Testing the pH at the surface of a concrete slab must be conducted in accordance with the current version of ASTM F710, not to exceed manufacturer’s requirements (ASTM F710 - Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring).
NOTE: Preparing the surface of a concrete slab for pH testing can be problematical. Make sure the concrete surface is adequately cleaned of adhesives, curing compounds etc. When pH readings are less than 7, it may be an indication of a residue remaining on the concrete surface. Also use care not to over clean the surface of the concrete, consequently removing the (usually) thin layer of carbonation. This can result in pH readings >12.