Definition of acid-free 

: not containing acid : having a neutral or alkaline pH acid-free paper Every evening, trained volunteers will place any personal items or memorabilia left at the replica wall in archival folders or acid-free boxes.— Brian Burnes, Kansas City Star, 22 Sept. 2010

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First Known Use of acid-free

1885, in the meaning defined above

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The first known use of acid-free was in 1885

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