Question 6 (a) The soil of an agricultural plot of land (size: 30 m by 50 m) was

Question

Question 6 (a) The soil of an agricultural plot of land (size: 30 m by 50 m) was suspected to be contaminated by cadmium (Cd) due to intensive use of phosphate fertilisers For these reasons soil samples were collected from this plot of land and analyzed by AAS. A composite random soil sample was processed, then taken into solution and analyzed by furnace AAS Propose a methodology to collect soil samples so as to have a composite random sample that would be representative of the whole agricultural plot of land (i) (i) How are soil samples generally processed before being taken into solution? (ii) Suggest how the composite soil sample could be taken into solution. Describe the different stages (from room temperature to the temperature of analysis) when a sample is analyzed by furnace AAS (iv) (v) (vi) Discuss fully the use of matrix modifiers during furnace analysis Illustrate your answer with examples. Discuss spectral, ionization and chemical interferences in AAS A mass of 1.8036 g of composite soil sample was taken into solution with a final volume of 25.0 cm3. After analysis by furnace AAS, it was found that the 25 cm3 solution contained Cd at a level of 0.15 mg/dm3 Calculate the level of Cd in the composite soil sample. Express your result in ppm. (vii) 2 22 +3 3 +33 marks] (b) Titanium and vanadium form coloured complexes with peroxide, and their concentrations can be determined spectrophotometrically. Standard solutions were prepared and the absorbances measured in a 1.00-cm cell and are given below: Absorbance Concentration (g/dm3) 0.06 0.18 Complex 410 nm 0.912 0.666 460 nm 0.615 0.900 Ti The absorbance of an unknown solution containing both titanium and vanadium complexes was measured in a 1.00-cm cell and found to be 0.566 at 410 nm and 0.503 at 460 nm. Determine the Ti and V contents in the unknown solution 7 marks]

Explanation / Answer

1.methodology

Divide the field into different homogenous units based on the visual observation and farmer’s experience.
Remove the surface litter at the sampling spot.
Drive the auger to a plough depth of 15 cm and draw the soil sample.
Collect at least 10 to 15 samples from each sampling unit and place in a bucket or tray.
If auger is not available, make a ‘V’ shaped cut to a depth of 15 cm in the sampling spot using spade.
Remove thick slices of soil from top to bottom of exposed face of the ‘V’ shaped cut and place in a clean container.

Mix the samples thoroughly and remove foreign materials like roots, stones, pebbles and gravels.
Reduce the bulk to about half to one kilogram by quartering or compartmentalization.
Quartering is done by dividing the thoroughly mixed sample into four equal parts. The two opposite quarters are discarded and the remaining two quarters are remixed and the process repeated until the desired sample size is obtained.
Compartmentalization is done by uniformly spreading the soil over a clean hard surface and dividing into smaller compartments by drawing lines along and across the length and breadth. From each compartment a pinch of soil is collected. This process is repeated till the desired quantity of sample is obtained.
Collect the sample in a clean cloth or polythene bag.
Label the bag with information like name of the farmer, location of the farm, survey number, previous crop grown, present crop, crop to be grown in the next season, date of collection, name of the sampler etc.

Collection of soil samples from a profile

After the profile has been exposed, clean one face of the pit carefully with a spade and note the succession and depth of each horizon.
Prick the surface with a knife or edge of the spade to show up structure, colour and compactness.
Collect samples starting from the bottom most horizon first by holding a large basin at the bottom limit of the horizon while the soil above is loosened by a khurpi.
Mix the sample and transfer to a polythene or cloth bag and label it.

2. Processing and storage

Assign the sample number and enter it in the laboratory soil sample register.
Dry the sample collected from the field in shade by spreading on a clean sheet of paper after breaking the large lumps, if present.
Spread the soil on a paper or polythene sheet on a hard surface and powder the sample by breaking the clods to its ultimate soil particle using a wooden mallet.
Sieve the soil material through 2 mm sieve.
Repeat powdering and sieving until only materials of >2 mm (no soil or clod) are left on the sieve.
Collect the material passing through the sieve and store in a clean glass or plastic container or polythene bag with proper labeling for laboratory analysis.
For the determination of organic matter it is desirable to grind a representative sub sample and sieve it through 0.2 mm sieve.
If the samples are meant for the analysis of micronutrients at-most care is needed in handling the sample to avoid contamination of iron, zinc and copper. Brass sieves should be avoided and it is better to use stainless steel or polythene materials for collection, processing and storage of samples.
Air-drying of soils must be avoided if the samples are to be analyzed for NO3-N and NH4-N as well as for bacterial count.
Field moisture content must be estimated in un-dried sample or to be preserved in a sealed polythene bag immediately after collection.
Estimate the moisture content of sample before every analysis to express the results on dry weight basis

4.)

involved in using atomic absorption spectroscopy (AAS) data to determine the concentration of a species in a solution are:

Step 1: Draw a calibration curve using the concentration and absorbance data for a set of standards.
Step 2: Use the calibration curve and the absorbance of the sample to "read off" the concentration of the species in the sample.
Step 3: If the original sample was diluted before being analysed, use the concentration of the diluted sample obtained from the calibration curve to calculate the concentration of the species in the original undiluted sample.

Atomic Absorption Spectroscopy can be used to measure the concentration of metals in :

mining operations and in the production of alloys as a test for purity
contaminated water, especially heavy metal contamination in industrial waste water
organisms, such as mercury in fish
air, such as lead

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