Unit 3: Chemical Analysis of Primary Nutrients in Organic Manures - Subjective Questions

Total nitrogen is the combined amount of organic nitrogen and ammoniacal nitrogen present in an organic manure.

Principle of the Kjeldahl method:

• The manure sample is digested with concentrated sulphuric acid in the presence of a catalyst.
• Organic nitrogen is converted into ammonium sulphate.
• The digestion reaction may be represented as:

• The digest is made strongly alkaline with sodium hydroxide, releasing ammonia:

• The liberated ammonia is distilled into boric acid or a measured quantity of standard acid.
• The trapped ammonia is determined by titration, and the nitrogen content is calculated from the volume of standard acid consumed.

Procedure:

1. Weigh a representative, finely ground manure sample into a Kjeldahl digestion flask.
2. Add concentrated sulphuric acid and a catalyst mixture, such as potassium sulphate with copper sulphate or selenium.
3. Heat carefully until frothing stops, and then digest strongly until a clear or nearly colorless solution is obtained.
4. Cool the flask, dilute the digest with distilled water, and transfer it quantitatively to the distillation unit.
5. Add excess sodium hydroxide to make the solution strongly alkaline.
6. Distill the liberated ammonia into boric acid containing a mixed indicator or into a known volume of standard acid.
7. Titrate the trapped ammonia with standardized acid. Run a reagent blank using the same procedure.
8. Calculate nitrogen after correcting the sample titre with the blank titre.

Important precautions:

• Avoid loss of ammonia during alkalization and distillation.
• Prevent bumping and carry-over of alkali.
• Continue distillation until ammonia recovery is complete.
• Use a homogeneous, moisture-corrected sample where results are required on a dry-matter basis.

Let:

• = titre for the sample in mL
• = titre for the blank in mL
• = normality of the standard acid
• = mass of manure sample in g
• = molar mass of nitrogen in g mol

The corrected volume of acid is mL. One equivalent of acid corresponds to one mole of ammonia and hence one mole of nitrogen.

Mass of nitrogen is:

Therefore:

or:

If an aliquot of the digest is used, the result must also be multiplied by the dilution or aliquot factor. For expression on a dry-matter basis, the moisture correction factor must be applied.

Functions of reagents:

• Sulphuric acid: Oxidizes the organic matrix and converts nitrogen into stable ammonium sulphate.
• Potassium sulphate: Raises the boiling point of the digestion mixture, thereby increasing the digestion temperature and accelerating oxidation.
• Catalyst: Copper sulphate, selenium or another suitable catalyst increases the rate of decomposition of organic matter.

Limitations:

• Conventional Kjeldahl digestion does not quantitatively determine nitrate and nitrite nitrogen unless a suitable pretreatment or modified procedure is used.
• Incomplete digestion causes low nitrogen recovery.
• Loss of ammonia during digestion, alkalization or distillation also produces low results.
• The method uses corrosive chemicals and requires careful control of heating, fumes and waste disposal.

• The manure sample is first digested so that organically bound and mineral phosphorus are converted into soluble orthophosphate.
• In an acidic medium, orthophosphate reacts with ammonium molybdate to form phosphomolybdic acid.
• Reduction of this complex produces an intense molybdenum blue color.
• Alternatively, a yellow vanadomolybdophosphate complex may be formed using ammonium vanadate and ammonium molybdate.
• Within the working range, color intensity follows the Beer-Lambert relationship:

where is absorbance, is molar absorptivity, is optical path length and is phosphorus concentration.

• Absorbance is measured with a spectrophotometer against a reagent blank.
• The phosphorus concentration is obtained from a calibration curve prepared using standard phosphate solutions.

Sample digestion:

1. Dry, grind and homogenize the manure sample.
2. Weigh a suitable quantity into a digestion vessel.
3. Digest it using an approved wet-acid mixture, such as nitric acid followed by perchloric acid under controlled conditions, or use a validated dry-ashing procedure.
4. Cool, dissolve the residue in dilute acid, filter if required and dilute to a known volume.

Color development and measurement:

1. Transfer a suitable aliquot of digest into a volumetric flask.
2. Add the prescribed molybdate-based color reagent.
3. Add reducing agent when the molybdenum blue method is used.
4. Dilute to volume, mix and allow the specified color-development time.
5. Measure absorbance at the wavelength prescribed by the selected method.
6. Prepare a reagent blank and a series of phosphate standards under identical conditions.
7. Determine phosphorus from the calibration curve and apply dilution, aliquot and sample-mass factors.

A certified reference material or spiked sample should be analyzed to verify digestion and recovery.

Molybdenum blue method:

• Orthophosphate first forms a phosphomolybdate complex in acidic medium.
• A reducing agent converts it into a blue-colored complex.
• It is generally more sensitive and is suitable for low phosphorus concentrations.
• Timing, acidity and reducing conditions require close control.

Vanadomolybdophosphate method:

• Orthophosphate reacts with vanadate and molybdate in acidic medium.
• A yellow vanadomolybdophosphate complex is formed.
• It is generally less sensitive but is convenient for samples containing comparatively high phosphorus concentrations.
• The developed color is usually stable over a practical measurement period.

In both methods, the final result depends on complete digestion, matrix control, blank correction and calibration with standard phosphate solutions.

Let:

• = phosphorus concentration obtained from the calibration curve in mg L
• = final volume of the digest in L
• = additional dilution factor, including any aliquot factor
• = sample mass in g

The mass of phosphorus in the digest is mg. Therefore:

The factor converts mg g into percent.

To express phosphorus as phosphorus pentoxide:

The conversion factor is obtained from molecular masses:

The reported basis, such as fresh weight or dry matter, must be clearly stated.

• The organic manure is digested or ashed to bring total potassium into solution.
• The solution is aspirated into a flame, where potassium compounds dissociate and potassium atoms become excited.
• Excited potassium atoms emit characteristic radiation as they return to lower energy levels.
• Potassium emission is commonly measured near .
• Over a suitable working range, emission intensity is related to potassium concentration.
• Standard potassium solutions are used to prepare a calibration curve.
• The concentration of the sample digest is read from this curve after blank correction.
• Where high concentrations or matrix effects occur, the sample must be diluted and matrix-matched standards should be used.

Procedure:

1. Prepare a representative, finely ground sample and weigh an appropriate test portion.
2. Digest the sample by an approved wet-digestion method or ash it under controlled conditions.
3. Dissolve the digest or ash in dilute acid, filter if necessary and dilute to a known volume.
4. Prepare a reagent blank and a series of potassium standards covering the expected concentration.
5. Start the flame photometer, select the potassium filter or wavelength and allow the instrument to stabilize.
6. Aspirate the blank and set the zero response.
7. Aspirate the standards and establish the calibration curve.
8. Aspirate the sample solutions, rinsing the system between measurements.
9. Dilute samples whose readings exceed the calibration range and repeat the measurement.
10. Calculate total potassium using the digest volume, dilution factor and sample mass.

Instrument performance should be checked periodically with an intermediate calibration standard.

Major sources of error include:

• Ionization interference: Potassium atoms may ionize in a hot flame, changing emission intensity.
• Spectral interference: Emission from other species or background radiation may affect the potassium signal.
• Matrix effects: Differences in acid content, dissolved salts and viscosity alter aspiration and atomization.
• Non-linearity: High potassium concentrations can fall outside the linear calibration range.
• Flame instability: Changes in gas pressure or fuel-to-oxidant ratio cause signal drift.
• Contamination: Potassium is common in glassware, dust, reagents and laboratory water.

Control measures:

• Use matrix-matched standards and a reagent blank.
• Dilute samples into the calibrated working range.
• Maintain stable flame and aspiration conditions.
• Use clean laboratory ware and high-purity reagents.
• Check calibration regularly with a quality-control standard.

Let:

• = potassium concentration of the measured solution in mg L
• = total digest volume in L
• = dilution factor
• = mass of sample in g

Then:

Potassium may be reported as potassium oxide. The conversion is:

because:

For example, if the manure contains K:

All dilution and aliquot factors must be included, and the result should specify whether it is expressed on an as-received or dry-matter basis.

• The manure is digested so that its sulphur is converted into soluble sulphate.
• Under controlled acidic conditions, sulphate reacts with barium ions supplied by barium chloride:

• The fine barium sulphate suspension produces turbidity.
• A stabilizing or conditioning reagent may be used to obtain uniform particle formation and prevent rapid settling.
• Turbidity is measured with a spectrophotometer or turbidimeter at the wavelength specified by the method.
• Sulphur concentration is determined from a calibration curve prepared using known sulphate standards.
• The sample and standards must have similar acidity, ionic strength, mixing time and standing time because these variables affect precipitate formation.

Digestion:

1. Weigh a homogeneous manure sample into a suitable digestion vessel.
2. Add an oxidizing acid mixture that does not introduce sulphate contamination.
3. Heat under controlled conditions until organic matter is completely oxidized and sulphur is converted to sulphate.
4. Cool, filter if required and dilute the digest to a known volume.

Turbidity development:

1. Transfer a suitable aliquot of the digest into a flask.
2. Adjust acidity and add the prescribed conditioning reagent.
3. Add barium chloride while mixing at a consistent rate.
4. Allow turbidity to develop for the specified time.
5. Measure the turbidity or absorbance against a reagent blank.
6. Treat sulphate standards in exactly the same way and construct a calibration curve.
7. Calculate total sulphur using the measured concentration, digest volume, dilution factor and sample mass.

Reagents, water and glassware must be checked for sulphate contamination.

Factors affecting accuracy:

• Incomplete digestion fails to convert all organic sulphur into sulphate.
• Sulphate contamination from reagents, water, glassware or digestion acids gives high results.
• Differences in acidity and ionic strength alter nucleation and particle growth.
• Unequal barium chloride addition, mixing or standing time changes turbidity.
• Rapid settling of barium sulphate causes unstable readings.
• Natural color or suspended particles in the digest may add to apparent absorbance.
• Sulphate concentrations outside the calibration range can give non-linear responses.

Quality-control measures:

• Use a reagent blank, duplicate, spike and reference material where available.
• Apply a sample-color correction when required by the method.
• Keep reagent volumes, mixing and timing identical for samples and standards.
• Measure all solutions within the validated concentration and time range.

Let:

• = sulphur concentration obtained from the calibration curve in mg L
• = total volume of digest in L
• = dilution or aliquot factor
• = sample mass in g

The percentage of sulphur is:

If the calibration curve gives sulphate concentration instead of sulphur concentration, sulphate must first be converted to sulphur:

To express sulphur as sulphur trioxide:

because:

The calculation must include blank correction and any dry-matter correction required for reporting.

• EDTA forms stable chelate complexes with calcium and magnesium ions:

where represents or and represents EDTA.

• For combined calcium and magnesium, the solution is buffered to approximately pH 10 and titrated with standard EDTA using Eriochrome Black T or a similar indicator.
• The endpoint changes from wine red to blue when EDTA has complexed the metal ions.
• Calcium may be determined separately at a higher pH using a calcium-selective indicator, such as murexide, under conditions where magnesium is precipitated as magnesium hydroxide.
• Magnesium is commonly calculated by subtracting calcium from the combined calcium-plus-magnesium result on a molar or equivalent basis.

Preparation of digest:

1. Digest or ash a known mass of manure and dissolve the mineral residue in dilute acid.
2. Filter if necessary and dilute to a known volume.

Calcium plus magnesium:

1. Pipette an aliquot of digest into a titration flask.
2. Add a pH 10 buffer.
3. Add Eriochrome Black T indicator.
4. Titrate with standardized EDTA from wine red to a clear blue endpoint.

Calcium alone:

1. Pipette a second equal aliquot.
2. Raise the pH to approximately 12 so that magnesium precipitates as .
3. Add a calcium-selective indicator such as murexide.
4. Titrate calcium with standardized EDTA to the prescribed endpoint.

Magnesium:

• Calculate magnesium from the difference between the moles of EDTA used for calcium plus magnesium and those used for calcium.
• Apply blank, aliquot, digest-volume and sample-mass factors before reporting the percentages.

EDTA titration:

• Uses complex formation and a visual indicator endpoint.
• Requires relatively simple and inexpensive equipment.
• Is suitable for moderate or high concentrations.
• Can be affected by colored digests, indistinct endpoints and other metal ions that react with EDTA.
• Magnesium is often obtained by difference, increasing combined uncertainty.

Atomic absorption spectrometry:

• Measures absorption of element-specific radiation by free atoms.
• Determines calcium and magnesium individually with greater sensitivity and selectivity.
• Requires calibration standards, a suitable flame and more expensive instrumentation.
• Can be affected by chemical, ionization and matrix interferences.
• Releasing or protective agents, such as lanthanum salts, may be used where phosphate interference affects calcium.

The selected method should be validated for the manure matrix and expected concentration range.

Let:

• = blank-corrected EDTA volume for calcium plus magnesium in mL
• = blank-corrected EDTA volume for calcium alone in mL
• = molarity of EDTA in mol L
• = total dilution or aliquot factor
• = sample mass in g

Because EDTA reacts with each metal ion in a molar ratio, calcium mass in the original sample is:

Thus:

The EDTA volume corresponding to magnesium is . Therefore:

and:

These expressions assume equal aliquots for both titrations. Different aliquot volumes must be normalized before subtraction.