Moisture Content, Water Content, Loss on Drying, Part 1: What Exactly Is Meant and How Are These Quantities Determined?

The presence of water in polymers, pharmaceutical substances or foodstuffs is often undesirable. A water content that is too high can reduce the shelf life of rice, make the processability of pharmaceutical products more difficult (for example tableting), or lead to quality problems with plastic products. The determination of the water content in different materials is therefore very important. Besides “water content”, the terms “moisture content” or “loss on drying” are also often used. In this article (Part 1), we will explain what the terms mean and discuss some of the techniques used to determine these quantities.

Introduction

In everyday English, by “moisture” we usually mean the presence of small amounts of water on the surface of a solid or weakly bound within a solid. Water present in the atmosphere is referred to as humidity. If “moisture content” refers explicitly to water and not to other substances, then the term “water content” should be used. The “water content” is then the quantity of water in the sample mass. “Moisture content” can however also refer to any other volatile substances (volatiles) including water in materials. In the context of techniques and methods for moisture content determination, the term “loss on drying” (LOD) is synonymous with “moisture content”. As shown in Figure 1, moisture content or water content can be determined by gravimetric, chemical, or physical methods. Gravimetric methods measure changes in mass of a sample after or during a defined temperature program using a balance. The total loss of mass is referred to as “loss on drying”. Whether this is exclusively water or whether other volatile substances or even decomposition products are released cannot be determined without further analysis. Gravimetrically determined moisture contents depend on the temperature program used and (to a lesser extent) on the relative humidity and pressure in the drying oven. So-called LOD analyses are always carried out using gravimetric methods; these methods always determine the total volatile content.

_{Moisture content, water content, loss on drying, Part 1: What exactly is meant and how are these quantities determined? | Thermal Analysis Application No. UC 511 | Application published in METTLER TOLEDO Thermal Analysis UserCom 51}