Cannabis is composed of a host of compounds including carbohydrates, flavonoids, alkaloids, waxes, cannabinoids, terpenes, and water. The water content in cannabis varies at different stages of development. Moisture content in properly-dried cannabis is estimated to be around 7–12%. The state of California requires it not be higher than 13%. Moisture content in uncured cannabis may exceed 70%.
Cannabis is highly susceptible to mold or fungal infestation. The moisture content is an indicator of potential microbial contamination which may pose a risk to the health of consumers. Extra moisture also affects the taste of cannabis and the shelf-life. Therefore, moisture analysis ensures that set moisture limits are not exceeded or undercut.
Water content in cannabis is measured and tested using two methods primarily: Karl Fischer (KF) titration or Loss on Drying (LOD). LOD involves weighing a sample, heating it in an oven, and measuring the weight loss. LOD is cheaper but is likely to be influenced by volatile organic carbons (VOCs) such as terpenes that may evaporate from the plant material and potentially cause miscalculations. KF titration, on the other hand, relies on a chemical reaction (see below) between water, sulfur dioxide, and iodine in a buffer solution to produce sulfuric acid and hydrogen iodide.  Iodine is added until there is no remaining water for the reaction. 
H2O + SO2 + I2 → SO3 + 2HI
Comparing LOD to KF
Research was conducted by Metrohm, a titration instrument manufacturer, to compare LOD to KF in analyzing moisture content in cannabis. Sixty-six cannabis flowers and plant tissue samples were analyzed using both methods.
Karl Fischer Analysis
The cannabis sample (0.5 g) was added directly into a vial which was sealed with an aluminum septum and placed on the sample processor.
The sample was placed in the company’s KF oven and analyzed at 150ºC, a temperature that allows the water to release without sample degradation. The authors explain that “[d]ry gas enters the titration vessel and, as water is vaporized, carries it to the titration vessel for analysis.” Because the experiment is not based on mass, terpene volatilization doesn’t influence the results, even at this high of a temperature.
Loss on Drying
The sample was weighed and then heated (unfortunately, the temperature used was not included). Once the water (and volatile compounds) evaporated, the sample was then reweighed and the difference in weight recorded.
The results showed that LOD samples returned a higher moisture content consistently which was attributed to the loss of volatile compounds such as terpenes. On average, moisture content in the LOD titration was 36% greater than in KF titrations.
From these results, it appears that the KF method of moisture analysis is more accurate as compared to the LOD method.
The researchers concluded that moisture analysis using the KF method is chemically specific to moisture and can be used accurately to analyze moisture content in cannabis. LOD, on the other hand, can show a higher moisture content consistently and is, therefore, not an accurate measurement of moisture content in cannabis.
Datamax, Public Domain
- Schöffski K, Strohm D. Karl Fischer moisture d In Meyers RA, ed. Encyclopedia of Analytical Chemistry. 2006. doi:10.1002/9780470027318.a8102.