CONDUCTOMETRIC TITRATION FOR CEC DETERMINATION
I. TEST PROCEDURE
A. Basis or Theory of Test
There are a number of methods for determining the cation exchange capacity of soils. All methods basically consist of saturating the cation exchange sites with a particular cation (e.g. , Ba++, Ca++, Na+, or NH4+), displacing this particular cation with another cation, and measuring the amount of the displaced cation in the leachate. The conductometric titration method offers the advantage of reliability and relative simplicity. The chemical reaction utilized is that between a barium saturated soil and a standardized sulfate titrating solution such as magnesium sulfate.
Ba soil + Mg++ + SO4 = BaSO4 + Mg Soil
Before the equivalence point, the conductance remains comparatively constant as the magnesium sulfate reacts to form insoluble barium sulfate and magnesium-soil. When all of the barium saturated soil has been titrated, the conductance of the suspension increases sharply as increments of the magnesium sulfate solution are added. The equivalence point of the reaction is obtained by plotting the data graphically, drawing in the two linear portions of the conductance curve, and taking the point of intersection as the equivalence point.
Conductometric titration eliminates many of the errors and difficulties associated with analytical determination of the displaced cation. The purpose of the conductance reading is simply to locate the equivalence point, not to ascertain the absolute conductivity of the suspension and titrating solution.
B. Test Method
The theory and test procedure for conductometric titration for exchange capacity are described in the following reference:
Mortland, M. M. and Mellor, J. L. (1954) Conductometric Titration of Soils for Cation-Exchange Capacity. Soil Science Society of America Proceedings, Vol. 18, Issue 4, pp. 363 – 364.
The same step-by-step test procedure as described in the proceeding reference is adopted:
- Secure a 4 to 10 gram (gr) sample (dry weight basis) of soil. Use higher sample weight (10 gr) for coarser textured or low plasticity soils.
- Disaggregate the sample in distilled water (use a 5:1 distilled water to solids ratio). Measure the pH of the soil suspension with a glass hydrogen electrode and pH meter.
- Transfer the soil suspension into a 7 cm diameter Buchner funnel mounted in a glass filtering flask. Use No. 40 or 42 Whatman filter paper and pre-wet the paper before transferring the suspension into the funnel.
- Leach the sample with 150 ml of a 1 N barium chloride solution over a period of approximately 4 hours.
ALKALINE SOILS: Use normal, unbuffered solution of barium chloride.
ACID SOILS: Use a normal solution of barium chloride buffered to pH 8.1 with triethanolamine. Further leach with 100 ml of a normal, unbuffered barium chloride solution.
- Wash the sample (still in the Buchner funnel) free of chloride with distilled water as indicated by a silver nitrate test on the leachate.
- Transfer the sample from the funnel to a 400 ml titrating beaker and add 100 ml of distilled water and 50 ml of ethyl alcohol.
- Titrate the soil-water-alcohol suspension with a 0.2 N magnesium sulfate solution. The suspension should be well stirred with a magnetic stirrer (w/ remote rheostat) when the titrating solution is added.
- Measure the conductance of the suspension with each added increment of the titrating solution. Use 1-ml increments initially and make sure the conductance has reached an equilibrium value before adding the next increment. Equilibrium will be slow near the end point.
- The endpoint of the titration is obtained from the intersection of the two straight lines (in a plot of conductance vs. milliliters of 0.2N titrating solution).
- Conductance values at the very beginning of the titration and near the endpoint should be neglected because they represent the effects of hydrolysis, solubility, or dissociation of the products. The more acute the angle between the two linear portions of the conductance curve, the more accurate the equivalence point.
The cation exchange capacity (CEC) of the soil is found from the following relationship:
CEC = 0.2 VE (100/WS)
Where CEC = cation exchange capacity, cmolc kg-1 dry soil
VE = equivalence volume of 0.2N magnesium sulfate.
WS = dry weight of soil sample, gr.
CATION EXCHANGE CAPACITY
Project _________________________________________ Date _______________
Job No. ___________ Run by________________ Chk’d by ________________
Boring No. __________ Sample No. ________________ Depth _____________ Sample Description ______________________________________________________________________ ____________________________________________________________________________________________________________________________________________
pH 6.7 (1:5 dilution)
Wt. of Jar _________ gr
Wt. of moist sample + jar _________
Wt. of dry sample + jar _________
Wt. of water _________
Wt. of dry soil (WS) 16.2 gr Water content (wc) _________ %
|Vol. of 0.2 N MgSO4 soln (ml)||Conductance (µmhos)|
Equivalence Vol. (VE) = _______ ml
CEC = 0.2VE (100/WS) = __________ cmolc kg-1