The Gravimetric Method

Two parameters must be measured to apply the BET equation, the pressure at the sample and the amount adsorbed at this pressure. There are tlnee conmron methods for measuring the amount of gas adsorbed, called the volumetric method, the gravimetric method and the dynamic method, of which the volumetric method is the connnonest [21],... 

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. 

The gravimetric method depends on the sampling of flowing, particulate-laden gas from different positions across the exhaust gas duct and the determination of the mass of the particulate material. The sample is collected over a certain time period from each point. The volumetric gas flow is measured. The result is obtained by the following steps ... 

It should be pointed out that the titrimetric methods described for the determination of antimony (Chapter 10) are to be preferred to the gravimetric methods as they are simpler, more rapid, and quite as accurate. 

Brenner et al. [ 169] applied inductively coupled plasma atomic emission spectrometry to the determination of calcium (and sulfate) in brines. The principal advantage of the technique was that it avoided tedious matrix matching of calibration standards when sulfate was determined indirectly by flame techniques. It also avoided time-consuming sample handling when the samples were processed by the gravimetric method. The detection limit was 70 ig/l and a linear dynamic range of 1 g/1 was obtained for sulfate. 

By this method the weighed dry product is dissolved in methanol and titrated with the Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows an steep increase in current, when the titration is terminated (dead-stop-titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observation completely, iodine can be produced by electrolyzation and the water content is calculated by Coulomb s law. Such an apparatus (e. g. Fig. 1.97.1 and 1.97.2) is available commercially. The smallest amount of water to be detected by such instruments is 10 pg. Wekx and De Kleijn [1.84) showed, how the Karl Fischer method can be used directly in the vial with the dried product. The Karl... 

The RM of the dried product was measured at 50 °C over P205 or in an oven with circulating air at 50 °C, or in the same oven at 90 °C over silica gel. Identical measurements were made with fresh bones. For NMR measurements, a known amount of D20 was added to the bone in a glass container. After equilibrium between DzO and H20 was reached, a known amount of the product was taken from the solution and studied in a Perkin Elmer NMR-spectrometer. In Fig. 3.23 the water contents of fresh and freeze dried bones are listed measured by NMR and the gravimetric methods at 90 °C. The data show that only a certain amount of the total water can be removed at 90 °C, while another amount is so... 

Compare Figure 3.1 with Figure 1.1. What is special about the gravimetric method of analysis ... 

A good number of pharmaceutical substances can be assayed gravimetrically. The gravimetric methods adopted vary according to the nature of the substance under determination. However, most of the substances being estimated gravimetrically fall into one or the other categories stated below, which would be discussed briefly with suitable examples ... 

In addition to the gravimetric method used by Rubel and Gentry and the fluorescence measurements of Ward and his coworkers, two methods of following gas/particle chemical reactions have been developed recently. 

Buoyancy corrections are only required by the gravimetric method. 

Though the protocol was written for a dry solid, it should also be mentioned that liquid samples can also be run (Firestone, 1998). Liquid samples can be added directly to the vessel itself or can be treated as a solid with an extraction step in the Karl Fischer solvent as presented in the protocol. The preference for moisture analyses is the gravimetric method (unit aj.i) largely due the higher costs of equipment and the use of chemicals in the Karl Fischer method. Currently one observes very few applications of the Karl Fischer method on food products published by the AOAC for nutritional labeling purposes. In particular, these food products are cocoa, cocoa products, confectionery coatings, and molasses (Sullivan and Carpenter, 1993). 

The accuracy and utility of the normal-phase separation is evident, but the gravimetric method of quantitation has several major drawbacks ... 

The gravimetric method specifies using preparative chromatographic columns, which must be prepared rather than obtained commercially. 

For the gravimetric method, which is the more exact, 25 c.c. of this solution are used, the procedure already given being followed (see General methods, p. 109). 

The colorimetric method of determining methyl alcohol in spirit is very rapid and if the conditions are faithfully adhered to, also very exact. It is especially advantageous when the methyl alcohol is present in small amount (up to 5-6%). When, however, the methyl alcohol occurs in larger proportions, the gravimetric method is to be preferred (see p. 258). 

Subsequent work on catalyst characterization via reduction studies has involved gravimetric or volumetric techniques. The gravimetric method measures weight loss (due to O loss), while the volumetric method measures amount of H2 consumed [and amount of water formed (O loss)]. In the latter case, the amount of H retained by the catalyst can also be determined (20). The gravimetric technique has also been applied to sulfided catalysts (27). From these measurements, the stoichiometric state of the catalyst can be determined. 

By this method the weighed dry product is dissolved in methanol and titrated with Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows a steep increase in current when the end-point of the titration is reached (dead-stop titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observa-... 

---