Dissolve Ca 25g Na2S2O3,H20 in 1 L H2O.
Boil gently 5 min and transfer while hot to storage bottle previously cleaned with hot chromic acid cleaning soln and rinsed with warm boiled H2O.
(Temper bottle, if not heat-resistant,before adding hot soln.)
Store soln in dark, cool place; do not return unused portions to stock bottle.
If solns less concd than 0.1 N are desired, prep.
by diln with boiled H2O.
(More dil. solns are less stable and should be prepd just before use.)