Reducing substances—
Transfer 1.0 g to a 250-mL conical flask, dissolve in 10 mL of water, and add 25 mL of
alkaline cupric citrate TS. Cover the flask, boil gently for 5 minutes, accurately timed, and cool rapidly to room temperature. Add 25 mL of 0.6 N acetic acid, 10.0 mL of 0.1 N iodine VS, and 10 mL of 3 N hydrochloric acid, and titrate with 0.1 N sodium thiosulfate VS, adding 3 mL of
starch TS as the endpoint is approached. Perform a blank determination, omitting the specimen, and note the difference in volumes required. Each mL of the difference in volume of 0.1 N sodium thiosulfate consumed is equivalent to 2.7 mg of reducing substances (as dextrose): the limit is 1.0%.
Assay—
Potassium stock solution—
Dissolve 190.7 mg of potassium chloride, previously dried at 105
for 2 hours, in water. Transfer to a 1000-mL volumetric flask, dilute with water to volume, and mix. Transfer 100.0 mL of this solution to a 1000-mL volumetric flask, dilute with water to volume, and mix. This solution contains 10 µg of potassium (equivalent to 19.07 µg of potassium chloride) per mL.
Standard preparations—
To separate 100-mL volumetric flasks transfer 10.0, 15.0, and 20.0 mL, respectively, of Potassium stock solution. To each flask add 2.0 mL of sodium chloride solution (1 in 5) and 1.0 mL of hydrochloric acid, dilute with water to volume, and mix. The Standard preparations contain, respectively, 1.0, 1.5, and 2.0 µg of potassium per mL.
Assay preparation—
Transfer about 180 mg of Potassium Gluconate, accurately weighed, to a 1000-mL volumetric flask, add water to volume, and mix. Filter a portion of the solution. Transfer 5.0 mL of the filtrate to a 100-mL volumetric flask, add 2.0 mL of sodium chloride solution (1 in 5) and 1.0 mL of hydrochloric acid, dilute with water to volume, and mix.
Procedure—
Concomitantly determine the absorbances of the
Standard preparations and the
Assay preparation at the potassium emission line of 766.5 nm, with a suitable atomic absorption spectrophotometer (see
Spectrophotometry and Light-scattering 
851
) equipped with a potassium hollow-cathode lamp and an air–acetylene flame, using water as the blank. Plot the absorbance of the
Standard preparation versus concentration, in µg per mL, of potassium, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration, in µg per mL, of potassium in the
Assay preparation. Calculate the weight, in mg, of C
6H
11KO
7 in the Potassium Gluconate taken by the formula:
20C(234.25 / 39.10),
in which
C is the concentration, in µg per mL, of potassium in the
Assay preparation, 234.25 is the molecular weight of potassium gluconate, and 39.10 is the atomic weight of potassium.
;)
