reading material :UNIT 8 Soil pH

SOIL Ph and CATION EXCHANGE CAPACITY LAB

Terminology:
Soil pH
Cation exchange capacity
Reserve acidity
Active acidity
H+ ion concentration
Buffer
Buffering capacity
Acid rain
Base saturation

Experimental Procedure

Part I: Determination of soil pH (active hydrogen ion content).

1. For soils No. 1 (acid) and No. 2 (neutral), mix 10 g soil and 10 mL deionized water in a small beaker or cup. Let stand 10 minutes, stirring occasionally.

2. Test pH of soil-water mixture with a pH meter.

---A pH electrode senses the difference between the H+ concentration in the soil solution and in a reference solution inside the electrode. Calibrate the pH meter with reference solutions of a known pH before using.

3. Convert soil pH to H+ concentration using Table 9.1 or the formula pH = -log [H+]. Record your results on the data sheet.

Part II: Determination of reserve hydrogen.

1. Suspend 2 funnels on a rack and place a clean Erlenmeyer flask under each one. Position a medium speed filter paper in each funnel (e.g. Whatman No. 2 paper).

2. In one funnel place 10 grams of soil No. 1 (acid) and in the second funnel place 10 grams of soil No. 2 (neutral). Form the soil in the funnels with a slight depression in the center.

3. Slowly add 15 mL of 0.5 N Ba (C2H3O2)2 (barium acetate) to each soil directly into the depression to assure that all filtrates go through the soil. Allow to drain. Barium replaces exchangeable hydrogen and acetic acid is formed in the solution.

                    2+                                            2+         2+               +                   -
CLAY  + Ba    +   2 C2H3O2 <------> Ba    +   Ba      +  2 H   +   2 C2H3O   

4. Slowly add 30 mL of deionized water in the same manner. Collect both the barium acetate leachate and water wash into the same flask.

5. Titrate the contents of each flask to determine the amount of hydrogen removed from the soil as follows: Add 10 drops of phenolphthalein to the filtrate, then add 0.01 N NaOH dropwise from a buret while continuously swirling the mixture. Continue adding NaOH until the mixture reaches the endpoint (i.e. a permanent pink color). Record the amount of NaOH added to each filtrate.

6. Prepare a blank by mixing 15 mL of 0.5 N Ba(C2H3O2)2 and 30 mL distilled water. Titrate as above and record the volume of NaOH required to reach the endpoint. It will be necessary to subtract the volume of NaOH used to neutralize the blank from the volume used to neutralize each soil's leachate.

7. Record your data, calculate the remainder of the data, and answer the questions on the question sheets.

Determination of Cation Exchange Capacity.

Cation exchange capacity of a soil can be measured by removing the cations present and determining their concentration. However, in a natural state the soil may contain ten or more different cations making this a difficult task. That approach can be simplified by filling the cation exchange sites with one cation while leaching the others from the soil, then testing for only the one cation. In this procedure, copper (Cu2+) cations are added to fill all the soil's exchange sites. Next, ammonium (NH4+) cations are added to displace the copper. After washing the copper from the soil, its concentration is determined by developing the characteristic blue cupric color in a basic solution and comparing the color intensity to standards of known concentration.

Figure 13-1. Cation exchange capacity determination begins by filling all exchange sites with copper, then displacing the copper with ammonium ions, and finally testing for the amount of copper removed from the soil.

Cation Exchange and Organic Matter

Cation exchange takes place on the surfaces of inorganic and organic soil colloids, i.e. clays and humus. Cations are bound to colloids by attraction to the negative charges originating within the colloid's structure. The ions held on these exchange sites are major factors in determining soil chemical and physical phenomena like fertility, acidity, salinity, environmental contamination, and aggregate stability.

The cation exchange capacity of a soil is determined by the amount of clay and humus and the type of clay present. The approximate cation exchange capacity (CEC) of individual colloids, measured in cmol c kg^ -1, are: montmorillonitic clays, 100; illitic clays, 30; kaolinitic clays, 10; and humus, 200. Humus, though usually present in small amounts compared to clays, can have a significant impact on total CEC by virtue of its own high exchange capacity. Soils with low CEC require more careful management to substitute for this deficiency. However, soils with high CEC may also present management problems associated with high clay content, unless a significant proportion of the total CEC originates from the organic fraction.

Cation Exchange Capacity.

1. Place 1.0 g of each dry soil sample into six separate, small, clean flasks. Add 10 mL 0.2 N Cu(C2H3O2)2 to each flask and mix by swirling for 1 minute. In this step copper cations are displacing all other cations on the exchange sites.

2. Prepare six funnels and filter papers to drain into beakers.

3. Pour the soil and solution into the funnel, washing any remaining soil into the funnel with deionized water in a wash bottle.

4. Wash the excess copper acetate from the soil by slowly pouring 30 mL of deionized water through the soil, letting each aliquot soak into the soil before adding more.

5. Discard all filtered solutions and place a clean, graduated cylinder under each funnel.

6. Add three, 5-mL portions of 1.0 N  NH4C2H3O2 slowly to the soil in the funnel, letting each aliquot soak into the soil before adding more. In this step ammonium cations are displacing copper cations from the exchange sites.

7. Leach deionized water through the soil until a total of 20 mL of solution is in the graduated cylinder. The copper ions that had occupied exchange sites are now washed into the cylinder.

8. Add 5 mL concentrated NH4OH to the graduated cylinder. As the solution becomes basic the blue cupric color will develop in proportion to the amount of copper present.

9. Determine the copper concentration by comparing the blue color of the sample to the blue color of standard copper solutions prepared by your instructor. This comparison can be done visually, or more precisely with a spectrophotometer at 540 nm and a standard curve.

10. Record the copper concentration and calculate the cation exchange capacity of each soil.

Data

I. Active Acidity

Enter the pH of the soils into this table and complete the calculations. For this problem, assume that the furrow slice of a hectare weighs 2,242,000 kg and the moisture content is 22 percent. Other values of interest: water weighs 1.0 kg L-1, the equivalent weight of H+ = 1 g eq-1, and the equivalent weight of CaCO3 = 50 g eq-1.

II. Reserve Acidity

Part III: Determination of Cation Exchange Capacity.

                                                        Low Organic  Matter           Medium Organic Matter                  High OrganicMatter
                                                         Natural Combusted              Natural Combusted                         Natural Combusted

1. Copper concentration in
leachate, cmolc Cu2+                          ______ ______                       ______ ______                           ______ ______

2. Cation exchange capacity,
cmolcg-1                                              ______ ______                        ______ ______                         ______ ______

3. Cation exchange capacity,
cmolckg-1                                            ______ ______                        ______ ______                         ______ ______

4. Exchange capacity attributable
to organic matter, cmolckg-1                    __________                              __________                            __________

5. Percent of exchange capacity
attributable to organic matter                     __________                              __________                          __________

6. Percent of exchange capacity
attributable to mineral colloids                    __________                             __________                           __________

Questions

1. Define the terms "reserve" and "active" acidity. Explain how each is affected by liming a soil.

2. How is the term buffering capacity related to active and reserve acidity?

3. Compare the buffering capacity of the two soils tested. Describe the relationship between buffering capacity of a soil and its cation exchange capacity (CEC)?

4. Soil A is a sandy loam and soil B is a clay loam. Both have a pH of 5.2:
a. Do they have the same amount of active acidity? Explain.
b. Would you expect them to have the same amount of reserve acidity? Explain.

5. Explain why active pH determinations are so useful in soil studies when at best they measure only a very small portion of the total hydrogen in the soil.

6. Acid rain deposits about 0.5 lb of H+ per acre per year on U.S. soils, mostly as H2SO4 and HNO3. How much CaCO3 would be needed (lb per acre) to neutralize this annual acid deposition? (Hint: the equivalent weights of H+ and Ca2+ are 1 g eq-1 and 50 g eq-1, respectively.)

7. Some forms of nitrogen fertilizer acidify soil. If 100 lb per acre of ammonia (NH3) were added and underwent the following reaction, how much acidity, H+, would be produced per acre each year? (Hints: One NH3 produces 1 H+. Ammonia weight 17 g mol-1 and H+ weighs 1 g mol-1, so the weight of H+ produced is 1/17th the weight of the ammonia added.)

NH3 + 2 O2 ----->H+   +  NO3- + H2O (Atomic weights: N = 14   and H+ = 1)

(Note: Because some NH3 is taken up directly as NH4+, not all NH3 undergoes nitrification so the acidity produced under Norman management is only about half of the theoretically possible value. Thus, ammonia fertilizer is not quite as acidifying as it first appears. However, net acidification of soils from ammonium, ammonia, and urea fertilizers and from N fixed by legumes is much greater than that from acid rain.)

8. Why does the acidification from acid rain or nitrogen fertilizer have a greater impact on light-colored, sandy soils than on dark-colored, clayey soils.

How much CaCO3 (limestone) should it take to neutralize the H+ produced from 100 lb of ammonia fertilizer?