Procedure
a. Preparation of dilution water: Place desired volume of water ( 3j) in a suitable bottle and add 1 mL each of phosphate buffer, MgSO4, CaCl2, and FeCl3 solutions/L of water. Seed dilution water, if desired, as described in 4d. Test dilution water as described in 4h so that water of assured quality always is on hand.Before use bring dilution water temperature to 20 + 3°C. Saturate with DO by shaking in a partially filled bottle or by aerating with organic-free filtered air. Alternatively, store in cotton-plugged bottles long enough for water to become saturated with DO. Protect water quality by using clean glassware, tubing, and bottles.
b. Dilution water storage: Source water ( 3j) may be stored before use as long as the prepared dilution water meets quality control criteria in the dilution water blank ( 4h). Such storage may improve the quality of some source waters but may allow biological growth to cause deterioration in others. Preferably do not store prepared dilution water for more than 24 h after adding nutrients, minerals, and buffer unless dilution water blanks consistently meet quality control limits. Discard stored source water if dilution water blank shows more than 0.2 mg/L DO depletion in 5 d.