Extractable Acidity

Reagents
Barium chloride (BaCl2.2H2O)
Triethanolamine N(CH2CH2OH)3
Hydrochloric acid (HCl)
Bromcresol green indicator

Mixed indicator (methyl-red and methylene blue)
Sodium carbonate anhydrous (Na2CO3)

Preparation of Reagents
Boil deionized water to remove dissolved carbon dioxide. This condition is reached when small, rapid bubbling ceases and large sporadic bubbling is observed. For 50 samples (capacity of shaker), boil 6 liters. Allow to cool to room temperature using carbon dioxide trap.

0.5N Barium chloride and 0.2N triethanolamine (TEA) buffer solution
In 14 liters boiled deionized water dissolve 1099.26g BaCl2.2H20 (61.07 g/L) using magnetic stirrer. Add 480.6mL TEA (26.7 mL/L) and 59.4mL concentrated HCl (3.3mL/L). Bring to 18 liter volume with boiled water. Adjust pH to 8.2 by adding concentrated HCl using burette and pH electrode.

0.2N HCl
Add 16.67mL concentrated HCl per liter deionized water using volumetric flask and make to volume.

0.2N Sodium carbonate
Add 21.20g anhydrous Na2CO3 per liter deionized water using volumetric flask and make to volume. A supply of Na2CO3 should be kept in the oven to maintain anhydrous condition.

Bromcresol green indicator
Dissolve 0.1g bromcresol green in 7.2mL 0.02N NaOH and dilute to 250mL with deionized water.

Mixed indicator (methyl-red and methylene blue)
Dissolve 1.250g methyl red and 0.825g methylene blue in 1 liter 90% ethanol.

Sample Preparation and Procedure
Sample weight is based on estimated potential acidity. Refer to KCl pH values for each sample:

10.0g for pH less than or equal to 4
15.0g for pH from 4 to 5
25.0g for pH greater than 5

Weigh samples, in duplicate, into 250mL Erlenmeyer flasks. Include duplicate blanks. Add 100mL buffer solution to each sample, swirl gently to ensure thorough wetting of the sample. Stopper tightly, load into shaker, and shake vigorously for 2 hours. Remove from shaker and allow to settle overnight.

Draw off 50mL of the solution using a pipette and a squeeze bulb (Safety Pipettor). A sliding scale (AMINCO) holding the pipette attached to a lab stand is useful for this endpoint. Drain the 50mL aliquot into 125mL Erlenmeyer flasks. Add 2 drops of bromcresol green and 8 drops of mixed indicator to each sample and blank.

Titrate samples with 0.2N HCl using lighted magnetic stirrer. The color changes from green to blue to purple. Some samples will go to a reddish-purple. The endpoint is reached when no detectable change in intensity of the color is observed.

Standardize the 0.2N HCl with 0.2000N Na2CO3. It is easier to use 0.200N NaOH standardized with 0.250N potassium hydrogen phthalate (51.058g potassium hydrogen phthalate per liter). Pipette 25mL sodium carbonate solution into four Erlemeyer flasks. Bring to a gentle boil to remove dissolved carbon dioxide and sharpen endpoint. Cover and allow to cool. Add 4 drops of phenolphthalein and 4 drops bromcresol green to each. Titrate with 0.2N HCl to a faint yellow. The endpoint will be reached when no detectable change in intensity of the yellow is observed.

Sample Calculations
Normality of HCl = (0.2000N) (mL Na2CO3)/mL HCl)
Meq/100g = (mL blank – mL titer) (N HCl) (100 mL/150 mL) (100)/(g of sample)

Additional Notes
The above glassware is washed with soap and brush immediately after extraction. Excess buffer solution can be stored and used later.

References
Soil Survey Methods Manual. August 1992. Pg. 361. U.S. Department of Agriculture.