Calcium ion stable photographic color developing concentrate and method of manufacture

A method of preparing a homogeneous, ready to use, sulfate-free, single-part color developing final concentrate comprising the steps of:

• A) mixing in water, a color developing agent present as a sulfate salt, an antioxidant for said color developing agent, alkali metal ions in at least stoichiometric proportion to said sulfate salt, and a photographically inactive water-miscible or water-soluble, hydroxy-containing straight-chain organic solvent that has a molecular weight of from about 50 to about 200, and is present in said final concentrate at a concentration wherein the weight ratio of water to said solvent is from about 15:85 to about 50:50, to form a water-insoluble alkali metal sulfate in a first solution,
• B) removing said water-insoluble alkali metal sulfate from said first solution,
• C) forming a second solution in water comprising:
• a buffering agent that is soluble in said organic solvent, and a polyaminopolyphosphonic acid that has at least five phosphonic acid groups in an amount to provide at least 0.005 mol/l in said final concentrate,
• D) prior to step C, adding to said first or second solution a diphosphonic acid that is either a hydroxyalkylidene diphosphonic acid or a salt thereof, or morpholinomethanediphosphonic acid or a salt thereof, said diphosphonic acid being present in said final concentrate at a concentration of at least 0.0001 mol/l,
• provided that the diphosphonic acid is diluted in water so that the pH is 6 or more prior to its addition to the first or second solution, and
• E) mixing said first and second solutions to form said final concentrate.