Field Laboratory CEC & BS by Hach Kit 1

extraction for CEC and BS
extraction for CEC and BS

This method was associated with a HACH kit.

Reagents

  • Sodium Hydroxide 0.11N — From Hach 12269–32 – Not listed on Hach site
  • Hydrochloric Acid 0.75N 2.79% — From Hach 12267–31 – Not listed on Hach site
  • Bromcresol Green-Methyl Red Indicator Solution — From Hach 23292–32
  • Buffer Solution Hardness 1 – From Hach 424–32
  • ManVer Hardness Indicator Hardness 2 — From Hach 425–32
  • EDTA Solution Strong — From Hach 12271–32 – Not listed on Hach site – See below for recipe.
  • Buffer Solution pH 8.1 — From Hach 12265–49 – Not listed on Hach site – See below for recipe.
  • Deionized H2O

Buffer pH 8.1 Recipe

Recipe from Hach Laboratories.
Replaces the Hach 8.1 Buffer that is no longer produced.

Ingredients needed for 1 L:

  1. KCl – 0.0745 kg (74.5 g)
  2. HCl – 3.11 mLs (density 1.2)
  3. TEA – 9 mLs (density 1.126)
  4. Toluene – 0.5 mLs

Procedure

  1. Add KCl and 800 mLs water to a large beaker.
  2. Add HCl with stirring.
  3. Slowly add TEA with stirring to a pH of 8.0 +/- 0.02.
  4. The pH may take a day to stabilize, be patient.
  5. Add toluene.
  6. Dilute to one liter with water.

EDTA Strong Solution Recipe

Recipe from Hach Laboratories

Ingredients needed for 1 L:

  1. Toluene, ACS Spectro Grade – 0.005 L
  2. Demineralized Water – 0.75 L
  3. Sequestrene NA4 – 0.02321 Kg
  4. Demineralized water, to make 1 L

Sequestrene NA4 is Tetrasodium EDTA salt (CAS no 64-02-8) from Hach 700534 CAS no 64-02-8

Procedure

  1. Add the Sequestrene and the Toluene to the first increment of demineralized water.
  2. Adjust the pH to 5.0 ± 0.5 with Hydrochloric Acid
  3. Dilute to volume with demineralized water.
  4. Mix thoroughly and control.

Equipment for Extraction

  • 20 ml syringes
  • cotton balls
  • mortar and pestle
  • 1.0g measuring spoon or scale

Terms

cation exchange capacity (CEC):

The sum of exchangeable bases plus total soil acidity at a specific pH, values, usually 7.0 or 8.0. When acidity is expressed as salt extractable acidity, the cation exchange capacity is called the effective cation exchange capacity (ECEC) because this is considered to be the CEC of the exchanger at the native pH value. It is usually expressed in centimoles of charge per kilogram of exchanger (cmolc –1) or millimoles of charge per kilogram of exchanger. See also acidity, total.

acidity, total:

The total acidity including residual and exchangeable acidity. Often total acidity is calculated by subtraction of exchangeable bases from the cation exchange capacity determined by ammonium exchange when the system is buffered to pH 7.0. Total acidity can be determined directly using pH buffer-salt mixtures (e.g. BaCl2 plus triethanolamine, pH 8.0 or 8.2) and titrating the basicity neutralized after reaction with a soil.


CEC Analysis Procedures

Blank Acidity for Buffer Solution

(Do this only if the buffer’s blank acidity is not written on the bottle.)

  1. Assemble the extraction setup by connecting two syringes with a small plastic connector. Make sure the plunger in the lower syringe is inserted all the way up, and that there is no plunger in the upper syringe (see drawing)
  2. Insert two cotton balls into the upper syringe and tamp down with a spatula.
  3. Add about 5–6 ml of buffer solution into the upper syringe and draw the liquid, through the cotton balls, into the lower syringe. Stop drawing when the liquid is just at the top of the cotton balls.
  4. Repeat step 3 until you have collected 20 ml of extract in the lower syringe.
  5. Disconnect the lower syringe and dispense the extract into a clean 50-ml Erlenmeyer flask.
  6. Using the 1.0 cc syringe and transfer cal, measure 1.00 cc of weak hydrochloric solution into the flask and swirl to mix.
  7. Add 2 drops of brom cresol green-methyl red indicator solution to the flask and swirl to mix.
  8. If a red color develops, add sodium hydroxide solution dropwise until color changes from red to blue.
  9. The number of drops of sodium hydroxide solution added to the extract through the blank (no soil) setup is equal to the blank buffer extractable acidity in meq/100 grams (or cmolc kg–1).
  10. This number should be subtracted from the soil acidity measurement results (see sample calculations).

Soil Acidity Measurement

  1. Assemble the extraction setup by connecting two syringes with a small plastic connector. Make sure the plunger in the lower syringe is inserted all the way up and that there is no plunger in the upper syringe (see drawing).
  2. Insert two cotton balls into the upper syringe and tamp down with a spatula.
  3. Using a clean mortar and pestle, grind approximately one teaspoon of air-dried soil. Using the 1.0g measuring spoon, measure 1 level scoop of air dried soil (1.0g) and put into the upper syringe barrel.
  4. Add about 5–6 ml of buffer solution and stir thoroughly to mix.
  5. Draw the extract through the soil and cotton pad by pulling back the plunger in the lower syringe.
  6. Repeat steps 4 and 5 until exactly 20 ml extract has been collected. Make sure that during the last extraction, the buffer solution left in the upper syringe just about covers the top of the cotton balls. You should have a clear or nearly clear extract if the soil did not go through the cotton balls during extraction.
  7. Disconnect the lower syringe and dispense the extract into a clean 50-ml Erlenmeyer flask.
  8. Using the 1.0 cc syringe and transfer cal, measure 1.00 cc of weak hydrochloric solution into the flask and swirl to mix.
  9. Add 2 drops of brom cresol green-methyl red indicator solution to the flask and swirl to mix.
  10. If a red color develops, add sodium hydroxide solution dropwise until color changes from red to blue. SAVE THIS SOLUTION for the calcium + magnesium procedure.
  11. The number of drops of sodium hydroxide solution added to the extract is equal to the extractable acidity in meq/100 grams (or cmolc kg–1). Subtract the acidity of the blank buffer from this number to get the soil acidity in meq/100 grams (or cmolc kg–1) (see calculations).

Calcium + Magnesium

  1. Add 10 drops of Hardness I solution to the solution retained from step 10 of the soil acidity procedure, and swirl to mix.
  2. Add 3 drops of Hardness II solution and swirl to mix.
  3. If a red color develops, add strong EDTA solution dropwise until the color changes from red to blue.
  4. The number of drops of strong EDTA solution added is equal to the meq/100g calcium + magnesium.

Calculation of Cation Exchange Capacity (CEC)

The cation exchange capacity (CEC) can be calculated from the following: 

CEC = Calcium + Magnesium + Acidity

The units are meq/100g (or cmolc kg–1). Sodium ( a gypsum requirement) is usually negligible, but should be included if present in significant amounts.


Calculation of Base Saturation (BS)

The percent base saturation can be calculated using the following: 

BS = [(Calcium + Magnesium) / CEC] x 100%

Where calcium + magnesium and CEC are expressed in meq/100 g.


Sample Calculations

Blank Acidity = 2 drops
Raw data for soil acidity = 4 drops

Soil acidity = Raw data – blank acidity = 4 drops – 2 drops = 2 drops

Calcium + magnesium = 18 drops

CEC = 18 + 2 = 20 meq/100 g.

BS = (18/20) x 100% = 90%

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