Exercise 6B: Determination of ECCE of Liming Sample

Effective calcium carbonate equivalent (ECCE) is an index that allows us to compare the relative acid-neutralizing ability of liming materials. This index takes into consideration both the chemical neutralizing potential of the liming material and the probable rate at which the material will react with the soil (availability). The rate of reaction for limestone materials is usually slow–a matter of months or years for complete reaction to occur. Some materials may take so long that they are considered ineffective.

The calcium carbonate equivalent (CCE) of the liming material is a chemical measure of its neutralizing power and is therefore determined chemically. The principle of the determination is simple. A sample of crushed limestone is treated with a known amount of standard acid that is more than enough to react with the acid-neutralizing materials in the limestone. The reaction is driven to completion by heating the solution to drive off the carbon dioxide produced. After the reaction is complete, the amount of acid remaining is determined by back titration with standard base. The calcium carbonate equivalent is the amount of acid neutralized by the sample calculated as a percentage of the amount that would have been neutralized by the same mass of pure calcium carbonate.

What portion of the calcium carbonate equivalent (CCE) in a particular liming material can be considered effective? The answer depends on the time period under consideration and on the size of the particles. In Iowa (Iowa Code, chapter 201A), a period of 3 years has been assumed to be reasonable. Particle size is measured by means of a set of sieves. Any material too large to pass through a 4-mesh sieve is considered to be ineffective because it does not react noticeably in 3 years. The fraction that passes through a 4-mesh sieve but not an 8-mesh sieve is evaluated as 10% effective in 3 years. The portion passing through the 8-mesh sieve but held on a 60-mesh sieve is evaluated as 40% effective. Any material that is fine enough to pass through the 60-mesh sieve should react completely within 3 years and is considered 100% effective.

The term “lime” can be used both as a noun (compounds that neutralize acid) and as a verb (the process of neutralizing acid). The term is applied to several different compounds that can neutralize acid. Limestones are normally dominated by calcium carbonate (calcite) but magnesium carbonate (magnesite) and calcium magnesium carbonate (dolomite) are also common constituents. Related compounds such as calcium and magnesium oxides or hydroxides also may be present or may be produced by special treatment. These other liming materials have lower molecular mass than calcium carbonate so they are stronger neutralizers on a mass-for-mass basis. As a result, some lime has a CCE greater than 100%. More commonly, though, the limestone contains enough inert impurities such as clay, quartz, and feldspar to lower its CCE below 100%. Materials with CCE values less than 70% are seldom used for liming purposes.

Total effectiveness of a limestone or other liming material is calculated by multiplying the availability by the CCE. For example, a limestone sample with a CCE of 80 and availability of 60 would have a total effectiveness of 48%. Because limestone usually is merchandised by the ton, the ECCE is often expressed as pounds of effective calcium carbonate equivalent per ton of liming material. In the example above, the ECCE would be 960 pounds of ECCE per ton. Iowa Code, chapter 201A, requires that the ECCE be reported on a dry weight basis.

 

Procedure

1. Your instructors have passed a few grams of the crushed limestone sample through a 20-mesh sieve for this exercise.

A 20-mesh sieve is a good size to use for this procedure. Fine material is used to assure complete reaction within a reasonable period of time. For commercial testing, any coarse material present would be crushed by hammer blows until it would all pass through the sieve for testing. Simple sieving, however, is usually adequate for this laboratory exercise. To be expressed on a dry-weight basis, the sample is first dried at 105^oC.

2. Weigh in duplicate 1.00 ± 0.05 g sample (record exact weight) of the < 20 mesh limestone sample into an acid-washed 250-mL Erlenmeyer flask. Return the leftover lime, both fine and coarse, to the bulk sample container.

The milliequivalent weight of calcium carbonate is 50 mg. A 1.00 g sample of pure CaCO₃ theoretically contains 20 meq (1000 mg/50 mg = 20) of acid-neutralizing power. Actual 1.0-g samples may contain anywhere from a little more to considerably less than 20 meq.

3. Add 40.0 ± 0.1 mL of standardized 1.0 N HCl to each flask. Record the exact strength and quantity of acid used.

An excess of fairly strong acid is used to assure complete reaction of any MgCO₃ present as well as CaCO₃ and any other acid neutralizers the sample may contain.

Primary reaction: 2HCl + CaCO₃ ó CaCl₂ + H₂CO₃

4. Heat the flask on a hot plate just below boiling for 30 min.

Many procedures for determining CCE heat at this stage just to the point of boiling and then maintain that temperature for a period of time. Heating drives off CO₂ gas from the secondary reaction below, allowing the reaction to be driven to completion.

Secondary reaction: H₂CO₃(l) ó H₂O(l) + CO₂ (g)

5. To each flask, add 50 ± 1 mL of distilled water and 5.0 ± 0.1 mL of 2 N BaCl₂ solution.

The CO₂ gas must be expelled from solution or it will interfere with the base titration in step 6. In this procedure, we will drive the reaction to completion and remove all CO₂ from solution by reacting the CO₂ with Ba to form BaCO₃, which precipitates. Bicarbonate (HCO₃^–) ions also are formed as intermediates. As carbonate precipitates and is removed from solution, bicarbonate will be converted to carbonate and will also precipitate. This procedure eliminates the need of an open flame or an extended time period on the hot plate.

6. Add 5 drops of phenolphthalein and titrate the unreacted acid with standardized 1 N NaOH.

Phenolphthalein is colorless in an acid solution but turns pink in an alkaline solution.

7. Record the normality and exact amount of NaOH used and calculate the CCE of the sample.

The acid neutralized per g of limestone sample equals the difference of the meq of HCl initially added in excess and the meq of NaOH required for the back titration, divided by the weight of sample analyzed. This amount taken as a percentage of the number of meq that would have been present if the sample were pure CaCO₃ (see note under step 2) is the CCE of the sample.

% CCE = 5.0045 x [(mL of HCl x N of HCl) – (mL of NaOH x N NaOH)] / Weight of the liming material (g)

Note:The 5.0045 is a correction factor that considers the meq wt of CaCO3 in the denominator and 100 (to convert to percentage) in the numerator:

Exact meq wt of CaCO₃ = 100.087/2 = 50.044 mg/meq

Per gram, 1000 mg/50.044 mg/meq = 19.982 meq

100/19.982 = 5.0045

8. Assume that 96% of the lime sample will pass through a 4-mesh sieve, 87% will pass an 8-mesh sieve, and 79% will pass a 60-mesh sieve. Use these values to calculate the ECCE of the material. Multiply the % CCE by an effectiveness factor to get ECCE.

Effectiveness Factor Based on Fineness_______

(total)

*Note in the introduction to Exercise 7 that “the portion passing through the 8-mesh sieve but held on a 60-mesh sieve is evaluated as 40% effective. All of the material passing an 8-mesh sieve also will pass a 4-mesh sieve; therefore, the factors are cumulative (0.1 + 0.3 = 0.4, or 40%). If the material passes all sieves, it is 100% effective (0.1 + 0.3 + 0.6 = 1.0 or 100%).

 

Name____________________

Date_____________________

Section__________________

Sample number________

Average % ECCE if:

• 96% finer than 4 mesh:
• 87% finer than 8 mesh:
• 79% finer than 60% mesh:

Notes or comments: