CHFEN 5503 Measuring wt% MoistureOVERVIEW The analyst often must determine the wt% moisture orvolatiles in a sample. This may be part of a materials characterization, or itmay be required in order to establish a weight basis for further analysis.Procedures for weight loss due to drying, decomposition, devolatilization,combustion, and ashing are similar, and guidelines are discussed below. Theseprocedures assume that a representative sample is available and has beenproperly stored prior to analysis.GUIDELINES [revised 1/18/00]The procedures described below are intended for general guidance whendetermining weight losses in order to calculate wt% water, volatiles,combustibles, etc.:1. Measurements for weight loss data usually require processing to a"constant weight." For a heating procedure the sequence is:a. weigh the sample in a tared crucible or evaporating dishb. heat at desired temperature for requisite timec. cool in a desiccatord. weighe. heat again at the same temperature (time may be less than in'a')f. cool in a desiccatorg. reweighThe heating and the cooling steps have a temperature and a timevariable, which is determined by the analyst or by some "procedure." Forexample, drying for water removal is often done at 105C "overnight."Overnight in a procedure is the equivalent of 16 hours. After step (d) or (g) above the decision to reheat depends on theprocedure and use for the data. When constant weight is required thenan interpretation of constant weight is also necessary. This to someextent depends on the precision of the balance but may also depend onthe precision required in the test. A rule of thumb for 5-10 g samples is +0.001 g.2. If a sample has volatiles that are flammable then the "drying" oven maybe fitted with a vacuum line and may also have a provision for nitrogenor another "inert" gas.3. The choice of temperature for these types of procedures depends on thenature of the material and the volatiles. Even for water, temperaturesabove 105C may be required in order to accomplish 100% removal.Constant weight is often attainable at 105C even when all water is notremoved. Sensitive materials may require low temperatures and/orvacuum drying.4. In some analysis procedures a compromise is required. For example,complete removal of combustibles on sands, soils, or mined ore occursonly at times and temperatures where carbonate decomposition alsooccurs. Our current procedure for wt% combustibles on sands and ores isto heat a 'dry' sample for 16 hours in a muffle furnace at 500C, cool in adesiccator, and weigh. (no