1.1 This method describes a procedure for determining the copper number of bleached and purified pulp, paper, and paperboard, except those containing calcium sulfite, zinc sulfide, melamine resin, or other copper-reducing nonfibrous materials. Papers containing such additives can be tested only if the amount and reducing power of the added material is known.
1.2 It has been well established that hydrolyzed or oxidized cellulose is capable of reducing certain metallic ions to lower valence states, and reactions of this type have served to detect damage to cellulose and to estimate the quantity of reducing groups.
1.3 The copper number may be regarded as an index of those impurities in paper, such as oxycellulose, hydrocellulose, lignin, and sugars, which possess reducing properties. It is useful for determining changes accompanying deterioration and may therefore be considered as a factor having an indirect bearing on the permanence of paper. It should not be applied to papers containing mechanical pulp or unbleached chemical pulp.