Moisture, determination

Anhydrous ammonia is normally analy2ed for moisture, oil, and residue. The ammonia is first evaporated from the sample and the residue tested (86). In most instances, the amount of oil and sediment ia the samples are insignificant and the entire residue may be assumed to be water. For more accurate moisture determinations, the ammonia can be dissociated into nitrogen and hydrogen and the dewpoint of the dissociated gas obtained. This procedure works well where the concentration of water is in the ppm range. Where the amount of water is in the range of a few hundredths of a percent, acetic acid and methanol can be added to the residue and a Karl Fischer titration performed to an electrometricaHy detected end point (89—92). 

It appears then that the establishment of practicable and accurate reference methods presents one of the major problems in the determination of moisture in foods. The present paper summarizes some of the important factors that govern the accuracy of moisture determinations in general, and reviews some of the recently published attempts to develop methods that might serve as useful reference standards for dehydrated foods, and especially for dehydrated vegetables and fruits. 

In studies on the moisture determination of corn by vacuum drying at 100° C., Sair and Fetzer (28) found that an error of about 0.5% in moisture content could be attributed to volatility of oils. The error was negligible in vacuum drying at 70° C. 

It is difficult to estimate the magnitude of the error due to insufficiently low humidity when distillation methods are used with organic liquids such as toluene (6, 28), xylene (6, 28), or chloroform (12). With organic liquids essentially immiscible with water and of high boiling point the error is probably very small. When methanol is used as an extraction solvent, as in the Fischer reagent method, the amount of unextracted water is undoubtedly some function of the concentration of water in the alcohol, but the error might be small because of substitution of adsorbed water by adsorbed alcohol (23, 34). This seems to be borne out by experiments of Schroeder and Nair (31), who deliberately added water to the alcohol to form a 0.5% water solution and found that the results of their moisture determinations were essentially the same as with anhydrous methanol, which contained about 0.05% water. 

Figure 4. Moisture Determinations with Fischer Reagent as Function of Time of Soaking in Methanol at 60° C. (J J)...

Table III. Comparison of Moisture Determinations on Lyophilized and Unlyophilized...

Figure 10. Moisture Determinations with Fischer Reagent...

Moisture Determinations. Because the moisture content of many fruits and vegetables is an index to maturity and quality, moisture determinations are employed extensively in the grading of raw fruits and vegetables, and in some cases— for example, tomato products—the finished item may be sold on the basis of moisture content. 

Ibid, E170 (Electrolytic Hygrometer Method) Ibid, E171-L to E172-L (Moisture Determination in Propellants and Explosives by Distillation Method) Ibid, E172-L to E173-L (Moisture Determination in Propellants and Explosives by the Karl Fischer Method)... 

Applications The coulometric Karl Fischer titration is a widely used moisture determination method (from ppm to 100%). In the presence of water, iodine reacts with sulfur dioxide through a redox process, as follows ... 

The near-IR technique has been used very successfully for moisture determination, whole tablet assay, and blending validation [23]. These methods are typically easy to develop and validate, and far easier to run than more traditional assay methods. Using the overtone and combination bands of water, it was possible to develop near-IR methods whose accuracy was equivalent to that obtained using Karl-Fischer titration. The distinction among tablets of differing potencies could be performed very easily and, unlike HPLC methods, did not require destruction of the analyte materials to obtain a result. 

X. Zhou, PA. Hines, K.C. White and M.W. Borer, Gas chromatography as a reference method for moisture determination by near-infrared spectroscopy, Anal. Chem., 70, 390-394 (1998). 

X. Zhou, P.A. Hines, and M.W. Borer, Moisture determination in hygroscopic drug substances by near infrared spectroscopy, J. Pharm. Biomed. Anal, 17, 219-225 (1998). 

The earliest applications for quantitative analysis of liquid samples and solid preparations entailed sample dissolution in an appropriate solvent. A number of moisture determinations in APIs and pharmaceutical preparations based on both reflectance and transmission measurements have been reported. Their results are comparable to those of the KF method. The high sensitivity provided by the NIR technique has fostered its use in the determination of moisture in freeze-dried pharmaceuticals. ° The noninvasive nature of NIR has been exploited in determination of moisture in sealed glass vials. " " ... 

R.M. Leasure and M.K. Gangwer, Near-infrared spectroscopy for in-process moisture determination of a potent active pharmaceutical iugredieut. Am. Pharm. Rev., 5(1), 103-104, 106, 108-109 (2002). 

Procedure (neutralizing value). At the same time as weighing the sample for moisture determination, weigh 0.5 g (AOAC 1 g, but 0.5 g for CaO or Ca(OH)2), or x g, of the prepared sample to the nearest 0.001 g, and transfer to a 300-ml (AOAC 250-ml) conical flask. Add 50 ml of 0.5 M hydrochloric acid, cover the flask with a watch glass and boil the contents gently for 5 min. Cool the mixture to room temperature, add 2 or 3 drops of the phenolphthalein indicator and titrate with 0.5 M sodium hydroxide solution to the end point of the indicator. 

Calculation. Subtract the blank reading from the sample reading in pg K ml read from the graph. This is equal to the g kg of potassium in the sample. Divide the pg K mM by 10 to obtain the % K in the sample. If the sample was not oven-dried, use the separate moisture determination to correct the result to K in DM. 

Subsequent reaction of the monosulfate with gypsum produces acicular crystals of ettringite. Monosulfate apparently does not contribute to expansion, whereas the formation of ettringite involves expansion. A recently issued patent [75] covers the use of prehydrated high-alumina cement (H-HAC), lime and gypsum mixtures. Particle type, size, thickness of protective coating, and presence of moisture determine the rate and extent of expansion [Fig. 6.11]. 

Moisture Determination in Propellants and Explosives by Distillation Method 102.1.3, Pre-ecrjked by. MIL-STD-286B (1 Dec 1967). Although this procedure is similar to the one described under Dynamite in Vol 5, p D1620-ljL, we are describing it because it uses trichloroethylene (in lieu of carbon tetrachloride) as a suivciK anu a awg lu ucu ui jug/ sample ... 

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