To a 250 ml beaker containing 1 ml of 9 M
H2SO4 and 5 ml of HCl conc., pour in quantitatively
by portionwise the combined filtrate and
washings while stirring vigorously to make a clear
solution. Boil to decompose the residual DDTC
and concentrate the volume of the analyte solution
to 60–70 ml. Add, while hot, 5–10 ml of
EDTA solution (note 8), adjust pH to 4 with
ammonia solution, then add one or two drops of
the screened indicator solution and sufficient
buffer solution till the indicator just turns green.
Finally, add 10–30 ml (note 9) of TDTC solution
dropwise with constant stirring.