AOAC Official Method 942.01 Copper in Fertilizers Short Volumetric Method

A. Reagents
(a) Sodium thiosulfate standard solution.—0.03M. Prepare daily by diluting 0.1M solution, 942.27 (see A.1.13). 1 mL 0.03M Na2S2O3 = 1.906 mg Cu.
(b) Starch solution.—Mix ca 1 g soluble starch with enough cold H2O to make thin paste, add 100 mL boiling H2O, and boil ca 1 min
while stirring.
(c) Bromocresol green indicator.—Dissolve 0.1 g tetrabromo-m-cresolsulfonphthalein in 1.5 mL 0.1M NaOH, and dilute to 100 mL with H2O.
B. Determination
Place 2 g test portion in 300mLErlenmeyer and add 10mLHNO3 and 5 mL H2SO4. Digest on hot plate to white fumes. If solution darkens, owing to organic matter, cool slightly, add little more HNO3, and digest again to white fumes, repeating operation if necessary until organic matter appears to be destroyed. Cool, add 50 mL H2O, boil ca 1 min, and cool to room temperature. Add bromocresol green, then NH4OH until indicator changes to
light green (pH 4). Cool again to room temperature, and if indicator changes back to more acid color, add NH4OH dropwise until indicator
becomes light green again, avoiding excess. Add 2 g NH4HF2, mix well, and let stand ca 5 min. Add 8–10 g KI, mix well, and titrate with standard Na2S2O3 solution to light yellow. Add ca 1 mL starch solution and continue titrating slowly until color is nearly same as just before addition of the KI and becomes no darker on standing 20 s. Report as % Cu.

Reference: JAOAC 25, 77, 352(1942).