Sampling precipitation water

Precipitation reactions have many applications. One is to make compounds. The strategy is to choose starting solutions that form a precipitate of the desired insoluble compound when they are mixed. Then we can separate the insoluble compound from the reaction mixture by filtration. Another application is in chemical analysis. In qualitative analysis—the determination of the substances present in a sample—the formation of a precipitate is used to confirm the identity of certain ions. In quantitative analysis, the aim is to determine the amount of each substance or element present. In particular, in gravimetric analysis, the amount of substance present is determined by measurements of mass. In this application, an insoluble compound is precipitated, the precipitate is filtered off and weighed, and from its mass the amount of a substance in one of the original solutions is calculated (Fig. 1.6). Gravimetric analysis can be used in environmental monitoring to find out how much of a heavy metal ion, such as lead or mercury, is in a sample of water. 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

To determine whether the mechanism for removal might be due to the precipitation of lead as lead hydroxide and subsequent removal by the resin acting as a filter, samples of water with a pH of 6-7 and a lead concentration of 25 /zg/L were prepared and filtered through a 0.45-/zm Metricel membrane filter. The results of this experiment were as follows lead concentration before passage, 26.1 /xg/L lead concentration after passage, 5.01 /xg/L. 

Lapse rates are commonly measured for precipitation, but the materials analyzed for isotope composition do not ordinarily sample precipitation directly. For fossils, isotope compositions are instead related to local surface water, e.g., rivers or lakes. Yet the catchment for a river may extend to much higher elevations than the local sampling site, i.e., the composition of local water is not necessarily linked to local elevation. Rather, the elevation estimate obtained reflects the hypsometric mean of the catchment (Rowley et al. 2001). 

This comparison of measured vs calculated data suggests that by ignoring the possible presence of primary sulfates [the concentration of which may not be insignificant at the Argonne sampling site (2)], the sulfate in precipitation water is formed from SO2 in the atmosphere, partly by aqueous-phase H2O2 oxidation and partly by aqueous-phase air oxidation. The relative deviations of the measured values from the two calculated values represent the relative amounts of each in the mixture. 

The precipitate gradually settles, taking down with it organisms and other suspended impurities. Thus, for example, Leeds 1 found that ail addition of 0-5 gram of alum per gallon 2 of a certain sample of water reduced its bacterial content from 8000 to 80 per c.c., that is, by 99 per cent. The precipitate also acts as a decolonser 3 and has been applied in this capacity for clearing the water at Antwerp. 

The total hardness of a given sample of water may be due in part to the presence of bicarbonate and in part to the presence of other soluble salts. When boiled, the normal carbonate is precipitated and, on account of the decrease in solubility of calcium sulphate with rise in temperature above 38° C., there is always a tendency for this substance to separate to some extent with the carbonate. This causes the deposit to form a coherent film on the containing vessel, whereas the pure carbonate gives a more or less powdery suspension. The boiled water is now softer than before, such hardness as it now possesses is termed permanent, whilst its temporary hardness is the difference between the total and permanent hardness, namely, that lost by boiling. 

How could you precipitate out contaminants to purify a sample of water ... 

The techniques used for sample collection and analysis are varied and are dictated by the concentration of the particular radionuclide and the instrumental sensitivity available for its measurement. For those radionuclides that are present in suflBciently high concentration, a 10-100 1. sample of water is collected, and the material is isolated and concentrated by co-precipitation and radiochemical separation. Several in situ... 

A sample of alkali metal chlorides was analyzed for sodium by dissolving a 0.800-g sample in water and diluting to exactly 500 mL. A 25.0-mL aliquot of this was treated in such a way as to precipitate the sodium as NaZn(U02)3(0Ac)9 6H2O. The precipitate was filtered, dissolved in acid, and passed through a lead reductor, which converted the uranium to U. Oxidation of this to required 19.9 mL of 0.100 M K2Cr207. Calculate the percent NaCl in the sample. 

The residue levels of pesticides in the water samples collected during the heavy rain season were the lowest of all the three seasons (Figure 16). This was attributed to the fact that the samples in this season were collected five months after the short rain season, and the large volume of precipitation water associated with the heavy rain season could have increased the dilution factor of the pesticides and thus lowering the concentrations in the samples. 

It is well known that, after its absorption, NOz forms nitric acid and nitrous acid in water. There is some indication that nitrite produced in this way is oxidized by dissolved 03 (Penkett, 1972). If neutralizing agents (ammonia, calcium carbonate etc.) are present, some nitrate salt is finally formed. It follows from this discussion that both S02 and N02 are oxidized in cloud water by atmospheric ozone. If this speculation is true a correlation should be found between the concentration of sulfate and nitrate ions in precipitation waters. Such a correlation was found in precipitation samples by Gambell and Fisher (1964) among others. However, correlations between any two species in rainwater must be considered with caution because the level of all ions is affected in a similar way by the precipitation intensity or quantity (see Subsection 5.4.1). Nevertheless the identical annual variations of the two ions in precipitation water (see Subsection 5.4.5) suggests that the two species are formed by some similar processes. 

For the investigation of the importance of the different wet removal processes, it is usual to compare the chemical composition of cloud and precipitation waters sampled simultaneously. This would appear to separate the effects of rain-out and wash-out. However, as we shall see, the results of such a comparison have to be considered with some caution. 

Soviet data also demonstrate that the sum of the concentrations of the different ions is independent of sampling location in the case of frontal precipitation systems, and is equal to about 6 mg 1 1 over the whole of the Soviet Union. The pattern shown in Fig. 50 is thus mostly produced by local precipitation systems, which are much more sensitive to local pollution. Petrenchuk and Selezneva (1970) argue that in frontal systems the rain-out of aerosol particles by condensation is the dominant mechanism in the control of precipitation water composition. 

The experiments show that the ordinary precipitation methods for the production of ferric phosphate give products which do not conform to the formula Fej(P04),. By digesting such samples with water very little is dissolved, but the material is decomposed to an extent depending upon the relative amounts of solid and solvent used. The amount of POj dissolved per gram of Fej(P04), varies from about 0.0026 gram removed by 5 cc. HjO to 0.0182 gram removed by 800 cc. HjO at the ordinary temperature. 

Fig. 6-22. Sample precipitation technique used to determine Kso for Fe3(P04)2(s,. From P. C. -Singer, ], Water Pollution Control Fed., 44 663 (1972). 1972 Water Pollution Control Federation, reprinted with permission.

Fig. 7 Gel Permeation Chromatogram of Hexanoylated, Steam Exploded Fiber Solids Precipitated in Water. A corresponding sample precipitated in isopropanol (good solvent for lignin ester) revealed an almost uniform distribution curve from which the low molecular weight constituents (i.e., lignin ester) were missing.

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