Quantitative Filter Paper
—See Filter Paper,Quantitative.

C6H4(OH)2·C6H4O2218.21—Green crystals having a metallic luster.Slightly soluble in cold water;soluble in hot water,in alcohol,and in ether.
Assay— Transfer about 450mg,accurately weighed,to a glass-stoppered flask,add 50mLof 1Nsulfuric acid and 3g of potassium iodide,insert the stopper in the flask,and shake until dissolved.Titrate the liberated iodine with 0.1Nsodium thiosulfate VS,adding 3mLof starch TSas the endpoint is approached.Each mLof 0.1Nsodium thiosulfate is equivalent to 5.405mg of quinone (C6H4O2).Between 49.0%and 51.0%is found.
Alcohol-insoluble matter— Dissolve 10g in 100mLof hot alcohol,filter through a suitable tared crucible of fine porosity,and wash with hot alcohol until the last washing is colorless.Dry at 105,cool in a desiccator,and weigh:the residue weighs not more than 1.0mg (0.010%).
Residue on ignition(Reagent test): not more than 0.050%,a 2.0-g test specimen being used.Save the residue.
Sulfate— Transfer 1g to a platinum crucible,add 10mLof hot water and 0.5g of sodium carbonate,evaporate to dryness,and ignite,protected from the sulfur in the flame,until the residue is nearly white.Cool,add 20mLof water and 1mLof 30percent hydrogen peroxide,boil gently for a few minutes,add 2mLof hydrochloric acid,and evaporate on a steam bath to dryness.Cool,dissolve the residue in 20mLof water,filter,and to the filtrate add 1mLof 1Nhydrochloric acid and 3mLof barium chloride TS:any turbidity produced within 10minutes does not exceed that in a control containing 0.2mg of added SO4and 0.5mg of sodium carbonate,1mLof 30percent hydrogen peroxide,and 2mLof hydrochloric acid previously evaporated on a steam bath to dryness (0.02%).
Heavy metals— To the residue retained from the test for Residue on ignitionadd 2mLof hydrochloric acid and 0.5mLof nitric acid,and evaporate on a steam bath to dryness.Dissolve the residue in 30mLof hot water containing 1mLof 1Nhydrochloric acid,cool,dilute with water to 40mL,and mix.Dilute 20mLof this solution (retain the rest of the solution)with water to 25mL,adjust to a pHbetween 3.0and 4.0by the addition of 1Nacetic acid or 6Nammonium hydroxide as necessary,dilute with water to 40mL,and add 10mLof freshly prepared hydrogen sulfide TS:any brown color produced does not exceed that in a control containing 0.02mg of added Pb (0.002%).
Iron á241ñ To 10mLof the solution retained from the test for Heavy metalsadd 2mLof hydrochloric acid,and dilute with water to 47mL:the solution shows not more than 0.01mg of Fe (0.002%).

—See p-Benzoquinone.