There are 3 ways to eliminate impurities in water
1, Distillation, the oldest method. Water is vaporized, leaving the impurities behind, then allowed to cool using a condenser to form liquid water again. Traditionally a copper condensor was used for its large heat transfer capabilrty. If a metal condensor or catch container is made of metal then the water may become contaminated with metal ions. A particular problem is with copper. Even ordinary glas is a problem in preparing water for conductivity experimnts. The sodium in ordinary glass leachs out. Only fused silica equipment can be used.
2 De-ionization, the water is passed throught a mixed resin ion exchange column. Metal ions are replaced with hydrogen ions and anions with hydroxide ions. The hydrogen and hydroxide ions combin to form water.
3 Reverse osmosis, water under great pressure is forced through a semipermeable barrier. Large ions cannot pass through leaving, in theory, pure water.
Iron and copper are common contaminants of the chemicals used for making developers. The wrorst offenders are probably sodium sulfite and sodium carbonate. So just using purified water may not help. A chelating agent is the best way to insure the storage life of ascorbate developers. Salicylic acid chelates iron and TEA does the same for copper.
With the right amount of iron III all the ascorbate in a solution can be destroyed in an hour or so. It can be that fast.