American Marine PINPOINT High-Precision pH Calibration Fluids pH 4.000 +/- 0.002 (for 10+, $1.30 each)
These come in single-use plastic envelope packets, and they are extremely accurate and field portable. Each packet contains enough fluid for a complete calibration (about 30 ml) and is intended to be discarded after use. pH monitor and controllers should be calibrated monthly.
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