Question

鮪魚糖好好吃

為什麼[A(org)]=n[An(org)]成立 (第三張圖equation 7.9)

感謝

Experiment 7
Experiment 7 The partition of Organic acid between Water
and Organic solvent
Objectives
Understand the partition of a solute between two immiscible solvents.
Introduction
A chemical analysis that is performed primarily with the aid of volumetric glassware (e.g.,
pipets, burets, volumetric flasks) is called a volumetric analysis. For a volumetric analysis
procedure, a known quantity or a carefully measured amount of one substance reacts with a
to-be-determined amount of another substance with the reaction occurring in aqueous
solution. The volumes of all solutions are carefully measured with volumetric glassware.
The known amount of the substance for an analysis is generally measured and available in
two ways:
1. As a primary standard: An accurate mass (and thus, moles) of a solid substance is
measured on a balance, dissolved in water, and then reacted with the substance being
analyzed.
2. As a standard solution: A measured number of moles of substance is present in a
measured volume of solution - a solution of known concentration, generally expressed as
the molar concentration (or molarity) of the substance. A measured volume of the
standard solution then reacts with the substance being analyzed.
The reaction of the known substance with the substance to be analyzed, occurring in
aqueous solution, is generally conducted by a titration procedure.
The titration procedure required a buret to dispense a liquid, called the titrant, into a flask
containing the analyte. A reaction is complete when stoichiometric amounts of the reacting
substances are combined. In a titration this is the stoichiometric point. In this experiment the
stoichiometric point for the acid-base titration is detected using a phenolphthalein indicator.
Phenolphthalein is colorless in an acidic solution but pink in a basic solution. The point in the
titration at which the phenolphthalein changes color is called the endpoint of the indicator.
Indicators are selected so that the stoichiometric point in the titration coincides (at
approximately the same pH) with the endpoint of the indicator.

The Partition of Organic Acid between Water and Organic Solvent
Standardization of a Sodium Hydroxide Solution
Solid sodium hydroxide is very hygroscopic;
therefore its mass cannot be measured to prepare a
molar
solution with an accurately-known
concentration (a primary standard solution). To
prepare a NaOH solution with a very well known
molar concentration, it must be standardized with an
acid that is a primary standard.
COOH
COOK+
potassium hydrogen phthalate
In this experiment, dry potassium hydrogen phthalate, KHC8H4O4, is used as the primary
acid standard for determining the molar concentration of a sodium hydroxide solution.
Potassium hydrogen phthalate is a white, crystalline, acidic solid. It has the properties of a
primary standard because of its high purity, relatively high molar mass, and because it is
only very slightly hygroscopic. The moles of KHC8H4O4 used for the analysis is calculated
from its measured mass and molar mass (204.22 mg/mol):
mass (g) KHCH404
mol KHC8H404
204.22 g KHC8H404
= mol KHC8H404
(7.1)
From the balanced equation for the reaction, one mole of KHC8H4O4 reacts with one
mole of NaOH according to the equation:
KHC8H4O4(aq) + NaOH(aq)
H2O(l) + NaKC8H4O4(aq)
(7.2)
In the experimental procedure an accurately measured mass of dry potassium
hydrogen phthalate is dissolved in deionized water. A prepared NaOH solution is then
dispensed from a buret into the KHC8H4O4 solution until the stoichiometric point is reached,
signaled by the colorless to pink change of the phenolphthalein indicator. At this point the
dispensed volume of NaOH is noted and recorded.
The molar concentration of the NaOH solution is calculated by determining the number
of moles of NaOH used in the reaction (Equation 7.2) and the volume of NaOH dispensed
from the buret.
molar concentration (M) of NaOH (mol/L) =
mol NaOH
L of NaOH solution
(7.3)
Once the molar concentration of the sodium hydroxide is calculated, the solution is said
to be "standardized" and the sodium hydroxide solution is called a secondary standard
solution.
Extr
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Extraction
Experiment 7
When a solute dissolves in a mixture of two immiscible solvent, an equilibrium of solute
dissolving in the two solvents is established. This equilibrium is quantified by a distribution
factor D, which is the ratio of solubility in the two solvents. For example, when bromine is
dissolved in a water and carbon tetrachloride (CCl4) (see Equation 7.4) mixture the distribution
factor D is given in Equation 7.5.
Br2(aq) Br₂(CC14)
ry
D =
n.
a
S
d
[Br₂ (CCI)]
[Br₂(aq)]
(7.4)
(7.5)
For some solute-solvent systems, the solute may have different forms in the two solvents.
For example, a solute may be a monomer A(aq) in water and it may be associated into 1
aggregate An(org) in organic solvent. The equilibrium is then given in equation 7.6.
nA(aq)
An(org)
The corresponding distribution factor is given in Equation 7.7.
D =
[A(org)]
[A(aq)]
(7.6)
(7.7)
Alternatively, the relation between the concentrations can also be represented by an
equilibrium constant Keq. (see equation 7.8)
Keg
=
[An(org)]
[A(aq)]"
(7.8)
The distribution factor D can be determined from the total amount of solute in each solvent
irrespective of the aggregation, whereas, the aggregation is essential in determining the
equilibrium constant. In some cases, the degree of association of solute in organic solvent can
be indirectly measured then equilibrium constant can also be evaluated.
In organic solvent, the total concentration of solute [A(org)] is n times that of [An(org)]. (See
equation 7.9)
[A(org)]=n[A, (org)]
Substitute into Equation 7.10
D =
[A(org)] n[A, (org)]
[A(aq)]
[A(aq)]
(7.9)
(7.10)
Substitute [An(org)] in Equation 7.8 into Equation 7.10, the distribution factor can be expressed
in equation 7.11
D=
nK [A(aq)]”
[A(aq)]
After taking log on each side of Equation 7.11, one obtains Equation 7.12.
(7.11)