Determinationof the

Sol's CMC

Contents Purposes and Demands

Principle Apparatus and Reagent

Procedure Data Records and Processing

Questions Attentions

退出

Purposes and Demands

Acquire the method to coagulate a sol.

Acquire the method to protect a sol. Master the determination of a sol’s

CMC and some important rules.

Principle

A sol has enormous surface and stron

g capacity of adsorption. Fe(OH)3 sol t

hat is the hydrolysate of FeCl3 in water

solution could adsorb Fe3+ ion to be po

sitive.

Kinds of sol in the same condition

carry the same electric charges. A sol

could exist steadily in solution because

of the electrostatic repulsion.

Once a sol’s condition is changed, such

as adding electrolyte, its electric

charges will reduce and even

eliminate. Because of the weakness of

electrostatic repulsion, the sol particles

will collide and coagulate with each

other and become bigger. This

process is called coagulation.

Under the certain temperature and pres

sure, the smallest concentration of ele

ctrolyte that can make a sol coagulate

visibly is called CMC (abbreviation of

Critical Micelle Concentration, the unit i

s mmol/L).

The coagulating value of CMC is due to

the sol’s properties and states.

CMC will be smaller depending on the

raise of temperature, thus, sometimes

we shall make a sol coagulate by

rising the temperature.

As the temperature is certain, the valu

e is due to the heteropolar valence of t

he ions that is adsorbed from the electr

olyte.

The heteropolar valence of the ions is h

igher, the value is smaller.

Also, the CMC of organic ions is much more

bigger than the one of inorganic ions, becau

se the adsorbing ability of the former is bett

er than that of the latter.

Adding the hydrophilic sol such as glutin can

be used to keep the hydrophobe such as Fe

(OH)3 existing steady in solution.

Apparatus and Reagent

Electric cooker glass-tube (20ml)Glass-tube framegraduated-flask (100ml)Pipette (10ml) graduate-flask (10ml)Pipette (1ml) pot bottle (150ml)Volumetric flask beaker (1000ml)

FeCl3 10% collodium

NaCl3 2.5M AgNO3 0.1M

NaSO4 0.05M Gultin 1%

Procedure

1. Preparation of a sol

Fill a 100ml volumetric beaker with

50ml of distilled water and heat it to

boiling point.

Pipette 2.5ml of FeCl3 solution to boilin

g water, immediately take the beaker fr

om electric cooker to cool, and make t

he hydrolyzation Fe(OH)3 prepared for

standby .

2. Determination of the CMC

It consists of two steps in the determination. First, set out a glancing value of CMC, and then determine a further accurate value.

1) Take six glass-tubes marked with “1 ，2 ， 3 ， 4 ， 5” and “contrast”. Pipette

10ml of 2.5M NaCl into tube “1” and 9

ml of distilled water into another five gl

ass-tubes.

Then extract 1ml of liquid from tube “1”

to “2”, Shaking up. And then operate

up to “5” like this in turn. Exact 1ml

liquid from “5” and drop.

Pipette 1ml of Fe(OH)3 sol prepared

into all of the six tubes hereinbefore.

Shake up and record the time. Make

the solution steady for 15min.

Observe the coagulation in all the tubes.

Compare the “contrast” with another five

tubes and find out the coagulated tube at

the latest.

The concentration of this tube is the

approximate value of CMC. Fill the data in

the following table.

Determination of CMC’s approximate value (C’)

number 1 2 3 4 5constr

ast

2.5M NaCl(ml) 10.00

Distilled water(ml)

9.00 9.00 9.00 9.00 9.00

Shaking up 1ml→ 1ml→ 1ml→ 1ml→ 弃去

Fe(OH)3 sol

(ml) 1.00 1.00 1.00 1.00 1.00 1.00

Found out the approximation of the CMC(C’).

2) Dilute the NaCl liquid of 2.5 mol/L to

prepare the NaCl solution of C’.

Calculate the volume on the basis of th

e formula: C1V1=C’V’.

Pipette accurately V’ ml of NaCl liquid

and make up this solution in 50ml of vo

lumetric flask.

3) Take another four clean glass-tubes

and mark down them with numbers“Ⅱ 、Ⅲ、Ⅳ、Ⅴ” and the tube of C’ for

“ ”. There will be a new row of six tubⅠes including the tube of “contrast”.

Pipette in tubes of “Ⅱ 、Ⅲ、Ⅳ、Ⅴ” respectively with 8 、 6 、 4 、 2ml of

NaCl solution of C’ and 1 、 3 、 5 、 7

ml of distilled water and shake up.

Pipette 1ml of Fe(OH)3 sol into all the t

ubes, Shake up and make the solution

steady for 15 min.

Then, find out the coagulated tube at the lat

est. Account the average value of the Cn of

the latest coagulated solution with the Cn+1

of the nearby tube which has not coagulated,

and the value is the CMC which can be exp

ressed to be C=1/2(Cn + Cn+1).

Finally fill in the table with experiment phen

omenon and results.

Determination of the CMC:

number Ⅰ Ⅱ Ⅲ Ⅳ Ⅴconstrast

CˊNaCl(ml) 

8.00 6.00 4.00 2.00 

Distilled water(ml) 

1.00 3.00 5.00 7.00 9.00

Fe(OH)3 sol(ml)  1.00 1.00 1.00

1.001.00

3. Repeat the complete procedure with electrolyte Na2SO4.

Data Records and Processing

The CMC of Fe(OH)3 sol coagulated with electrolyte NaCl:

Cˊ= m mol/l C=1/2(Cn+ Cn+1)= m mol/l

number １ ２ ３ ４ ５ Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ

C(NaCl) mmol/L

                  

coagulating                   

The CMC of Fe(OH)3 sol coagulated with electrolyte Na2

SO4:

Cˊ= m mol/l C=1/2(Cn+ Cn+1)= m mol/l

The CMC ratio of using NaCl to Na2SO4 is:

number １ ２ ３ ４ ５ Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ

C(Na2SO4)

mmol/L                    

coagulating                   

Questions

What’s the meaning of CMC?

What’s the key step in the experiment?

Attentions Waste liquid is asked for callback

together.

Pipettes should not be confused

because each one has their respective

use.

Chart Ⅱ － 20 － 1 ζpotential of Chart Ⅱ － 20 － 2 influence of the concentration Diffuse double layer of electrolyte to ζpotential

C4>C3>C2>C1