Analyzing CBD Crude for Ethanol: The Loss on Drying Method

A scale, a clean dish and controlled heat will tell you how much ethanol is left in your crude, no HPLC required.

DocumentTechnical Note
MethodLoss on drying
Revision2026.1
First published31 December 2019
Knowing the ethanol concentration at each step of an ethanol extraction and purification process is how you quantify and optimize it. HPLC will give you exact ethanol, cannabinoid and terpene percentages, but it takes specialized equipment and expertise. Loss on drying is inexpensive, simple, and a standard way to measure volatile content in pharmaceuticals. This procedure is adapted from USP <731> Loss on Drying.

Equipment

  • A scale with enough resolution that its error is small. 10 mL of ethanol (0.789 g/mL at 20°C, 68°F) weighs about 7.9 g. With a scale good to plus or minus 0.02 g and three weighings, the result is within about plus or minus 0.8 percent, which is plenty for process work.
  • A crude vessel that is inert and heat resistant, such as a borosilicate glass petri dish, clean and dry.
  • An oven or hot plate with good temperature control and ventilation. On a hot plate, tent aluminum foil over the dish to let vapor out and keep dust off. An oven is easier.
  • A thermometer or K-type thermocouple to confirm the sample temperature.
  • PPE: safety glasses, tongs, heat resistant gloves.
  • A laboratory notebook and pen. See Maintaining a Laboratory Notebook.

Method

  1. Clean the dish: wash with dish soap and water, rinse with deionized water, and air or oven dry. Do not wipe the inside, which leaves fibers behind.
  2. Weigh the empty dish. This is the dry mass.
  3. Add crude to a depth of 5 to 10 mm (0.2 to 0.4 in), 1 to 10 g depending on your scale.
  4. Weigh the dish and crude. This is the initial mass.
  5. Heat above the boiling point of ethanol, 78.2°C (172.8°F), but below 105°C (221°F). Start around 200°F (93°C) and adjust. Record the temperature before every weighing.
  6. Weigh every 30 minutes or so until the mass stops decreasing.
  7. Record the final mass.
Keep it below 105°C (221°F). Above that, THCA begins to convert and volatile terpenes are lost, so the mass drop no longer measures only ethanol.

Calculation

Ethanol % = (1 − (final mass − dry mass) / (initial mass − dry mass)) × 100

Mass percent, not volume or molar percent. Dry mass: the clean, empty dish. Initial mass: dish plus crude before heating. Final mass: dish plus crude once weight loss has stopped.

  • Uncertainty. A common rule for digital scales is plus or minus 2 of the last displayed digit, so a scale reading to 0.01 g carries plus or minus 0.02 g per weighing. Three weighings give plus or minus 0.06 g overall.
  • Use enough crude for reasonable accuracy, but not so much that drying takes all day.

Why it is worth doing

Ethanol content changes how crude behaves. At 10 percent ethanol and 60°C (140°F) you may be able to pump crude out of a falling film evaporator, while at 5 percent and the same temperature it may be too viscous for the pump. Measuring ethanol at each step lets you set those parameters on numbers instead of feel.

Health and safety

Ethanol is a USP <467> Class 3 residual solvent, considered low toxicity. The OSHA exposure limit is 1,000 ppm over 8 hours. Work in a well ventilated area, do not breathe the vapor, and handle hot glass with tongs and gloves.

Sources. USP <731> Loss on Drying, USP 35-NF 30 (2012); USP <467> Residual Solvents, USP 30-NF 25 (2007); NIOSH Pocket Guide to Chemical Hazards, Publication 2005-149 (2007). N.B. Oler Technical Note, Analyzing CBD Crude for Ethanol (2019).

Questions about your system? Call the shop at 541-330-6409, Monday to Thursday, 8 to 4 Pacific, or email info@nboler.com.
Back to blog