Volatilization losses, from samples

The acidification of seawater samples is required for the storage of seawater in order to prevent the precipitation, adsorption of trace metals on the container walls or volatilization loss from solution. Generally, nitric acid is added to seawater to bring the pH of the solutions to below 1.5. The addition of acids immediately after collection on board ship may be preferable for sample storage rather than addition before analysis in the laboratory. 

In generating titration curves, care should be taken that any reactions are allowed to go to completion (check for pH stabilization over about ten minutes if slow reactions are observed). The material to be titrated should be at the same temperature as the effluent to be treated, and care should be taken to avoid the loss of volatile components from samples to the atmosphere. The reagent used should be the main reagent to be used for neutralization. If problems are experienced in metering small quantities of lime-based reagents, caustic (NaOH) can be used near neutral to get sufficient resolution of the titration curve—at least one point per unit pH change should be generated. 

Although the volatility of Hg requires that care be taken with soils to prevent contamination of samples or, rarely, Hg loss from samples, the sampling techniques... 

In order to test rate laws, a must be determined as a function of time using an appropriate experimental technique. If the reaction involves the loss of a volatile product as shown in Eq. (8.1), the extent of reaction can be followed by determining the mass loss either continuously or from sample weight at specific times. Other techniques are applicable to different types of reactions. After a has been determined at several reaction times, it is often instructive to make a graph of a versus time before the data are analyzed according to the rate laws. As will be shown later, one can often eliminate some rate laws from consideration because of the general shape of the a versus t curve. 

It should be noted that hydrolysis of these pesticides is expected to occur simultaneously with volatilization for the pesticides studied (Table I). Over a 7 day experiment, however, only malathion and mevlnphos would be expected to hydrolyze to a significant extent. We determined the loss rate of mevlnphos to be 0.0016 0.0002 hr l (tjj = 18 days), and of malathion to be 0.011 0.001 hr-1 (t j = 2.6 days) at 22 2°C, at pH 8.2+0.2 for a model evaporation pond by daily sampling of duplicate pesticide solu-Xlons (covered to prevent volatilization) for 7 days and plotting log concentration versus time. For both of these pesticides, then, degradation was a much more important route of pesticide loss from water than volatilization. The relatively slow loss rate of the other pesticides could not be determined in our 7 day... 

Grover et al. ( 7) recently measured the volatilization of 2,4-D isooctyl ester after application to a wheat field at 0.5 kg/ha (acid equivalent). He reported that total vapor losses of the isooctyl ester over a 5-day sampling period were 93.5 g/ha or 20.8% of the amount applied. The crop canopy intercepted 77% of the applied ester and thus acted as the major source of vapor loss. He found that the 2,4-D ester losses from the soil surface occurred only when the soil surface was moist, i.e., after a rainfall event or in the early hours of the morning following the disposition of dew. The ester was rapidly hydrolyzed to the acid on the wheat plants and in the soil, particularly when surface soil moisture was available. 

Passive samplers are widely used in monitoring volatile organic chemicals (VOCs) in groundwater. Such samplers have the potential to reduce costs of monitoring from the high levels associated with the use of pumps to sample the test wells. Moreover, the risk of loss of volatile analytes during sample transport and storage is substantially reduced once the compounds are accumulated in the sampler sorption phase. 

Losses from volatilization of the analyte can be minimized by restricting the temperature at which ashing takes place. For determination of lead, copper, zinc, cadmium, and iron in foodstuffs, for example, good recoveries of the analytes were obtained by heating the samples slowly to 450°C and holding this temperature for 1 hour. A collaborative study showed no significant losses of the analytes under these ashing conditions [94],... 

To determine whether the loss from leaf surfaces was matched by a corresponding enrichment of the volatile components in the air, air samples were collected at several points within and toward the edges of the field using Hivol samplers containing XAD-4 resin. 

For some elements, fit-for-purpose sample preparation is required in order to obtain reliable analytical results. Iodine is a well-known case, its determination being complicated by loss of iodide (as HI) from HNO3 solutions, memory effects due to volatilization in the sample introduction system and by matrix effects... 

It is again significant that the value of 7.6 mmoles of N02 per gram of weight loss of sample obtained over the last period from 397 to 492 hours of decomposition for run No. 2 cannot be obtained from any decomposition reaction one may write for N02C104. It is, however, nearly theoretical for the decomposition of NOCIO4 to volatiles ... 

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