Bluelab� pH 4.0 calibration solutions are essential for use with all Bluelab� products. Bluelab� Solutions are manufactured specifically for Bluelab� products and are referenced to high laboratory standards to ensure your Bluelab� products last longer and maintain accuracy. High quality solutions formulated for calibrating and/or testing pH metres.Bluelab� pH 4.0 calibration solutions... a must in every grower's toolbox.