CHFEN 5503Measuring wt% Moisture
OVERVIEWThe analyst often must determine the wt% moisture or volatiles in a sample. This may be part of a materials characterization, or it may 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. These procedures assume that a representative sample is available and has been properly stored prior to analysis.
GUIDELINES [revised 1/18/00]
The procedures described below are intended for general guidance when determining 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 dish
b. heat at desired temperature for requisite time
c. cool in a desiccator
d. weigh
e. heat again at the same temperature (time may be less than in 'a')
f. cool in a desiccator
g. reweigh
The heating and the cooling steps have a temperature and a time variable, which is determined by the analyst or by some "procedure." For example, drying for water removal is often done at 105C "overnight." Overnight in a procedure is the equivalent of 16 hours.
After step (d) or (g) above the decision to reheat depends on the procedure and use for the data. When constant weight is required then an interpretation of constant weight is also necessary. This to some extent depends on the precision of the balance but may also depend on the 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 may be fitted with a vacuum line and may also have a provision for nitrogen or another "inert" gas.
3.The choice of temperature for these types of procedures depends on the nature of the material and the volatiles. Even for water, temperatures above 105C may be required in order to accomplish 100% removal. Constant weight is often attainable at 105C even when all water is not removed. Sensitive materials may require low temperatures and/or vacuum drying.
4.In some analysis procedures a compromise is required. For example, complete removal of combustibles on sands, soils, or mined ore occurs only at times and temperatures where carbonate decomposition also occurs. Our current procedure for wt% combustibles on sands and ores is to heat a 'dry' sample for 16 hours in a muffle furnace at 500C, cool in a desiccator, and weigh. (no reheating)