Transcript CH13.ppt

CHAPTER
Titrimetric Methods;
Precipitation Titrimetry
Chapter13 p
Titrimetry
• Volumetric titrimetry
• Coulometric titrimetry
• Gravimetric titrimetry
Chapter13 p 337
§13A
Some terms used in volumetric titrimetry
• Standard solution: a reagent of exactly
known concentration that used in a
titrimetric analysis
• Titriation: a process in which a standard
reagent is added to a solution of an analyte
until the reaction between the analyte and
reagent is judged to be complete.
Chapter13 p 338
• Back-titration: a process in which the excess
of a standard solution used to consume an
analyte is determined by titration with a
second standard solution.
Chapter13 p
Equivalence Points and End Points
• Equivalent point: the point in a titration when the
amount of added standard reagent is exactly
equivalent to the amount of analyte.
• End point: the point in a titration when a physical
change occurs that is associated with the condition
of the chemical equivalence.
• Indicators are often added to the analyte solution to
produced an observable physical change at or near
the equivalence point.
Chapter13 p
• Titration error (Et) : the difference in
volume or mass between the equivalence
point and the end point
Et = Vep – Veq
Vep : the actual volume of reagent required to
reach the end point
Veq : the theoretical volume to reach the
equivalence point
Chapter13 p
Indicators
• Physical change:
 the appearance or disappearance of a color
 the change in color
 the appearance or disappearance of turbidity
Chapter13 p
Typical setup for carrying out a
titration. The apparatus consists of a
buret, a buret stand and clamp with a
white porcelain base to provide an
appropriate background for viewing
indicator changes, and a wide-mouth
Erlenmeyer flask containing a
precisely known volume of the
solution to be titrated. The solution is
normally delivered into the flask
using a pipet, as shown in FIGURE 222.
Figure 13-1
The titration process.
Chapter13 p 339
Detail of the buret
graduations. Normally, the
bret is filled with titratnt
solution to within 1 or 2
mL of the zero position at
the top. The initial volume
of the buret is read to the
nearest ± 0.01 mL. The
reference point on the
meniscus and the proper
position of the eye for
reading are depicted in
figure 2-21.
Figure 13-1
The titration process.
Chapter13 p 339
Before the titration begins.
The solution to be titrated, an
acid in this example, is placed
in the flask and the indicator
is added as shown in the
photo. The indicator in this
case is phenolphthalein,
which turns pink in basic
solution.
Figure 13-1
The titration process.
Chapter13 p 339
During titration. The titrant
is added to the flask with
swirling until the color of
the indicator persists. In the
initial region of the titration,
titrant may be added rather
rapidly, but as the end point
is approached, increasingly
smaller portions are added;
at the end point, less than
half a drop of titrant should
cause the indicator to
change color.
Figure 13-1
The titration process.
Chapter13 p 339
Figure 13-1
The titration process.
Titration end point. The end point is achieved when the barely
perceptible pink color of phenolphthalein persists. The flask on
the left shows the titration less than half a drop prior to the end
point; the middle flask shows the end point. The final reading of
the buret is made at this point, and the volume of base delivered
in the titration is calculated from the difference between the
initial and final buret readings. The flask on the right shows what
happens when a slight excess of base is added to the titration
mixture. The solution turns a deep pink color, and the end point
has been exceeded. In color plate 9, the color change at the339
end
point is much easier to see than in this black-and-white version.
Primary Standards
◎Primary Standard: a highly purified compound that
serves as a reference material in volumetric and
mass titrimetric method
1. High purity
2. Atmospheric stability
3. Absence of hydrate water
4. Modest cost
5. Reasonable solubility in the titration medium
6. Reasonably large molar mass
Chapter13 p
• Secondary standard: a compound whose
purity has been established by chemically
analysis and that serves as the reference
material for a titrimetric method
Chapter13 p
§13B
Standard Solution
•
1.
2.
3.
4.
Standard Solution
Be sufficiently stable
React rapidly
React more or less complete
Undergo a selective reaction
Chapter13 p
• Two basic methods are used to establish the
concentration of such solutions:
(1) Direct method～～ careful weighed quantity of
primary standard is dissolved in a suitable solvent
and dilute to exactly know volume.
(2) Standardization: the titrant to be standardized is
used to titrate
a weighed quantity of a primary standard
a weighed quantity of a secondary standard
a measured volume of another standard solution
Chapter13 p
§13C
Volumetric Calculations
◎Some Useful Algebraic Relationship
 definition of mole
nA =
mA
A: A species
MA
mass A (g)
mol =
molar mass A (g/mol)
Chapter13 p
◎Some Useful Algebraic Relationship
definition of molar concentration
cA =
nA
M =
V (L)
mole A
V (L)
mol = V (L) x cA (mol A/ L)
V: the volume of the solution
Chapter13 p
Calculating the Molarity of Standard Solutions
Ex 13-1
Describe the preparation of 2.000L of 0.0500M
AgNO3 (169.87g/mol) from the primary-standardgrade solid
請敘述由一級標準品的AgNO3 (169.87g/mol)
固體，製備出2.000L of 0.0500M 的溶液
Chapter13 p 342
Ex 13-2
A standard 0.0100M solution of Na+ is required to
calibrate a flame photometric method to determine
the element. Describe how 500mL of this solution
can be prepare from primary standard Na2CO3
(105.99g/mol)
焰色光度計可利用0.0100M的Na+溶液當作測試Na+濃度的
校正溶液。請敘述如何由一級標準品的Na2CO3
(105.99g/mol)固體，製備出500ml的上述溶液
Chapter13 p
Ex 13-3
How would you prepare 50.0mL portions of standard
solution that are 0.00500M, 0.00200M, and
0.00100M in Na+ from the solution in Ex13-2?
請敘述如何由Ex 13-2的溶液製備出50 mL的 0.00500M,
0.00200M, and 0.00100M 的Na+溶液
Chapter13 p
Treating Titration Data
◎Calculating Molarities from Standardization Data
Ex13-4
A 50.00mL portion of an HCl solution required
29.71mL of 0.01963M Ba(OH)2 to reach an end
point with bromocresol green indicator. Calculate
the molarity of the HCl.
50.00mL的HCl溶液需要29.71mL，0.01963M的Ba(OH)2來
達到反應終點，試計算HCl的莫耳濃度
Chapter13 p
Ex 13-5
Titration of 0.2121g of pure Na2C2O4 (134.00 g/mol)
required 43.31mL of KMnO4. What is the molarity
of the KMnO4 solution?
0.2121g的Na2C2O4溶液需要43.31mL，KMnO4來達到反應
終點，試計算KMnO4的莫耳濃度
Chapter13 p
◎Calculating the Quantity of Analyte from Titration
Data
Ex13-6
A 0.8040g sample of an iron ore is dissolved in acid.
The iron is then reduced to Fe2+ and titrated with
47.22mL KMnO4 solution. Calculate the results of
the this analysis in terms of (a) %Fe (55.85g/mol) (b)
%Fe3O4 (231.54g/mol)
將0.8040g的鐵礦溶解於酸液中。鐵礦被還原成Fe2+，並
用47.22mL，0.02242M的KMnO4滴定之。由此滴定分
析結果計算鐵礦中(a) %Fe (55.85 g/mol) (b) %Fe3O4
(231.54 g/mol)
Chapter13 p
Ex 13-7
A 100.0mL sample of brackish water was made
ammoniacal, and the sulfide it contained was
titrated with 16.47mL of 0.02310M AgNO3. The
analytical reaction is
+
2-
2Ag + S
Ag2S(s)
Calculate the concentration of H2S in the water
in parts per million.
100.0mL具有臭味的水溶液樣品氨化，且利用16.47mL，
0.02310M的AgNO3滴定水溶液中硫化物的量，試計算水
溶液中H2S的濃度為？ppm
Chapter13 p
Ex 13-8
The phosphorus in a 4.258g sample of a plant food
was converted to PO3- and precipitated
as Ag3PO4
4
through the addition of 50.00mL of 0.0820M
AgNO3. The excess AgNO3 was back-titrated with
4.86mL of 0.0625 M KSCN. Express the results of
this analysis in terms of %P2O5
4.258g農作物樣品中的磷化合物成分與水作用形成磷酸根，
之後利用0.0820M，50.00mL AgNO3滴定而形成磷酸銀
沈澱。過多的AgNO3利用0.0625M，4.06mL的KSCN反滴
定之。請計算P2O5的含量。
Chapter13 p
Ex 13-9
The CO in a 20.3L sample of gas converted to CO2 by
passing the gas over iodine pentoxide heated to 150oC :
The iodine was distilled at this temperature and was
collected in an absorber containing 8.25mL of 0.01101M
Na2S2O3
The excess Na2S2O3 was back-titrated with 2.16mL of
0.00947M I2 solution. Calculate the concentration in
milligrams of CO (28.01g/mol) per liter of sample.
20.3L的CO氣體通過150oC的I2O5會反應形成CO2氣體，
形成的碘蒸汽在150oC的溫度下進行蒸餾並利用0.01101M,，8.25mL
Na2S2O3予以吸收，多餘的Na2S2O3需要2.16mL，0.00947M的I2
進行反滴定，試計算在樣品中，CO氣體的濃度（單位：mg/L）
Chapter13 p
§13D
Gravimetric Titrimetry
• Weight or gravimetric titrimetry
～～ the mass of titrant is measured.
Chapter13 p
Calculations Associated with
Weight Titrations
• Weight molarity (MW) : the number of moles of
reagent in 1 kg solution
weight molarity =
mole A
solution (kg)
0.1 Mw NaCl(aq) ～～
= 0.1 mol of the NaCl in 1 kg of solution
= 0.1 mmol in 1g of the solution
Chapter13 p
Advantages of Weight Titrations
• Calibration of glassware and tedious cleaning to ensure
proper drainage are completely eliminated.
• Temperature corrections are unnecessary because weight
molarity does not change with temperature, in contrast to
volume molarity.
• Weight measurements can be made with considerably
greater precision and accutacy
• Weight titrations are more easily automated than are
volumetric titrations.
Chapter13 p
§13E
Titration Curves in Titrimetric Methods
•
End point～～physical change that near equivalent
point
Two most widely used end point
(1) changes in color due to the reagent, the analyte, or
an indicator
(2) change in potential of an electrode that responds
to the concentration of the reagent or the analyte
Chapter13 p
Types of Titration Curves
• Titration curve: plots of a concentration-related
variable as a function of reagent volume.
• Two general types of titration curves:
sigmoidal curve
linear segment curve
Chapter13 p
The p-function of analyte is plotted as a
function of reagent volume
Measurements are made on both sides
the equivalent point
Figure 13-2
Two types of titration curves.
Chapter13 p 351
Concentration Changes during Titrations
• The equivalent point in a titration is characterized by
major changes in the relative concentrations of reagent
and analyte.
• Example:
Ag+ + SCN-
AgSCN(s)
Chapter13 p 351
Chapter13 p 351
Figure 13-3
Titration curve for the
titration of 50.00 mL
of 0.1000 M AgNO3
with 0.1000 M
KSCN.
Chapter13 p 352
§13F
Precipitation Titrimetry
• Precipitation Titrimetry: based on the reactions that
yield ionic compounds of limited solubility (mid1800s)
slow rate of formation of most precipitates
most important precipitating reagent is AgNO3,
used to determination of the halides, the halide-like
anion,
Argentometric methods
Chapter13 p
Precipitation Titration Curves
Involving Silver Ion
• Ag+ + (halides)Ag (halides) (ppt)
• To construct titration curves, three type of
calculations are required
preequivalence
equivalence
postequivalence
Chapter13 p
Ex 13-10
Perform calculations needed to generate a titration
curve for 50.00mL of 0.0500M NaCl with 0.1000M
AgNO3 (for AgCl, Ksp = 1.82 x 10-10 )
Ag+ (aq) + Cl- (aq)
AgCl (s)
試計算以50.00mL, 0.0500M NaCl與0.1000M AgNO3滴定時
的滴定曲線 （以AgNO3加入體積為X軸，pAg 為Y軸）
Chapter13 p
Chapter13 p 354
◎ The Effect of Concentration on
Titration Curve
Figure 13-4
Titration curve for A,
50.00mL of 0.0500 M
NaCl with 0.1000 M
AgNO3, and B,
50.00mL of 0.00500 M
NaCl with 0.0100 M
AgNO3.
Chapter13 p 355
◎ The Effect of Reaction Completeness
on Titration Curve
Figure 13-5
Effect of reaction
completeness on
precipitation titration
curves. For each curve,
50.00m of a 0.0500 M
solution of the anon was
titrated with 0.1000 M
AgNO3. Note that smaller
values of Ksp give much
sharper breaks at the end
point.
Chapter13 p 356
◎ Titration Curves for Mixtures of Anions
Titration of 50.00mL solution (0.05M I-, 0.0800M Cl-)
with 0.1000M AgNO3
Ag+ (aq) + I- (aq)
Ag+ (aq) + Cl- (aq)
AgI (s)
Ksp = 8.3 x 10-17
AgCl (s) Ksp = 1.8 x 10-10
How much iodide is precipitated before appreciable
amount of AgCl form.
Chapter13 p
[Ag+] [I-]
[Ag+] [Cl-]
8.3 x 10-17
=
1.82 x 10-10
= 4.56 x 10-7
[I-] = 4.56 x 10-7 [Cl-]
After 25.00mL of titrant have been added
-
cCl = [Cl ] =
50.00 x 0.0800
= 0.0533 M
50.00 + 25.00
[I-] = 4.56 x 10-7 x 0.0533 = 2.43 x 10-8 M
Chapter13 p
The percentage of I- unprecipitated:
no. mmol I- = (75.00 mL) x (2.43 x 10-8 M)
= (75.00 mL) x (2.43 x 10-8 mmol/ mL)
= 1.82 x 10-6
original no. mmol I- = (50.00mL) x (0.05 mmol/ mL) = 2.50
I- unprecipitated =
1.82 x 10-6
x 100% = 7.3 x 10-5 %
2.50
Chapter13 p
Figure 13-6
Titration curves for
50.00mL of a solution
0.0800 M in Cl- and
0.0500 M in I- or Br-.
Chapter13 p 357
• As Cl- begins to precipitate,
Ksp = [Ag+] [Cl-] = 1.82 x 10-10
[Ag+] =
1.82 x 10-10
= 3.41 x 10-9
0.0533
pAg = - log( 3.41 x 10-9 ) = 8.47
Chapter13 p
• After 30.00 mL of AgNO3 had been added
-
cCl = [Cl ] =
50.00 x 0.0800 + 50.00 x 0.0500 - 30.00 x 0.100
50.00 + 30.00
= 0.0438 M
[Ag+] =
1.82 x 10-10
= 4.16 x 10-9
0.0438
pAg = 8.38
Chapter13 p
Chapter13 p
◎ Indicators for Argentometric
Titrations
Three types of end points are encountered in
titrations with AgNO3 (silver nitrile)
1. Chemical
2. Potentiometric
3. Amperometric
Chapter13 p
chemically indicator
• The color change should occur over a
limited range in p-function of the reagent or
the analyte
• The color change should take place within
the steep portion of the titration curve for
the analyte
Chapter13 p
Chromate Ion: The Mohr Method
• Sodium chromate (Na2CrO4)
~ ~ determination of Cl-, Br-, CN~ ~ form a brick-red silver chromate (Ag2CrO4)
titration reaction
Ag+ + Cl-
AgCl (s)
white
indictor reaction
2Ag+ + CrO42-
Ag2CrO4 (s)
red
Chapter13 p
The silver concentration at chemical equivalence :
[Ag+] = √Ksp = √ 1.82 x 10-10 = 1.35 x 10-5 M
[CrO42-] =
Ksp
1.2 x 10-12
=
[Ag+]2
(1.35 x 10-5)2
= 6.6 x 10-3 M
Chapter13 p
Adsorption Indictor: The Fajans Method
• Adsorption Indictor: an organic compound
that tends to be absorbed onto the surface of
the solid in a precipitate titration
Fluorescein
Chapter13 p
Iron (III) Ion: The Volhard Method
• Silver ions are titrated with a standard solution of
thiocyanate ion:
Ag+ + SCN-
AgSCN (s)
• Iron (III) serves as the indictor:
Fe3+ + SCN-
FeSCN2+
red
Chapter13 p
Chapter13 p 362