NaOH is highly hygroscopic, meaning that it absorbs water from the air. Therefore, over time the titrant will become more dilute as it absorbs water. NaOH can be standardized by titrating into a sample of potassium hydrogen phthalate (KHP) of known concentration. For example, titrate 0.05 N KHP with 0.1 N NaOH to an endpoint, and using the volume of NaOH added, the precise concentration of NaOH can be calculated. For more information on standardizing NaOH titrant, please refer to Standard Methods 2310. To limit the amount of water being absorbed, a glass drying tube with a cotton ball inserted should be used to prevent moisture going into the tube.