Re: How does Solochrome black indicator work in an edta- calcium ion titration?

The EDTA indicator that I am familiar with is Eriochrome Black. From your 
description and the sound of the name Solochrome Black is probably very 
closely related and works in a similar way. 

Eriochrome Black forms complexes with a large number of metals, including 
calcium, but not with sodium, ammonium, nor potassium. Most of these metal 
complexes are a strong wine red colour. When the dye is not complexed with 
a metal, and when it is in a solution around pH 10, it has a blue-grey 
colour. (it is actually also an acid base indicator, and has different 
colours in acidic or very alkaline solutions, so it is usual to work with a 
well buffered solution). The metal complexes with Eriochrome Black form and 
dissociate reversibly and fairly rapidly.

EDTA is a molecule that also forms complexes with nearly all metals. It 
forms particularly strong complexes, because it has six points of 
attachment that allow the molecule to wrap itself around the metal ion like 
a fur coat and attach at all 6 of the octahedral co-ordination sites.

When you start titrating water containing calcium ion with EDTA, you first 
put in the Eriochrome Black (or Solochrome Black), and you see the 
characteristic wine red colour, because the dye attaches itself to the 
metal ions. But as you add more and more EDTA, all of the calcium ions 
gradually put on their fur coats, and become unavailable for bonding to the 
dye. When enough EDTA has been added to coat all of the metal ions present, 
there is nothing left for the dye to complex with, and so the red colour is 
lost and replaced with the blue/grey. This is, of course, the endpoint of 
the titration.

One important point is that when you do an EDTA titration in this way, it 
is in no way specific for calcium. You are simply determining the sum total 
of all metal ions other than sodium or potassium in the solution! So to 
make the procedure specific for calcium, steps must be taken to precipitate 
out any other metals that might be present. Using a high pH will get rid of 
a lot of them as hydroxides -- especially things like iron and aluminium -- 
but you really need to take more special steps to stop magnesium from 
interfering. Either that, or you could regard the result as a 'calcium plus 
magnesium' determination.

Eriochrome black only forms a fairly weak calcium complex, and you often do 
not get a good endpoint with calcium. Solochrome black might be better in 
that regard. One variant of the titration procedure is to use a back-
titration. You add a measured excess of EDTA to the calcium-containing 
solution, and then back-titrate by adding magnesium chloride solution until 
you start to see the wine red colour from the magnesium complex. That means 
that the measured amount of EDTA you added was used up in part by the 
calcium ions originally present in the solution, and in part by the 
measured amount of magnesium ions you added in the back titration.

As far as the ion selective electrode is concerned, yes, I do know more or 
less how it works, but is that really so very helpful to you? (All right, 
I'm just having a little joke with the wording of your question!)
There are a couple of web sites I managed to find, which are probably only 
slightly helpful.  

A Polish site explaining ISEs  and

Another explanation of ISEs 

I'll try very briefly to amplify the explanations. The potential of an 
electrochemical cell varies slightly with the concentrations of the species 
involved in the oxidation and reduction reactions associated with the cell.
The variation is only fairly small: between 50 and 60 millivolts per factor 
of 10 in concentration for a single electron transfer redox system, and 
only half as much if two electrons are transferred. But the voltage 
difference, although small, is reliably and reproducibly measurable to give 
a concentration result at a 5% accuracy sort of level. This is adequate for 
most quality control and/or environmental monitoring.

The "ion selective" part comes in at the level of designing an 
electrochemical cell whose emf is sensitive to the concentration of only 
one ion, and unaffected by the presence or concentrations of other ions. 
This is generally done by designing a membrane which allows passage of 
water, but not ions in general. An organic complexing agent is incorporated 
into the membrane that will complex with only one sort of ion -- calcium in 
this case -- and allow calcium ions to pass through the membrane (but not 
magnesium or ferrous or zinc, etc. nor any of the anions) This is the 
ionophore. Inside the membrane there is a solution with high ionic strength 
of inert ions -- typically potassium chloride, or something like that, and 
the electrode itself. Passage of calcium ions through the membrane creates 
a charge imbalance which leads to a potential difference between the inner 
and outer surfaces of the membrane, which then contributes to the potential 
difference between the working electrode and the reference electrode.