End-point selection

Typically, selection of the major test end-point to be used in the investigation is based on the compliance needs or the objective of the investigation. For acute exposures, survival is often the end-point selected for performance of the toxicity assessment. Although chronic exposures often use survival, growth expressed as dry weight, or some measure of reproductive potential (i.e. fecundity, offspring production) is the major test end-point. Survival of the test organisms (expressed... 

Chlotpyrifos 0,03 0.1 100 Plasma. RBC ChEI dose and end point selected from weight of evidence analy.sis of 90-day and 2-year studies in dogs and rats Dog Chronic 0.0003... 

Fig. 3. gives kinetics of recovery for density inhibited cells exposed to H2O2. Here, 4 hr of delay before trypsinization was sufficient to show the maximum recovery. The presence of benzamide clearly inhibited this recovery process. Fig. 4. shows the yield of mutants due to UV irradiation of density inhibited cells. The phenotypic end point selected for the study was resistance to 8-azaguanine. Immediately after UV exposure, cells were diluted for mutation expression or given a delay time before mutation... 

The properties of straight run diesel fuels depend on both nature of the crude oil and selected distillation range. Thus the paraffinic crudes give cuts of satisfactory cetane number but poorer cold characteristics the opposite will be observed with naphthenic or aromatic crudes. The increasing demand for diesel fuel could lead the refiner to increase the distillation end point, but that will result in a deterioration of the cloud point. It is generally accepted that a weight gain in yield of 0.5% could increase the cloud point by 1°C. The compromise between quantity and quality is particularly difficult to reconcile. 

In this experiment students standardize a solution of HGl by titration using several different indicators to signal the titration s end point. A statistical analysis of the data using f-tests and F-tests allows students to compare results obtained using the same indicator, with results obtained using different indicators. The results of this experiment can be used later when discussing the selection of appropriate indicators. 

The two forms of the indicator, HIn and In, have different colors. The color of a solution containing an indicator, therefore, continuously changes as the concentration of HIn decreases and the concentration of In increases. If we assume that both HIn and In can be detected with equal ease, then the transition between the two colors reaches its midpoint when their concentrations are identical or when the pH is equal to the indicator s piQ. The equivalence point and the end point coincide, therefore, if an indicator is selected whose piQ is equal to the pH at the equivalence point, and the titration is continued until the indicator s color is exactly halfway between that for HIn and In. Unfortunately, the exact pH at the equivalence point is rarely known. In addition, detecting the point where the concentrations of HIn and In are equal maybe difficult if the change in color is subtle. 

Procedure. Select a volume of sample requiring less than 15 mL of titrant to keep the analysis time under 5 min and, if necessary, dilute the sample to 50 mL with distilled water. Adjust the pH by adding 1-2 mL of a pH 10 buffer containing a small amount of Mg +-EDTA. Add 1-2 drops of indicator, and titrate with a standard solution of EDTA until the red-to-blue end point is reached. 

Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibility of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes. 

Finding the End Point Potcntiomctrically Another method for locating the end point of a precipitation titration is to monitor the change in concentration for the analyte or titrant using an ion-selective electrode. The end point can then be found from a visual inspection of the titration curve. A further discussion of potentiome-try is found in Chapter 11. 

There are two types of fluoride lon-selective electrodes available [27] Onon model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Onon model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride lon-specific electrode can be used for direct measurement [2S, 29] or for potenPometric titration with Th" or nitrate solutions, with the electrode as an end point indicator... 

There may be several TSs (and therefore at least one minimum) between the two selected end-points. Some algorithms may find one of these, and the two connecting minima can then be found by tracing the IRC path, or all the TSs and intermediate minima may be located. Methods which generate a sequence of points along the whole reaction path (e.g. CPR and SPW) are examples of the latter behaviour. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

The temperature at which the fuel is boiled off or vaporized at the refinei y is known as the end point temperature listed m ASTM test D86, while ASTM spec D975 uses a 90 percent boiling point or distillation temperature to determine its suitability to vaporize. However, a number of major heavy-duty, highspeed diesel engine manufacturers specify that prior to selecting a diesel fuel you should ensure that a 95 percent distillation temperature is considered to ensure better combustion. 

Fields of Application for ion-selective electrodes o Routine-analytical work in the laboratory (direct or as end-point indicators application frequency in industry 30%) o Clinical analyzers for Na+, K.+, Ca2+, pH, pC02, etc. o Process analyzers... 

Given three add-base indicators—methyl orange (end point at pH 4), bromthymol blue (end point at pH 7), and phendphthalein (end point at pH 9)—which would you select for the following acid-base titrations ... 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

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