Gravimetric moisture measurements

There are several errors involved in gravimetric moisture measurements. One error is the assumption that h is zero in an ordinary oven. This effect can be minimized by using a vacuum oven or a strong desiccant such as phosphorus pentoxide. Another problem is the evaporation of volatile wood constituents, if present, to give a higher apparent moisture content in the wood. A third problem in accurate moisture measurement is the effect of sample moisture history (8). 

The methods for the determination of residual moisture currently used at the Center for Biologies Evaluation and Research at the U.S. FDA are the gravimetric (loss on drying) method, the Karl Fischer method, and the thermogravimetric (TG) and thermogravimetric/mass spectrometric (TG/MS) method. Current work in progress involves the use of vapor pressure moisture measurements to provide additional information about residual moisture content and its interaction with the components of the freeze-dried final container and its contents. 

The gravimetric method measures surface moisture and loosely bound water of hydration [17]. The gravimetric method was described in the U.S. Code of Federal Regulations [15] before 1990. In the CBER/FDA laboratory the gravimetric (loss on drying) test [2,44] is performed in a humidity- and... 

Industrial Hquid chlorine is routinely analy2ed for moisture, chlorine, other gaseous components, NCl, and mercury foUowing estabHshed procedures (10,79). Moisture and residue content in Hquid chlorine is determined by evaporation at 20°C foUowed by gravimetric measurement of the residue. Eree chlorine levels are estimated quantitatively by thiosulfate titration of iodine Hberated from addition of excess acidified potassium iodide to the gas mixture. 

The gravimetric method is accepted as the most accurate humidity-measuring technique. In this method a known quantity of gas is passed over a moisture-absorbing chemical such as phosphorus pent-oxide, and the increase in weight is determined. 

As the gas stream proceeds through the sampling apparatus, the particulate matter is trapped on a filter, the moisture is removed, and the volume of the sample is measured. Upon completion of samphng, the collec ted material is recovered and sent to a laboratory for a gravimetric determination or analysis. 

Class and porcelain are particularly susceptible to adsorption of atmospheric moisture on exposed surfaces. Containers to be heated in gravimetric procedures (e.g. oven-dry matter or ash content) should therefore be pre-heated to the same temperature as that procedure and cooled in a desiccator before measuring the tare weight. 

Alternatively, aw may be measured by isopiestic equilibration. In this method, a dehydrated sorbent (e.g. microcrystalline cellulose) with a known moisture sorption isotherm (section 7.4) is exposed to the atmosphere in contact with the sample in an enclosed vessel. After the sample and sorbent have reached equilibrium, the moisture content of the sorbent can be measured gravimetrically and related to the nw of the sample. 

Measure the moisture content of the sample gravimetrically in an air or vacuum oven... 

Microwave moisture analyzer, gravimetric measurement of water, 9-11 Milk... 

A moisture balance calibrated to indicate an 8% moisture content can be used to monitor weight loss and may be useful until experience is gained. The calibration can be performed as a gravimetric determination on a sample analyzed in parallel. This measurement can also be used to estimate an 8% moisture content relative to the initial sample weight. 

Moisture sorption kinetics by neat epoxies were measured using gravimetric analysis using a Mettler balance which was accurate to +0.05 mg. This technique was described in detail elsewhere 137). Another method was used to monitor the sorp-... 

Water vapor at room temperature will not penetrate well-defined crystallites but will be adsorbed in the amorphous regions. Consequently, moisture sorption measured gravimetrically at a given relative vapor pressure and temperature has been used to determine order in cellulosic materials. In the case of Valentine [252] and Jeffries [253], the fraction of ordered material was obtained by correlating moisture sorption with values obtained by the deuterium... 

The freeze-dried product has to have a residual moisture content (RM) between alj and am, e.g., 0.7-1.1%, measured by a method, e.g., the gravimetric method at +50°C [4]. The dried cake should be white, with no visual shrinkage, and mechanically stable when slightly shaken. It will withstand a maximum tern-... 

Some of the traditional methods used for the characterization of bar soaps are moisture, free acid or free alkalinity, total soap, and chloride (salt). Moisture determinations are typically run by Karl Fischer titration. However, under certain circumstances, moisture can be measured gravimetric ally. The measure of free fatty acid or free alkalinity is typically done via a simple pH titration using phenophtha-lein or potentiometric end point. Total soap is generally done using an extraction/ gravimetric procedure the soap sample is acidified in an ethanol-water mixture, the fatty acids are extracted into a hydrocarbon solvent, and the amount of extracted... 

Attaining and measuring an equilibrium weight of sorbed water vapor. Gravimetric measurement of moisture uptake can occur continuously or discontinuously. Continuous measurement usually involves placing a sample on a balance in a temperature- and relative humidity-controlled environment. Microbalances in closed systems have been used successfully for this purpose, " and commercial systems are now available that can accurately and precisely control relative humidity and simultaneously monitor sample weight. 

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