电导法测定弱电解质电离常数电导法测定弱电解质电离常数及难溶盐溶解度及难溶盐溶解度Determination of Weak Determination of Weak

Electrolyte Ionization Constant and Electrolyte Ionization Constant and solubility of Hardly Dissolved Salt by solubility of Hardly Dissolved Salt by

Conductivity MethodsConductivity Methods

Content

Purposes and Demands

Principle

Apparatus and Reagent

procedure

Data Records and Processing

Questions

Attentions

Demonstrated

Purposes and Demands

1.  To determine the ionization constant of acetic acid.2. To determine the solubility of calcium sulphate.3. Master the operation of conductivity detector.

Principle

1 ） Determination of weak electrolyte ionization constant.

At certain temperature, the equilibrium of acetic acid ionization in water can be written as follow

HAc = H+ + Ac －

Initial c 0 0eg c(1-α) cα cα

So, ionization constant could be expressed as:

1

2c

cK

Ionization degree α in formula ① is related with molar conductivity. That is:

Λ——molar conductivity when concentration is c Λ 0——ultimate molar conductivity if solution is dil

uted illimitably

Λ0 = L++ L– （ the sum of positive and negativ

e ion molar conductivity) can be lookup on literatures.

o

cK 1000

1

K —— conductivity (s/cm or ms/cm orμs/cm). It can be determined through experiment.

C —— concentration (mol•L-1 or N • L － 1)

Adding equation .② into .①, we obtain that

　

④

Draw a graph of cΛm against 1/Λ and get a beeline of which slope is:

slope = Λ 0 2 × Kc

Look for Λ0 on literatures, and account out Kc.

实验原理

coco KKc

12

2 ） Determination of hardly dissolved salt’s solubility

Kc = Ksolution – KH2O

Ksolution —— conductivity of solution

KH2O —— conductivity of water

Apparatus and Reagent

Conductivity detector 1Super thermostatic trough 1Conical flask （ 250ml ） 2Conical flask （ 50ml ） 3Pipette (20ml) 2Beaker (100ml) 10.1000M HAc solutionCaSO4(A.R)

Procedure

1 ） Determination of weak electrolyte ionization constant

1. Regulate the temperature of thermostatic trough at 25℃±0.1℃. Put two conical flasks one of which is filled with 120-130ml conductive water and another with 40ml HAc(0.1M) into thermostatic trough, and maintain the temperature at this level for 20 min.

2. Clean a platinum black electrode with distilled water and blotted it up by the use of filter paper (don’t touch platinum -black). Put the electrode into the conical flask filled with 40ml HAc and detect the conductivity.

Remove l of HAc solution and complement with 20ml conductive water that has been kept temperature constant, shaking up. After temperature is kept constant for 5 min, determine the conductivity of the solution above.

Repeat the same dilution procedure to confect other five HAc solution of different concentration showed in the table below.

Determine six conductivities of these solutions and complete the table.

2 ） Determination of hardy dissolved salt’s solubility1. Weigh 1g CaSO4 solid, put it into 100ml beaker and dissolve with distilled water. Heat the solid-liquid mixture up to boiling point.

Drop off superstratum liquid. Repeat the same procedure twice and think about the meaning of this operation.

2. Dissolve the solid adequately with 80ml distilled water and heat it up to boiling point. Cool the liquid down to room temperature, collect clear topper liquid and take it to thermostatic trough.

Fill the beaker with 40ml distilled water. Repeat the above procedure twice. When the temperature keeps constant, measure the conductivity of these three saturated CaSO4 solutions.

3. Get 40ml of conductive water which has been kept temperature constant and measure its conductivity.

Figure out the conductivity of CaSO4 by the formula we have learned.

HAc浓度 电导率 电离度 平衡常数C＝0.1M K(Ω - 1 cm﹒ - 1)

×1×1/2×1/4×1/8

×1/16×1/32

摩尔电导率molcm

cK 21)

1000(

)(1 1 molcm

c Lmol /(121 molcm )(

o

)1

(2

cK

（ 1 ） Determination of weak electrolyte ionization constant by conductivity methods

Data Records and Processing

Draw a graph of cΛm against 1/Λ and calculate its slope.And as we know: +Λ0

– （ literatures ）＝ 349.8 ＋ 40.9 ＝ 390.7(Ω-1cm-1mol-1)Then, Kc would be figure out, that is Kc =

实验数据记录和处理

2 ） Determination of solubility of hardly dissolved salt 1. Conductivity of water

times 1 2 3 averager

Conductivity of water （ S•cm-

1 ）

实验数据记录和处理

2. conductivity of CaSO4 saturated solution

times 1 2 3 average

conductivity of CaSO4 （ S•cm-1 ）

Kcaso4=Ksolution-Kwater

Questions

1. Why should we use the thermostatic trough in the experiment?2. Why can’t we determine conductivity of solution with direct current?3. Why do we have to wash CaSO4 solid and use superstratum liquid for determination in the preparation process of CaSO4 saturated solution?

Attentions

1. Conductance electrode must be below liquid-level.

2. Determination must be operated in constant temperature.

3. Conductivity detector must be proofread before each determination.

Outline in the use of DDS-11A conductivity detector

1. Switch on the electrical source and allow it warm-up for several min (until pointer turns stable).

2. Turn the constant knob to be electrode constant (it’s been checked and determined by the factory).

3. Emendation. Determine conductivity of H2O on low r

ound and HAc is on high round. Turn the knob, driving the pointer point to full scale namely ↗1.0.

4. Determination. Choose proper measuring range

from large range to small one and try to control the pointer pointing to the full scale in order to get accurate data.

That just is: 104→103→102……10……1……0.1

5. Reading and record. The data marker is in the form of ×10x

×10-6(s/cm). X stands for 4 , 3 or 2 and 1 in the measuring range.