Analysis

the condenser was 2cm above the surface of the H3BO3 solution. The distillation flask was attached to the distillation apparatus. Then about 50mL of 40% NaOH solution was poured though the distillation flask through the opened stopcork. Distillation was immediately commenced and continued until 50mL distillate was collected. NH4-boric solution was titrated with 0.01M H2SO4 using micro burette. The green colour changed to pinkish blue marking the end point. A blank sample (distilled water) was ran for distillation and titration (Keeney and Nelson, 1982).

Calculations:

Milliequivalent of NH4 in 10ml of extract = (T-B) x 2M (3.22) Where T = sample titration

B = blank titration M = molarity

3.7 Determination of Heavy Metals Concentration in Samples

Instruments used for analysis:

Determination of heavy metals contents of each sample solution and the blank were analyzed for the extractable heavy metals using Atomic Absorption Spectrophotometer (AAS) Unican 939/959 model.

3.7.2 Preparation of working standard from stock solutions

Before determination of the heavy metal concentration in each sample solution, a calibration curve of heavy metal was prepared using aliquots from standard stock solution (Analar grade) of metal ions Fe²⁺, Pb²⁺, Cu²⁺, Cr²⁺, Co²⁺, Cd²⁺, Ni²⁺, Mn²⁺, Zn²⁺, V²⁺ and Hg²⁺. This was obtained by using metals salts from a mixture of commercially available 100mg/kg stock solutions. The preparation procedures for each metallic element are as described below:

Iron (Fe) 1000 ppm

1g of Iron II ammonium sulphate was dissolved in 20mL of 5M HCl and 5 mL of HNO3. The mixture was heated until completely dissolved. It was filtered and the volume made up to 1l with distilled water.

Cobalt (CO) 1000ppm

2.2032g of anhydrous cobalt (II) chloride was dissolved in distilled water to a volume of 1 litre.

Manganese (Mn) 1000ppm

1.8g of potassium tetraoxomanganate was dissolved in a 450ml distilled water in 1l conical flask and was heated to 4-5 hrs at 70 - 80C.

thereafter it was filtered hot through glass-fire into a 500 ml volumetric flask

previously washed thoroughly. When cooled 2ml conc. H2SO4 was added and finally made up to 500ml mark with distilled water.

Zinc (Zn) 1000ppm

A clean 100mg 30 – mesh zinc metal was dissolved in a slight excess of either Hydroiodic acid and then diluted to 1 litre with distilled water.

Chromium (Cr) 1000ppm

Approximately 1.0g of Cr metal was weighed and dissolved in 50mL of concentrated HNO3 acid. This was diluted to one litre in a volumetric flask with distilled water and a standard solutions of 0.05, 0.10, 0.15 and 0.20 ppm were prepared from the stock solution.

Cadmium (Cd) 1000ppm

1.0g of Cd metal was dissolved in 2mL of 5M HCl acid and 2 drops of concentrated HNO3 acid. This was diluted to one litre in a volumetric flask with distilled water. From the stock solution a standard solution of 0.5, 0.10, 0.20 ppm was prepared and a calibration curve was also prepared.

(Ademoroti, 1996/AOAC (1980).

Copper (Cu) 1000ppm

Accurately weighed 1.0g of copper metal was dissolved in 50mL of 5M HNO3 acid. This was diluted to one litre with distilled water. (Ademoroti, 1996/AOAC (1980).

Vanadium (V) 1000ppm

Vanadium metal (1.0g) was dissolved in 25mL of concentrated HNO3

acid and diluted to one litre in a volumetric flask with distilled water. From the stock solution a standard solution was prepared at different dilution.

(Ademoroti, 1996/AOAC (1980).

Magnesium (Mg) 1000ppm

1g magnesium ribbon was dissolved in 20mL 5MHCl. The solution was made up to 1 litre. (Ademoroti, 1996/AOAC (1980).

3.7.3 Principles of AAS

AAS spectrophotometer Unicam 939/959 model was installed on a level and stable platform. The burner was placed under a vent and the correct hollow cathode lamp for each heavy metal (Fe, Pb, Cu, Co, Cr, Cd, Mn, Zn, V and Hg) was chosen, installed and aligned in the instrument. Monochromator at the correct instrument wavelengths as specified by the manufacturers for each element were set as Fe (249.5nm), Pb (281nm), Cu (323 nm), Co (206.5nm), Cr (353nm), Cd(258.8nm), Ni (232nm), Mn(278 nm), Z(212nm) and V (246nm) and Hg (315mm). The manufacturer’s instruction was also followed for setting instrument detection limit (IDL) between 0.002 – 0.01mg/kg for selection of the instrument slit and setting of light source current. An aliquot of standard stock solution of desired element was then plugged into the instrument curvette for straight reading. This was to reduce detrimental effects of overlapping spectral interferences on element

quantification during metal analysis. The flame was lighted with fuel and oxidant (air-Acetylene) regulated while the burner was adjusted for stability and maximum absorption. Photometer was balanced and the standards for working curves were run.

The intensity of light was caused by the presence of the free unexcited atoms. This occurred when the light beam was directed through the flame into monochromator and then onto detector from which the light intensity absorbed was measured. The measured light intensity absorbed was proportional to the concentration of the required element in any of the sample (water, soil, sediment, fish). The concentrations of standards were plotted against the absorbance. This gave the calibration curve.

The concentration of each heavy metal in each sample was determined by measuring the absorbance and extrapolated from the calibration curve.

Duplicate and blanks were employed to test precision, accuracy and reagent purity used in digestion procedure.

Calculation:

Heavy metal concentration mg/l 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 𝑜𝑓 𝐴𝐴

𝑣𝑜𝑙 (𝑚𝑙)𝑠𝑎𝑚𝑝𝑙𝑒 x ¹⁰⁰

𝑣𝑜𝑙 (𝑚𝑙)𝑎𝑙𝑖𝑞𝑢𝑜𝑡 (3.26) Where: AA is reading of atomic absorption of heavy metal.