Cation exchange capacity (CEC) measurements

Material required

  • Cobalt hexamine powder
  • Centrifuge
  • Spectrometer
  • Pipette 
  • Agate motor and pestle to make powder

Step 1 :

Take at least 2g of powdered sample. Put the sample in oven for about 24 hours at 60-70 C°. The sample should be sieving at 2 mm (elimination of the coarse elements, only the fine fraction is kept).

Prepare cobalt hexamine solution. Take 4.4580 g of cobalt and put it in 1 liter of distilled water. Shake it for a moment then cover it with aluminum paper and keep it for at least 24h.

Fig 1: Cobalt hexamine solution (a) shaking well to mix the cobalt hexamine powder with distilled water (Magnetic stirrer mixer), (b) prepared solution covered with aluminium foil to avoid sunlight.

Figure 1

Step 2 :

Take out the samples from the oven. Measure its weight.

Take 2g (measured weight) and 40 mL of the cobalt solution in a falcon tube. Put the falcon tubes in touring device for 30 tours per min for 8 hours.

Fig 2: (a) powdered sample mixed with cobalt hexamine solution  (b) Mixing of the sample using tube rotator mixer

Figure 2

Step 3 :

Centrifuge the tubes at 4600 speed for 10 minutes at temperature 20 C°.

Fig 3: Centrifuge machine and sample prepared for centrifuge

Figure 3

Step 4 : (Calibration)

For calibration, prepare 4 or 5 solutions of water and cobalt hexamine i.e. 0 % (only water), 25% (1.8mL of water and 0.6 mL of cobalt hexamine), 50% (1.5mL of water and cobalt hexamine each), 75% and 100% (cobalt hexamine). (Use the centrifuge tubes only).

Fig 4: Solutions prepared in centrifuge tubes for the calibration

Figure 4

Step 5 :

Turn on the spectrometer atleast 15 minutes before starting the measurements. Set the wavelength at 472 nm.

Calibrate it with distilled water, this will give almost 0.00.

Measure the CEC of the witness which is the cobalt solution without sample (100% cobalt hexamine solution), it is usually around 0.936[the value of witness].

Now, take 3 mL of prepared sample (Figure 3) from the falcon after centrifuge using the filter and syringe (to filter the grains).

Fig 5: Syringe and filter used.

Put the sample in spectrometer and then take the value appears on the digital screen [the value of absorption].

Figure 5

Use the following formula to get value of CEC.

CEC=[  (value of witness-value of absorption)/(value of witness )]∗50∗V/m∗100

V = volume of cobalt hexamine i.e. 0.04 Lm = mass of the powdered sample

Value of absorption= value displayed on spectrometer when the prepared solution (from fig. 3) is used.

Value of witness= 100% cobalt hexamine solution

Precautions:

Avoid direct contact of eyes and hands to the cobalt hexamine powder and solution. Wear gloves and glasses.