DETREMINATION OF TOTAL ORGANIC CARBON IN SEDIMRNTS
Determination of organic carbon was of considerable value in assessing
organic pollution for the investigation of where other methods of estimating
oxygen demand tend to be unreliable on account of the presence of high salt
concentrations.
Titrimetric method was used to determine the total organic carbon in the
sediment sample. The principle was similar to what obtains with the COD
determination. The only difference was that sediments were being oxidized here
and that no reflux was carried out.
10 g of the sediment sample was dried to constant weight and then grinded
to fine powder in a mortar. 0.2 g of the fine sample was weighed into 500ml flask.
10 ml 0.5 M K2Cr2O7 was added and spined gently. 20 ml conc. H2SO4 was then
added rapidly but avoiding splashing and directing the acid into suspension and
immediately spined gently until the reagents were mixed but becoming vigorous
later and this was done for 1 min. The flask was allowed to stand for 30 min.
200 ml distilled water was added and then 10 ml conc. H3PO4 cautiously
to avoid splashing before cooling. 3 drops of ferroin indicator solution was later
added and titrated to wine-red colour end point with 0.25 M iron (II) ammonium
sulphate (The colour changed from blue to deep-green and then to wine –red as
the final end point). A blank determination was carried out as above but without
the sample. The iron (II) ammonium sulphate titre reading was noted.
Calculation
mg total organic carbon/g sample = (Vb-Vs) X M X 16000
wt. of sample in g.
Where Vb = ml iron (II) ammonium sulphate used for blank.
VS = ml iron (II) ammonium sulphate used for sample.
M = Molarity of iron (II) ammonium sulphate
TOTAL PHOSPHATES IN SEDIMENTS
About 10 g of the sediment sample was grinded to constant weight and
then to fine powder in a mortar. 2.0 g of the powder sample was weighed into 250
ml clean beaker and 25 ml conc. HNO3 was added for pre-oxidation, it was then
covered with a sizeable glass and spined to mix well. The mixture was set on a
hot plate regulated to 200 0C and placed in a fume chamber, and heated for 30
min. When all the brown fumes produced initially had ceased, the beaker was set
down from the hot plate, covered with glass and continue the digestion for about
90 min.
Thick white fumes formed initially were subsiding as the digestion
progressed and finally left colourless residue behind. All the organic and
inorganic phosphate was then converted to the orthophosphate. The beaker was
then set down and cooled very well. Some distilled water was added to the
residue, stirred carefully and neutralized with 6 M NaOH added dropwisely. The
solution is filtered through whatman filter paper No. 42 into 50 ml volumetric
flask. The residue was washed into the filter paper with distilled water. Then the
filtrate was made up to 50 ml mark and shaked.
About 20 ml of the filtrated solution was transferred with the aid of pipette into
another 50 ml volumetric flask; noting the dilution factor the absorbance was
measured against a blank at the appropriate wavelength.
CYANIDE
Cyanide occurs in dangerous concentrations in wastewaters from electroplating,
gold mining, case-hardening of steel, gas from cassava industry will contain
HCN.
Cyanides are known to be toxic to animal life and humans.
Titration is adopted to determine cyanides.
Procedure
50ml 1N NaOH was poured into the gas washer and diluted with distilled water so
that the solution covers the spiral. the system is then connected to a water
aspirator and the suction adjusted to maintain an air-flow rate through the unit
just below the point of bubble coalescence in the spiral. About 1 bubble per
second was satisfactory.
The sample was put into the flask. In case of portable water meeting the WHO
drinking water standards for CN, a sample volume of 500ml was took.In case of
wastewater that may contain more than 10mg/1CN, a known aliquot diluted to
250ml was used and the dilution factor noted.
Through the air- inlet thistle tube, 20ml HgCl2 solution was added and 10ml
MgCl2 solution .the tube was washed with distilled water , and the air flow
allowed to mix the flask contents for 3 minute. 5ml conc. H2SO4 was slowly and
carefully introduced into the distilling flask. wash the air-inlet tube was washed
with distilled water again. the flask was heat cautiously to secure good reflux.
After refluxing for 1 hour, the heater was turned off but the air flow was
maintained for an additional 15 minutes. the pinch clamp was opened to drain the
gas washer contents into a 250ml volumetric flask and diluteed to the mark by
washing the scrubber. the gas washer was refilled with another 50ml 1M NaOH.
the reflux-distillation was repeated on the same sample for 1 hour. the CN
concentration in both distillates was determined first by titration to ascertain the
magnitude.
Titration
an aliquot of the distillate was taken; adjusted to pH 11, if necessary, with 1M
NaOH. The aliquot is diluted to 250ml or some other convenient volume to be
used for all titrations. 0.5ml indicator solution was added, Titrated with standard
silver nitrate titrant to the first colour from yellow to salmon (orange-pink). a
blank containing the same amount of alkali and water was also titrated.
N.B.
(1)
It is advisable to adjust the sample volume of the strength of
AgNO3, solution needed will not be more than 10ml.
(2)
If the sample is not distilled, a rough estimate of the CN may be
obtained by adjusting the pH to 11 above with NaOH and titrate as given above.
Calculation
CN (mg/l) = (A-B) X 1,000
ml. original sample
X
ml. aliquot
Where A= ml.standard AgNO3 for aliquot,
B= ml. standard AgNO3 for blank
250