2nd Exam - ACID-BASE TITRIMETRY Flashcards
substances which can act either as an acid or as a
base (can either donate protons or accept protons
depending on the environment), an example of
which is water.
Amphiprotic Substances
leveling effect of water
solvent effect
Standard
solutions of strong acids and bases are used
extensively for determining analytes that are
themselves acids or bases or analytes that can be
converted to such species by chemical treatment
Theory of Neutralization Titrations
The standard solutions employed in the neutralization
titrations are __________ or ________ because these substances react more completely with
an analyte than do their weaker counterparts and thus
yield sharper end points
strong acids (SA) or strong bases (SB)
is an observable physical change that occurs at or near the equivalence point Two most widely used end points involve
- a change in the color due to the reagent, the analyte or an indicator
- a change in the potential of an electrode that responds to the concentration of the reagent or the analyte
end point
consist of a plot of reagent volume as
the horizontal axis and some function of the analyte or
reagent concentration as the vertical axis
titration curves
important observations are
confined to a small region (typically 0 1 to 0 5
mL) surrounding the equivalence point
sigmoidal curve
measurements are made
on both sides of, but well away from the
equivalence point Measurements near equivalence
point are avoided The vertical axis is an
instrument reading that is directly proportional to
the concentration of the analyte or reagent
linear-segment curve
The ______ offers the advantages of speed and convenience
The _________ is advantageous for reactions that are complete only in the presence of a goodly excess of the reagent or analyte
sigmoidal type ;
linear segment type
most commonly used weak acids/bases
HCL, HClO4, H2SO4, NaOH, KOH
The hydronium ion in an aqueous solution of a strong
acid has two sources:
the reaction of the acid with water
the dissociation of water
To derive the titration curve for a solution of strong acid with a strong base, three types of pH calculations are required
pre-equivalence
equivalence
post equivalence
the solution consists of the excess acid
and the hydronium ion concentration is derived directly from the
concentration of the excess acid
pre-equivalence point
the hydronium ion and hydroxide ion are
present in equal concentrations and the hydronium ion concentration is derived directly from the ion product constant of water
equivalence point
the solution consist of the excess base
and the hydronium ion concentration is derived directly from the
concentration of the excess base and the ion product constant of water
post equivalence point
When a weak acid is titrated with a strong base or
a weak base with a strong acid,
consisting of a conjugate acid/base pair is formed
-resists changes in pH
buffer solution
Henderson-Hasselbalch Equation
pH = pKa + log [C. Base]/[W. Acid]
pOH = pKb + log [C. Acid]/[W. Base]
The Effect of Dilution on Buffer Solutions
The pH of a buffer solution remains essentially
independent of dilution until the concentration of the
species it contains is decreased to the point where
approximations become invalid
defined as the number of moles of a strong acid or
a strong base that causes 1 00 L of the buffer to
undergo a 1 00 unit change in the pH
buffer capacity
( T or F )A buffer solution of any desired pH can be
prepared by combining calculated quantities of a
suitable conjugate acid/base pair
T
COMMON TYPES OF ACID BASE INDICATORS
- Phthalein indicators (phenolphthalein): 8-10
- Sulfonphthalein indications (Phenolsulfonphthalein / phenol red): 6.8-8.3
- Azo indicators (methyl orange: 3.1-4.4 and methyl red: 4.1-6.1)
that occurs when the pH at which the
indicator changes color differs from the pH at chemical
equivalence
systematic errors
that originates from the limited ability of
the eye to distinguish reproducibly the intermediate
color of the titration
random
error
Variables that influence the behavior of indicators
- Temperature
- Ionic strength of the medium
- Presence of organic solvents and colloidal particles
Reagents for Neutralization Titrations
- Standard Acid Solutions
- Standard Base Solutions
- prepared by diluting an approximate volume of concentrated reagent, commonly used is HCl
- standardized against weighed quantities of sodium carbonate, and other primary standards [TRIS or THAM tris-(hydroxymethyl)aminomethane)
Standard Acid Solutions
- sodium hydroxide NaOH is the most commonly prepared standard base solution KOH and Ba(OH) 2 are also encountered
- standardized using weak organic acids, most commonly used is potassium hydrogen phthalate (KHP)
Standard Base Solutions
APPLICATIONS OF NEUTRALIZATION TITRATIONS
- Elemental Analysis
- Determination of Inorganic Substances
- Determination of the organic functional groups
- Determination of Salts
elements
susceptible to this type of analysis are nonmetallic and
include carbon, nitrogen, chlorine, bromine, and fluorine
elemental analysis
element determined using the most common method known as Kjeldahl method, which is based on a neutralization titration
Here, the sample is decomposed in concentrated acid to convert the nitrogen to ammonia The liberated ammonia is distilled, collected in an acidic solution, and determined by a neutralization titration Procedure is straightforward and readily adapted for routine analysis
Nitrogen
element determined by burning the sample in a stream of oxygen The sulfur dioxide formed during oxidation in collected by distillation into a dilute solution of hydrogen peroxide The sulfuric acid is the titrated with a standard base
Sulfur
determined by the conversion of ammonia
with strong base followed by distillation The ammonia is collected and titrated as in the Kjeldahl method
Ammonium salts
here, ions are first reduced to ammonium ion by Devarda’s alloy, and introduced into a strongly alkaline solution of the sample in a Kjeldahl flask The ammonia is then distilled and titrated
Nitrates and Nitrites
neutralization titrations are employed to determine the equivalent weight of the purified organic acids. A base range indicator such as phenolphthalein is required.
carboxylic and sulfonic acid groups
Aliphatic amines are titrated with a solution of a strong acid. Many amines are titrated in nonaqueous solvents which enhance their basicity.
amine groups
determined by saponification using a
standard base and the excess is titrated with a standard acid
ester groups
determined by esterification which produces acetic acid The acetic acid is then titrated with a standard solution of alcoholic sodium or potassium
hydroxide
hydroxyl groups
determined with a solution of hydroxylamine hydrochloride In the process, HCl is liberated and is titrated with a base
carbonyl groups
The total salt content of a solution can be accurately
and readily determined by an acid/base titration The
salt is converted to an equivalent amount of acid or a
base by passage through a column packed with an ion
exchange resin
Determination of salts
