Re: What is in solochrome black indicator solution?

Erin,  although I'm no expert in analytical chemistry I struck lucky with  
your question.  My decades old textbook on Quantitative Inorganic Analysis 
affectionately known as "Belcher & Nutten" ( Butterworth's Scientific 
Publications, 1960 ) came up with something.  Solochrome Black T is also known 
as Erichrome Black T ( It should be Erin-chrome shouldn't it ! ).  It's an 
azo-naphthol dye with a naphthol on both sides of the azo link, one of which has 
both a sulphonate and nitrate substituent.  It's an acid-base indicator changing 
from wine-red to blue at a pH of 6.3 and from blue to orange at pH 11.3.  It is 
used in conjunction with EDTA for metal ion analysis.  It complexes with the 
metal ion, forming a weaker complex than with EDTA so the metal ions can be 
titrated with the EDTA solution.  The complex is red and the dye reverts to blue 
when the metal is complexed with EDTA provided the pH is in the right range.

B&N recommends a pH of 10 which can be obtained with ammonia/ammonium chloride 
buffer.  Calcium forms a weaker complex than magnesium and the titration forms 
calcium-EDTA first and only reaches an end-point when both Ca and Mg ions have 
been complexed by the EDTA.  Some metals form complexes with are too strong and 
the indicator action is lost.  To overcome this a "masking agent" is used to 
take out the interfering ions.  Cyanide can be used but triethanolamine is a lot 
safer !  

The formula for the indicator dye given in B&N is 0.2g indicator dye in 15 mls 
of triethanloamine and 5 ml ethanol.  Triethanolamine is a low-melting ( about 
20 C I believe ) and mixed with ethanol it will be liquid at room temperature.

Good Luck