from 5 to 5 25 percent na 2 ta and 2 5 to 3 0 percent sodium hydroxide application_number US 96850778 A description the present invention comprises a contacting at a temperature of from 20 u00b0 c up to 100 u00b0 c an aqueous solution containing 5 to 6 weight percent sodium hydroxide the inner wall surfaces of elements of apparatus used in production of terephthalic acid by the steps from the oxidation of p xylene with air in the presence of a liquid phase of acetic acid containing dissolved components of catalysis through separation of solid terephthalic acid from acetic acid mother liquor and drying the separated terephthalic acid wherein said contacting the amount of the sodium hydroxide solution used dissolves as the disodium salt substantially all of the terephthalic acid adhering to said walls and leaves unreacted sodium hydroxide in an amount of from 2 to 3 weight percent of the solution b adding sodium hydroxide to the solution of disodium terephthalate and unreacted sodium hydroxide to increase its concentration to the range of from 12 up to 14 weight percent whereby from 94 percent up to substantially all of the dissolved disodium terephthalate precipitates c separating the aqueous solution containing 12 to 14 weight percent sodium hydroxide from precipitated disodium terephthalate d dividing the separated solution of 12 to 14 sodium hydroxide into a major portion of.