Liquid samples exercises

EXAMPLE 8.4 Sample exercise Estimating the solubility of a gas in a liquid... 

EXAMPLE 8.11 Sample exercise Predicting the vapor pressure of a mixture of two liquids... 

Liquids and powders cannot be cleaned, so they are handled in clean containers, avoiding contamination from the container wall. For liquid samples, care should be exercised to avoid loss of fluid when the sample is transferred in and out of the container. In the case of powder (or powderized) samples, the observer should be certain that a truly representative sample has been prepared. This is especially important if the main sample under analysis is not homogeneous. 

Many different spray techniques have been used to introduce liquid samples into an analytical gap. One of the early successful uses of this technique was by Lundegardh in 1928 with agriculturally important samples. Atomizers have been used to spray the sample into the analytical gap as a liquid aerosol and excellent analytical results can be obtained if care is exercised to keep operating parameters constant. 

As we pointed ouf in Sample Exercise 19.10, some care is necessary in fhe choice of unifs. We will express AG° in kj/mol. Eor fhe reacfanfs and producfs in fhe equilibrium-consfanf expression, we use fhe following conventions Gas pressures are given in afm solution concenfrations are given in moles per lifer (molarify) and solids, liquids, and solvenfs do nof appear in fhe expression. (Section 15.3) We illusfrafe fhe use of Equation 19.23 in Sample Exercise 19.13. 

The density of a liquid can be determined easily by weighing an accurately known volume of liquid. This procedure is illustrated in Sample Exercise 1.13. 

If necessary, cool a portion of the sample to at least 20F(11 C) below the anticipated flash point. Exercise adequate care to avoid contamination of the sample with coolant liquid or vapors. Fill the glass cup with the cooled sample to a depth approx l/S inch(3-2mm) below the edge with the... 

However, care must be exercised in using molecular sieves for drying organic liquids. Appreciable amounts of impurities were formed when samples of acetone, 1,1,1-trichloroethane and methyl-r-butyl ether were dried in the liquid phase by contact with molecular sieves 4A (Connett Lab.Practice 21 545 1972). Other, less reactive types of sieves may be more suitable but, in general, it seems desirable to make a preliminary test to establish that no unwanted reaction takes place. For the principles of synthesis and identification see R. Szostak Molecular Sieves, Chapman Hall, London 1988, and for structure, synthesis and properties see R.Szostak Handbook of Molecular Sieves, Chapman Hall 1992. 

As a preliminary, ferric sulfate is made by the oxidation of ferrous sulfate. Dissolve 100 g. of ferrous sulfate in 100 cc. of boiling water, to which has been added before heating 10 cc. of sulfuric acid. Add concentrated nitric acid portionwise to the hot solution, until a diluted sample gives a reddish-brown (not black) precipitate with ammonia. This will require about 25 cc. Boil the solution down to a viscous liquid to get rid of excess nitric acid, dilute to about 400 cc., and add the calculated weight of ammonium sulfate. The crystallization is conducted as in the former exercise, preferably under 20°. By the addition of potassium sulfate, the corresponding potassium iron alum may be secured. In this case, it is necessary to concentrate the solution until there is about four parts of water to one of the hydrated alum and cool to about zero to secure crystallization. Both of these alums are amethyst in color, the potassium salt being much less stable and having a rather low transition point. 

A cardinal rule of trace analysis by GC (and almost certainly by any other method of measurement) requires that standardization involve quantities of the test materials equal to, or closely approximating the quantities to be measured in samples. In the case of standard materials that are liquids or solids, a properly measurable weight or volume of the material can be appropriately diluted. To avoid the use of excessive solvent volumes, serial dilutions can be made, as long as reasonable care is exercised. 

In the case of liquid oils, care should be exercised so that no aeration of the oil occurs before sampling. Solid fats should not be melted beforehand and the sample should be taken from the center of the mass (Hendrikse et al., 1994). It is essential, of course, to deaerate all the solutions used in the protocol, as the presence of oxygen can lead to further formation of peroxides. 

Neale, R. S., Ind. Eng,. Chem., Prod. Res. Dev., 1980, 19, 634 This substituted silane (intended as an explosive plasticiser), though relatively stable to heat and impact of a hammer, is exceedingly sensitive to compressive shock. Great care must therefore be exercised in syringing samples of the viscous liquid. 

When a sample of Na[Zn2(CH3)2H3] xTHF is exposed to high vacuum at room temperature, both dimethylzinc ( Caution. Spontaneously flammable in air.) and THF are slowly evolved. After 3 hr of pumping at room temperature, followed by about 1 hr of pumping at 80°, the evolution of volatiles, which are collected in a trap cooled with liquid nitrogen, should cease. If not, more pumping is required. Care must be exercised in adjusting... 

While luminescence in vapor-deposited matrices accordingly should be a powerful technique for detection and quantitation of subnanogram quantities of PAH in complex samples, it suffers from two major limitations. First, it is obviously limited to the detection of molecules which fluoresce or phosphoresce, and a number of important constituents of liquid fuels (especially nitrogen heterocyclics) luminesce weakly, if at all. Second, the identification of a specific sample constituent by fluorescence (or phosphorescence) spectrometry is strictly an exercise in empirical peak matching of the unknown spectrum against standard fluorescence spectra of pure compounds in a hbrary. It is virtually impossible to assign a structure to an unknown species a priori from its fluorescence spectrum qualitative analysis by fluorometry depends upon the availabihty of a standard spectrum of every possible sample constituent of interest. Inasmuch as this latter condition cannot be satisfied (particularly in view of the paucity of standard samples of many important PAH), it is apparent that fluorescence spectrometry can seldom, if ever, provide a complete characterization of the polycyclic aromatic content of a complex sample. 

In activation analysis, as in all trace element determinations, care must be exercised to avoid contamination. One of the advantages of activation analysis is that the manipulation prior to irradiation of the sample is minimized. Contamination by inactive material after irradiation is unimportant, provided it does not take place in amounts sufficient to affect the chemical yield. The restrictions imposed on neutron activation by selfshielding mentioned above may necessitate the use of small samples and either liquid or solid dilutions of the pure standard material to overcome the difficulty. 

Based on the most widely accepted meanings of leaching and digestion, the former is always desirable when possible. Partial dissolution of the sample with complete removal of the target analytes provides a less complex liquid and the possibility to avoid interferences. It should be noted, however, that complete dissolution of the analytes must be ensured if the target analytes are to be quantified with adequate precision. Special care must be exercised in applying a method developed with spiked matrices to natural samples — rather than with real samples as opposed to prepared or artificial samples since the opposite of real samples would be imaginary samples . 

---