Cupric Tartrate, Alkaline, Solution (Fehling’s Solution)
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
Copper solution (Solution A): Transfer 34.639 g of cupric sulfate to a 500-mL volumetric flask, and dissolve in and dilute with water to volume. Filter, if necessary.
Alkaline tartrate solution (Solution B): Transfer 173 g of potassium sodium tartrate and 50 g of sodium hydroxide to a 500-mL volumetric flask, and dissolve in and dilute with water to volume. Filter, if necessary.
Just before use, prepare Cupric Tartrate, Alkaline, Solution by mixing equal volumes of Solution A and Solution B.
• Standardization
Standard stock solution: Transfer 9.5 g of Sucrose to a 1-L volumetricflask, dissolve in 100 mL of water, add 5 mL of hydrochloric acid, and store for 3 days at 20°–25°. Dilute with water to volume. This solution is stable for several months.
Invert sugar solution: Immediately before use in standardizing the Cupric Tartrate, Alkaline, Solution, transfer 25 mL of Standard stock solution to a 100-mL volumetric flask, and dilute with water to volume.
Procedure
Apparatus: Mount a ring support on a ring stand 1–2 inches above a gas burner, and mount a second ring 6–7 inches above the first. Place 6-inch open-wire gauze on the lower ring to support a 400-mL conical flask, and place a 4-inch watch glass with a center hole on the upper ring to defiect heat. Attach a 50-mL buret to the ring stand so that the tip just passes through the watch glass centered above the flask.
Place an indirectly lighted white surface behind the assembly for observing the endpoint.
Standardization: Transfer 20.0 mL of the Cupric Tartrate, Alkaline, Solution to a 400-mL flask containing a few boiling chips, and add 15 mL of water and 39.0 mL of Invert sugar solution. Mix by swirling at ambient temperature, and immediately place the flask on the wire gauze of the Apparatus. Adjust the burner so that the boiling point of the solution is reached in about 2 min. Boil gently but steadily for 2 min. As boiling continues, add 3–4 drops of methylene blue solution (1 in 100). Complete the titration within 1 min by adding the Invert sugar solution dropwise until the blue color disappears. Allow a 5-s reaction time between drops at the end of titration. Adjust the Cupric Tartrate, Alkaline, Solution for the correct amount of copper (equivalent to 100 mg of invert sugar), and restandardize if the total volume of Invert sugar solution is more or less than 40.0 mL.
[Note—If this volumetric solution is used in a qualitative application such as pH adjustment, dissolution medium, or diluent, its standardization is not required.]
