Analytical Methods:Carbonate Titration

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Carbonate Analysis: Titration Method
This method follows Rowell (1994).

This is a two-phase analysis: 1. The soil is mixed with a known amount of hydrochloric acid (HCl) causing the dissolution of the carbonate (CaCO3 and creating Calcium chloride (CaCl2), water and carbon dioxide.

CaCO3 + 2HCl &#8658; CaCl2 + H2O + CO2

2. The amount of acid left over is measured by titrating it with sodium hydroxide (NaOH) to produce sodium chloride (NaCl) and water. Adding phenolphthalein indicator to the solution causes it to turn pink when all the acid has reacted.

HCl + NaOH &#8658; NaCl + H2O

Equipment and Chemicals

 * 2 Molar Hydrochloric acid (2M HCl)
 * 0.1 Molar Sodium hydroxide (0.1M NaOH). Carefully weigh out 4.00g of NaOH, add 750 ml of distilled water and stir until the pellets are dissolved. Pour the solution into a 1l glass volumetric flask. Use a small amount of distilled water to swill the beaker and add the rinsings to the volumetric flask until the solution reaches the volume mark on the flask neck. This is 0.1M NaOH.
 * Phenolphthalein indicator solution
 * Distilled water


 * 2 mm mesh sieve
 * Mortar and pestle
 * Scientific balance
 * Hot plate
 * Burette and clamps
 * Filter papers
 * Filter funnels
 * 100 ml volumetric flasks
 * 250 ml conical flasks
 * 20 ml glass pipette
 * 10 ml glass pipette
 * Pipette bulb
 * Dropping pipette
 * 50 ml measuring cylinder

Method

 * 1) Sieve the air-dried soil and retain the less than 2 mm fraction. Grind this roughly in the pestle and mortar.
 * 2) Weigh out 10 g of the ground soil. Recording the exact weight (M) to 0.01 g.
 * 3) Transfer the soil to a conical flask and pipette in 20 ml of 2M HCl.
 * 4) Allow the sample to react until the effervescence stops. Then place the flask on a hotplate and boil gently for 10 minutes.
 * 5) Allow to cool, then filter the solution into 100 ml volumetric flasks. Make up to volume (mark on neck of flask) with distilled water.
 * 6) Pipette 10 ml of this solution into a clean conical flask. To this add 50 ml of distilled water using a measuring cylinder and a few drops of phenolphthalein indicator.
 * 7) Clamp the burette so that it is vertical with enough space below to place the conical flask. Fill the burette with 0.1M NaOH to the 0 ml mark, and place a piece of white paper underneath the burette. Stand the flask holding the solution on the paper under the tap of the burette.
 * 8) Controlling the burette tap with one hand and holding the conical flask in the other, slowly allow the NaOH to run into the flask, swirling the flask all the time to mix the two solutions. Stop as soon as the solution in the flask develops a permanent pink colour. Record the volume of NaOH (V) you have added by reading off the scale on the side.
 * 9) Repeat steps 5-7 using another 10 ml fraction from the same sample. Then average the two results.

Calculation
the mass of CaCO3 that reacted is: Y x (100.1 / 2) = Z g of CaCO3
 * 1) The number of moles of NaOH you have used in the titration is: 0.1 x (Vaverage / 1000) = W x 10 -3 mol NaOH
 * 2) In the equation equal numbers of moles of NaOH and HCl react together so W also = W x 10-3 mol HCl in the 10 ml of fraction.
 * 3) This 10 ml was drawn from 100 ml of solution so the number of moles in the 100 ml of solution is: (W x 10-3) x 10 = X mol HCl left over from the reation with the carbonates in the soil.
 * 4) The 20 ml of 2M HCl originally added to the soil contained 0.04 mol HCl. Therefore the number of mols of acid that reacted with the carbonate is: 0.04 - X = Y mol HCl
 * 5) The molecular mass of CaCO3 is 100.1 g. 2 mols of HCl react with 1 mol (100.1 g) of CaCO3. Therefore,
 * 1) As a percentage the M g of soil analysed contains Z x (100/M) = % CaCO3 in the air-dried soil.

For example if 10 g of soil were analysed and 12 ml of 0.1M NaOH was used in the titration.
 * 1) 0.1 x 12 = 1.2 x 10-3 mol NaOH
 * 2) 1.2 x 10-3 mol NaOH react with 1.2 x 10-3 mol HCl in 10 ml fraction used
 * 3) In the total 100 ml of solution is 1.2 x 10-3 x 10 = 1.2 x 10-2 or 0.012 mols HCl left over from the reaction.
 * 4) The amount of HCl that reacted with the soil is 0.04 - 0.012 = 0.028 mol HCl.
 * 5) This reacted with 0.028 x (100.1 / 2) = 1.401 g of CaCO3 in the 10 g of soil
 * 6) This means that the percentage CaCO3 in the air-dried soil is 1.4 x (100 / 10) = 14.01%

Safety
Some of the chemicals used in the method are toxic or highly corrosive. Always consult chemical safety data sheets and follow appropriate health and safety protocols before handling or using chemicals.

Many chemical saftey data sheets are available here from this Oxford University based database.