This is a large bottle of buffer solution, and is intended for those of you that do a lot of measuring. It is more cost effective to buy this bottle of buffer for frequent use over the small single use packs.
This is the necessary 4.01 calibration buffer for which you will use as a reference to calibrate your meter. The Hanna App can save up to 5 different buffers, but a 2-point calibration is all that is necessary. Start with the 7.01 buffer, and once the app gives you a stable reading save it, and then proceed to the 4.01 calibration buffer.