Consider a soil with CEC = 35 cmolc/kg holding the following amount of exchangeable cations: 13 cmolc/kg of Ca2+, 6 cmolc/kg of Mg2+, 4 cmolc/kg of K+, 3 cmolc/kg of Na+, 3 cmolc/kg of H+, and 6 cmolc/kg of Al3+ please show all work Calculate its percent acid saturation (%A.S.) and calculate its percent base saturation (%B.S.)

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13 Dec 2019

Consider a soil with CEC = 35 cmol_c/kg holding the following amount of exchangeable cations: 13 cmol_c/kg of Ca²⁺, 6 cmol_c/kg of Mg²⁺, 4 cmol_c/kg of K⁺, 3 cmol_c/kg of Na⁺, 3 cmol_c/kg of H⁺, and 6 cmol_c/kg of Al^{3+ please show all work}

^{Calculate its percent acid saturation (%A.S.) and calculate its percent base saturation (%B.S.)}

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Cation exchange capacity (CEC) is the sum of all exchangeable cations (e.g., Na+, Ca2+, K+) that a soil can adsorb. These cations are positively charged ions and are held on the negatively charged surfaces of soil particles (e.g., clay and organic matter) and can be replaced by other positively charged ions in the soil solution. Since they are readily exchanged with other cations, they are also easily available to plants. The CEC of a soil is dependant on the composition (e.g., types and amounts of clay and organic matter) of the soil (permanent charge) and the pH (pH-dependent charge).

The proportion of the CEC occupied by non acid-forming cations (Ca2+, Mg2+, K+ and Na+) is termed the percent base saturation. It is important to note that these cations are not bases and do not necessarily form bases. Cations including Al3+ and Fe3+ are considered acid-forming cations as they can hydrolyze water. The
percent base saturation can be calculated as shown in Equation 1.

Consider the method presented in your textbook for determining the CEC for a soil. Similar measurements were conducted for soil samples collected from a Gray Luvisol in BC (forested area; see Table 1 a brief summary of the soil properties). For these measurements, 100 g subsamples of oven-dried soil from each horizon were leached with 400 mL of a NH4+ salt solution (leachate 1). The NH4+ displaces all of the cations adsorbed to soil surfaces. Leachate 1 was analyzed for Ca, Mg, K and Na (Table 2). The excess NH4+ remaining in the soil was removed using alcohol and then the sample was leached again with a 400 mL of a K+ salt solution (leachate 2). Similar to above, the K+ displaces all of the NH4+ from the soil surfaces.

Determine the CEC of the soil sample collected from each soil horizon presented in Table 2.

A student wishes to analyze a seawater sample to determine the amounts of Na+ , Ca2+ and Mg2+ present. She begins with 50.00 mL sample of seawater that she dilutes to 500.00 mL. She removes a 50.00 mL aliquot of the diluted solution, which she analyzes for total cations present by exchanging the cations for H+ on an ion exchange column and titrating the liberated acid with 0.5000 M NaOH. The amount of NaOH required to reach the endpoint was 22.48 mL.

The student then titrates another 50.00 mL aliquot of her diluted seawater solution with 15.31 mL of 0.0100 M EDTA solution at a pH of 10 to the Erio T endpoint.

The third titration, performed on a 50.00 mL aliquot of the diluted seawater solution involves precipitating Mg2+ as the hydroxide and titrating the Ca2+ remaining in solution with 0.010 M EDTA solution. Only 5.15 mL of EDTA were required to reach the end point.

Summary of titration data:

Original seawater = 50.00 mL; diluted seawater = 500.00 mL

Vol. of titration aliquot = 50.00 mL

First titration: [NaOH] = 0.5000 M; V NaOH = 22.48 mL

Second titration: [EDTA] = 0.0100 M; VEDTA = 15.31 mL

Third titration: [EDTA] = 0.0100 M; VEDTA = 5.15 mL

From the titration data, calculate the following (show work):

A). mmol of Ca2+ present in the third 50.00 mL titration aliquot - ans.__________

B). mmol of Ca2+ present in the original 50.00 mL seawater sample - ans.__________

C). mg of Ca2+ present in the original 50.00 mL seawater sample - ans.__________

D). mmol of Ca2+ and Mg2+ present in the second 50.00 mL titration aliquot - ans.__________

E). mmol of Mg2+ present in the 50.00 mL titration aliquot - ans.__________

F). mmol of Mg2+ present in in the original 50.00 mL seawater sample - ans.__________

G). mg of Mg2+ present in in the original 50.00 mL seawater sample - ans.__________

H). meq of Ca2+ and Mg2+ present in the 50.00 mL titration aliquot - ans.__________

I). total meq of Na+ , Ca2+ and Mg2+ present in the 50.00 mL titration aliquot - ans.__________

J). meq of Na+ in the 50.00 mL titration aliquot - ans.__________

K). meq of Na+ present in the original 50.00 mL seawater sample - ans.__________

L). mg of Na+ present in in the original 50.00 mL seawater sample - ans.__________

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