The pH 4.01 buffer is available in a 500ml bottle, one of the most commonly used buffer solutions. It is extremely stable and represents the mid-way point in the acid range of the pH scale - ideal for those requiring an accurate acid reading, particularly those working in water purification plants, the food industry, and wherever the pH is expected to be acidic. Buffer solutions should be stored at 25◦C, away from direct sunlight for accurate readings.