Spermiogenesis or Spermateleosis or metamorphosis of spermatid
Acid base.pptx
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13. Finding the End Point with a Visual Indicator
Acid/Base Indicators
Many naturally occurring and synthetic compounds exhibit colors that depend on the pH
of the solutions in which they are dissolved.
An acid/base indicator is a weak organic acid or a weak organic base whose
undissociated form differs in color from its conjugate base or its conjugate acid form.
For example, the behavior of an acid-type indicator, HIn, is described by the equilibrium
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16. Selecting and Evaluating the End Point
The equivalence point occurs when stoichiometrically equal amounts of
analyte and titrant react.
For example, if the analyte is a triprotic weak acid, a titration with NaOH
will have three equivalence points corresponding to the addition of one,
two, and three moles of OH– for each mole of the weak acid.
An end point for a titration is determined experimentally and represents
the analyst’s best estimate of the corresponding equivalence point.
17. Where Is the Equivalence Point?
It has been shown that for most acid–base titrations the inflection point, which corresponds to
the greatest slope in the titration curve, very nearly coincides with the equivalence point.
The principal limitation to using a titration curve to locate the equivalence point is that an
inflection point must be present. Sometimes, however, an inflection point may be missing or
difficult to detect.
The inflection point is visible, for acid dissociation constants larger than 10-9, but is missing
when Ka is 10–11 (Smaller).
18. Another situation in which an inflection point may be missing or difficult to
detect occurs when the analyte is a multiprotic weak acid or base whose
successive dissociation constants are similar in magnitude.
let’s consider the titration of a diprotic weak acid, H2A, with NaOH. During the
titration the following two reactions occur.
19. • In general, separate inflection points are seen
when successive acid dissociation constants Differ
by a factor of at least 500.
Ka1 is approximately 20,000 times larger
than Ka2, shows two very distinct
inflection points.
dissociation constants that differ by
a factor of approximately 690.
Ka values differ by a factor of only 27,
20. Locating Titration End Points from pH Measurements
pH electrode and pH meter allow the direct measurement of pH as a function of
titrant volume.
The end point can be taken as the inflection point of the titration curve. With a
sigmoid-shape titration curve,
The inflection point is the steepest part of the titration curve where the pH
change with respect to volume is a maximum.
This point can be estimated visually from the plot. The first derivative, which is
approximately ▲pH/▲V, is the slope of the titration curve
21. Figure 9.14d shows a typical
result. This method of data
analysis, which converts a
portion of a titration curve
into a straight-line, is a Gran
plot.
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23. Finding the End Point by Monitoring Temperature
• The reaction between an acid and a base is exothermic.
• Thermometric titration curve (Figure 6) consists of three distinct
linear regions.
• Before adding titrant, any change in temperature is due to the
cooling or warming of the solution containing the analyte.
• Titration branch - Adding titrant initiates the exothermic acid–base
reaction, resulting in an increase in temperature.
• After the equivalence point, any change in temperature is due to the
difference between the temperatures of the analytical solution and
the titrant.
24. Figutre 7: Thermometric titration curves
showing curvature at the intersection of
the titration and excess titrant branches
25. • Actual thermometric titration curves (Figure 7) frequently show
curvature at the intersection of the titration branch and the excess
titrant branch
• due to the incompleteness of the neutralization reaction, or
excessive dilution of the analyte during the titration
• The problem is minimized by using a titrant that is 10–100 times
more concentrated than the analyte,
• When the intersection between the two branches shows curvature,
the end point can be found by extrapolation (Figure 7).
26. • For example, the titration of boric acid, H3BO3, for which Ka is 5.8 *
10–10
, yields a poorly defined equivalence point (Figure 8). The
enthalpy of neutralization for boric acid with NaOH, however, is only
23% less than that for a strong acid (–42.7 kJ/mol for H3BO3 versus –
55.6 kJ/mol for HCl), resulting in a favorable thermometric titration
curve (Figure 9).
Figure 9: Titration curves for 50.00 mL of 0.0100 M
H3BO3 with 0.100 M NaOH determined by
monitoring temperature.
Figure 8: Titration curves for 50.00 mL of 0.0100 M
H3BO3 with 0.100 M NaOH determined by
monitoring pH.
27. Selecting and Standardizing a Titrant
• Most common acid–base titrants are not readily available as primary
standards and must be standardized
• Standardization is done by titrating a known amount of an appropriate
acidic or basic primary standard.
• The majority of titrations involving basic analytes, whether conducted
in aqueous or nonaqueous solvents, use HCl, HClO4, or H2SO4 as the
titrant.
• Since the concentrations of concentrated acids are known only
approximately, the titrant’s concentration is determined by
standardizing against one of the primary standard weak bases.