Use the 4.0 collaboration solution MA9004 to make sure you're maintaining a proper pH reading and the 4.0 range on your pH meters.
We offer the Milwaukee Instruments MA9004. This pH solution allows you to dip the end of your probe into it, and recalibrate your meter. This is sometimes called buffer solution in the water industry. The 4.0 calibration solution can be used with many other meters on the market.