Range of an indicator

The pH range of an indicator does not have to be equally distributed on either side of the indicator s piQ. For some indicators only the weak acid or weak base is colored. For other indicators both the weak acid and weak base are colored, but one form may be easier to see. In either case, the pH range is skewed toward those pH levels for which the less colored form of the indicator is present in higher concentration. 

The pH range of an indicator chosen for a titration must coincide with the point at which the pH is changing very rapidly. 

To be useful in a titration, the pH range of an indicator must lie within the vertical section of the graph. The pH range of a variety of acid-base indicators is given on p. 20 of the SQA Data Booklet. 

Why does the typical acid/base indicator exhibit its color change over a range of about 2 pH units 14-4. What variables can cause the pH range of an indicator to shift ... 

Assuming that the end of the interval is reached when 91% of the indicator is present in the alkaline form, the transformation range of an indicator in saturated solution lies between the hydro-... 

How can you estimate the pH range of an indicator s color change Why do some indicators have two separate pH ranges ... 

On account of the difficulty of detecting a small intensity of one colour in the presence of another, the useful range of an indicator is limited to... 

Dilute solutions of nominally 0.001 M NaOH and HGl are used to demonstrate the effect of an indicator s color transition range on titration error. Potentiometric titration curves are measured, and the indicator s color transition range is noted. Titration errors are calculated using the volume of titrant needed to effect the first color change and for a complete color change. 

In this experiment the effect of a mixed aqueous-organic solvent on the color transition range of common indicators is investigated. One goal of the experiment is to design an appropriate titrimetric method for analyzing sparingly soluble acids and bases. 

With 0.1M solutions, the ideal pH range for an indicator is limited to 45-9.5. Methyl orange will exist chiefly in the alkaline form when 99.8 mL of alkali have been added, and the titration error will be 0.2 per cent, which is negligibly small for most practical purposes it is therefore advisable to add sodium hydroxide solution until the indicator is present completely in the alkaline form. The titration error is also negligibly small with phenolphthalein. 

The second approach consists of estimating sequentially the values Fx(zit (N)) for a series of cut-off values z covering the range of variability of the concentration values, usually I0 max 100%] The key idea is to interpret the conditional cdf Fx(z (N)) as the conditional expectation of an indicator transform I(x z) which can be estimated from the corresponding indicator data Indeed, consider the indicator transform ... 

Figure 10 shows the absorbance response of an indicator to pH change. As can be seen, with the use of an indicator, the pH measurements can be made only over a range of about two pH units, i.e. pKa 1. Beyond this range, the change in absorbance with pH becomes small and the error in pH measurements will be large. 

Despite increasing the usable range of the indicators, the MAXCOV plots yielded cusps rather than complete peaks. The estimated base rates were. 085 for unsmoothed and. 083 for smoothed plots. The range of estimates was small (.02). However, this is likely to be an underestimate because it is based on four estimates derived from four aggregated plots averaged over three subanalyses with the same input indicator. This atypical presentation makes the findings difficult to interpret. 

Reversible, non-competitive inhibition of polymerase is also afforded by a series of N-benzoyl pyrrolidines. Substitution on the benzoyl moiety with a para-trifluoromethyl group is optimal in this series. Bulky, hydrophobic groups at the 2-position of the pyrrolidine ring increase activity, and the 5-position tolerates a wide range of substituents, indicative of a solvent exposed portion of the inhibitor. Compound (+)-38, containing a 2-thienyl moiety at the 5-position, has an IC50 of 190 nM in the enzyme assay while its enantiomer is almost 100-fold less active [83]. 

Sensitivity. The measured fluorescence parameter of an indicator should be sensitive to changes of analyte in the desired concentration range, as summarized in Table 10.1 for a number of analytes. The indicator should have high extinction coefficient for efficient excitation and high quantum yield for a good signal-to-noise ratio. 

For an indicator to be useful in detecting the end-point of an acid-base reaction, the pH range of the indicator must fit into the vertical part of the titration curve for that reaction. The vertical part of the titration curve will include the pH of the salt solution at the end-point. 

A term used in statistics to indicate the range of an interval within which the true value for a parameter hes, given a specified confidence level. 

An option to consider in using the WA technique is tolerance weighting. The rationale for this approach is that taxa occurring over a narrow range of an environmental gradient should be better indicators than taxa with broader tolerances. Consequently, taxa with narrower tolerances should be weighted... 

The measurement range of an instrument depends on a number of instrumental and sample factors and must be determined for each system. Most commercial instruments have an indicator that shows when the droplet concentration is in the appropriate measurement... 

Trap desorption. The choice of the thermal desorption apparatus is critical in order to avoid contamination and to be able to work with aroma compounds in a wide range of retention indices. In all systems, problems can be encountered due to reactive compounds or cold spots within the analyzer. It is recommended that all transfer lines, valves, or surfaces in contact with the volatile compounds be made of an inert material such as fused-silica or deactivated glass-lined stainless steel. Even more ideal are systems that do not have long... 

A FIGURE 15.4 Some common acid-base indicators and their color changes. The color of an indicator changes over a range of about 2 pH units. 

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