use an Ion-Selective electrode
http://www.nico2000.net/Book/Guide1.html
http://en.wikipedia.org/wiki/Ion_selective_electrode

What is the merit of the two approachs(complexmetric titration and emf measurements)?

EDTA titration is cheap, easy and the limiting accuracy is your glassware and technique, as many of the inherit errors of using an indicator can be nullified by your technique. Such techniques are blank and back titrations. You can quickly, cheaply and easily make multiple runs and infer using statistical methods. Titration calibration curves are easily made. You can also perform indirect titrations and use masking agents and compare the results using various statistical calculations and get fairly accurate results. The down side is you have to have a titratiable amount of analyte (which in many cases you do not), and you must know the “general” concentration. There is such a thing as too little to accurately titrate with EDTA and too much. Also, (back to the question what is special about EDTA) EDTA behaves differently at varying pH’s, and in some solutions you just can not use it at all because of the pH or possible change in pH (or other factors, what is special about EDTA?).


Ion-Selective electrodes are much much easier then EDTA titration and much quicker. You do not have to use any of your analyte, you just dip the electrode in the analyte and you have a reading. The down side is they are Ion-Selective and while some work for multiple ions, in say the water hardness determination you might need two separate electrodes; which are not all that expensive but compared to EDTA they are quite expensive (and very very fragile and easily damaged). Another down side with Ion-Selective electrodes is they are not very accurate at just measuring the concentration. You can get better results with EDTA titration (and proper technique) then you can just dipping the Ion-Selective electrodes into your analyte to try and measure the absolute concentration. That is to say, just dipping the Ion-Selective electrode in your analyte is only somewhat accurate at determining the concentration of the solution; it depends on how accurate you need for each situation (and how much you can spend on electrodes) They are however extremely extremely accurate at measuring the change in concentration. If you perform a titration but use the Ion-Selective electrode to determine the end point it will be many factors more accurate then an indicator. In doing this you are not limited to EDTA, you can use many reagents, so when EDTA is not a good candidate you can use other reagents. You are also eliminating the indicator error, and having to find an indicator that works with the analyte at the particular pH ranges.

[Borek]

The down side is they are Ion-Selective and while some work for multiple ions, in say the water hardness determination you might need two separate electrodes

It is even worse, as their selectivity is finite - other ions can interfere with your readings.

Look at the Nicolsky-Eisenman equation - while it is here in the form suitable for pH electrode, it will look similar for every ion selective electrode. Ca and Mg have similar chemical properties, so they will interfere one with the other. It is not necesarilly bad thing when you want to determine water hardness, but you have to know what you are doing.

Same about EDTA titration - Mg and Ca are getting complexed almost the same way, you may separate them only in high pH, precipitating Mg(OH)₂.