End-point of a titration

Personal Errors Finally, analytical work is always subject to a variety of personal errors, which can include the ability to see a change in the color of an indicator used to signal the end point of a titration biases, such as consistently overestimating or underestimating the value on an instrument s readout scale failing to calibrate glassware and instrumentation and misinterpreting procedural directions. Personal errors can be minimized with proper care. 

A colored compound whose change in color signals the end point of a titration. 

B. l0-Bromo-ll-hydroxy-10,ll-dihydrofarnesyl Acetate [2,6-Dodeca-diene-1,11-diol, 10-bromo-3,7, -trimethyl-, 1-acetate, (E,E)-]. Farnesyl acetate (29 g., 0.11 mole) is dissolved in 1 1. of /erf-butyl alcohol (Note 4) contained in a 3-1. Erlenmeyer flask. Water is added (500 ml.), and the solution is cooled to about 12° using an external ice water bath. Maintaining this temperature, rapid magnetic stirring is begun, and more water is added until a saturated solution is obtained. The second addition of water may be rapid initially, but the saturation point must be approached carefully, like the end point of a titration. A total of about 1200 ml. of water is required for the above amounts of farnesyl acetate and ferf-butyl alcohol. The solution must remain clear and homogeneous at about 12°, and if the saturation point is accidentally passed by adding too much water, ferf-butyl alcohol should be added to remove the turbidity. 

Indicator a substance that has an intense color in acid or base solution and another color in the other type of solution. Indicators are used to determine when the end point of a titration has been reached. 

Operator Effects An analyst may read a meter or scale consistently high or low. There will be some variability in judging the end-point of a titration. 

Ion-selective electrodes are particularly useful for monitoring the disappearance of an ion during a titration. In many cases it is not necessary to calibrate the instrument because there is often a significant change in the potential at the end-point of a titration. However, some electrodes have a slow response time and care must be taken to ensure that titration is not performed too quickly. 

Explain which of the following indicators could be used to detect the end-point of a titration involving solutions of (a) hydrochloric acid and sodium hydroxide,... 

Gran plot A graph such as the plot of Vb 10 ph versus Vb used to find the end point of a titration. Vb is the volume of base (titrant) added to an acid being titrated. The slope of the linear portion of the graph is related to the dissociation constant of the acid. 

The end point of a titration is the point at which the concentrations of the acid and base forms of the indicator are equal [HIn] = [In-]. When we substitute this equality into the expression for Kln, we see that at the end point [H30+] = Kln. That is, the color change occurs when... 

The end point of a titration, the region of the steepest rise in the titration curve, can be determined experimentally if an instrument is available for measuring the pH after each addition of base. A simpler means is to introduce into the solution a small amount of an indicator chosen so that its range lies within the steep vertical portion of the curve. This ensures a sharp color change at the end point. 

End point (of a titration) — An experimental point in the progress of a titrimetric reaction located where the reaction between the -> analyte and reagent is considered as complete. Ideally, it should be coincident to the - equivalence point [i]. 

The location of the end-point of a titration by using either the first or second derivative of the titration data was discussed in Chapter 9. These methods use only the data points near the end-point. Another approach. Gran s method, makes use of the complete data set. It is useful when either (i) the inflection at the end-point is poorly defined or (ii) data at the end-point is missing. 

At the end point of a titration of 25 mL of 0.300 M NaOH with 0.200 M HNO3, what would the concentration of sodium nitrate in the titration flask be ... 

End point of a titration -> Stage of a titration where the color of the indicator in the analyte is... 

The end point of a titration should not come as a surprise to the analyst. Before a single drop of titrant has been added, an estimate of the endpoint volume should be carried out. For a simple forward titration, like the citric acid example above, the approximate titre is given by... 

Although production of each carbonate ion uses up two hydroxide ions, the uptake of carbon dioxide by a solution of base does not necessarily alter its combining capacity for hydronium ions. Thus, at the end point of a titration that requires an acid-range indicator (such as bromocresol green), each carbonate ion produced from sodium or potassium hydroxide will have reacted with two hydronium ions of the acid (see Figure 16-1) ... 

For the purpose of investigating the relation between the potentio-metric and stoichiometric end points of a titration the assumption will be made that K , is negligible compared with KAC and equation (17) can then be put in the form... 

Valik19 made differential potentiometric titrations of aspartic acid, one series of results being given in curve (c) of Fig. 6. The volume of solution of alkali necessary to titrate tlie second acid dissociation for which the ionization constant is 2.5 X 10"10 should be exactly equal to that for the first unless there is a difference between the potentiometric and stoichiometric end points. Within the rather large limit of error, this was found to be true, but the end point could not be located with accuracy due to the flatness of the curve, as shown above. Differences between the stoichiometric and potentiometric end points are predicted for titrations of weak acids or weak bases. Such a difference increases the weaker the acid or base, but the difficulty of locating the end point also increases. It may be safely concluded that within the accuracy to which the potentiometric end point of a titration can be established it is identical with the stoichiometric end point. 

Potassium chromate is used as an indicator in titrations for the determination of chloride ion. At the end point of a titration of an aqueous solution of a metal chloride salt (e.g. NaCl) against silver nitrate solution in the presence of potassium chromate, red Ag2Cr04 precipitates. Give equations for the pertinent reactions occurring during the titration, and, using relevant data from Table 6.4, explain how the indicator works. 

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