DETERMINATION OF NITRATE IN RAW AND PORTABLE WATER

NOTE: TEST 2 date : 8,November, 2007

PRINCIPLE.

Nitrate reacts with sodium salicylate under strongly acidic conditions. Addition of excess sodium hydroxide solution produces the sodium salt of the organic nitro complex. This nitro compound is soluble in water and produces a strong yellow solution. The intensity of the color is proportional to the amount of nitrate in the sample and is measured at 410 nm on a spectrophotometer. A calibration graph relates absorbance to nitrate concentration.

RANGE

0 – 80 mg/l using a 5 ml aliquot of sample. The range may be extended by taking a smaller sample volume or by diluting the sample.

REAGENTS

Sodium salicylate (0.5 % w/v).
Dissolve 0.5  0.01g of laboratory reagent grade sodium salicylate in 100 ml of distilled water. Store in a cool place but prepare every 10 days.

Sodium hydroxide (25 % w/v).
Dissolve 25  0.1g of sodium hydroxide (NaOH) in 70 ml of distilled water. When cool, dilute to 100 ml with distilled water

Sulphuric acid (Concentrated). Analar grade S.G. 1.84

Standard Nitrate (stock) solution 1 ml = 500 g NO3.
Dissolve 0.815  0.001g of Analar potassium nitrate previously dried at 110oC in approximately 100 ml of distilled water. Quantitatively transfer to a 1000 ml volumetric flask and dilute to the mark with distilled water. Mix well.

Standard Nitrate (working) solution (A), 1 ml = 50 g NO3.
Pipette 10.0 ml of standard nitrate (stock) solution into a 100 ml volumetric flask, dilute to the mark with distilled water. Mix well.

Standard Nitrate (working) solution (B), 1 ml = 5 g NO3.
Pipette 10.0 ml of standard nitrate working solution ‘A’ into 100 ml volumetric flasks, dilute to the mark with distilled water. Mix well.

APPARATUS

Water bath
UV/visible range spectrophotometer.
General laboratory glassware.

Calibration procedure

Into a series of 100 ml pyrex beakers, dispense using a micro-burette the volume of standard nitrate working solutions shown in the table below;

Vol. of standard nitrate (working solution in ml) wt NO3 (g) Measure in the following size cell (mm)
SOLUTION (B)
0.00 0 (Blank) 40
1.00 5 40
2.00 10 40
5.00 25 40
10.00 50 40

SOLUTION (A)
2.00 100 10
4.00 200 10
6.00 300 10
8.00 400 10

Continue as under procedure 6.2 to 6.7

For each size cell, construct a calibration graph relating absorbance to weight of nitrate.

PROCEDURE

Pipette 5.00 ml of sample into 100 ml pyrex beaker. If more than 80 mg/l nitrate is expected, pipette a smaller aliquot or dilute sample accordingly. Note sample volume (v) used and dilution (d).

Add using a pipette, 2.0 ml of sodium salicylate solution (0.5% w/v). Mix well and evaporate to dryness on a water bath.

Remove from water bath and add using a pipette and safety pipette filler 1.0 ml of concentrated sulphuric acid. Mix well and allow to stand for 10 minutes.

Dilute to approximately 50 ml with distilled water.

Add using a measuring cylinder 10 ml of sodium hydroxide solution (25 % w/v). Mix thoroughly.

When cool, quantitatively transfer the solution to a 100 ml volumetric flask and dilute to the mark with distilled water. Stopper and mix well.
Measure the absorbance on a spectrophotometer against the blank (100 % transmission) in either a 10 mm cell or 40 mm cell at a wavelength of 410 nm: note the reading

If it is thought that a significant error may be introduced by color or turbidity in the sample, a duplicate sample should be treated exactly as under 6.1 to 6.7 but omitting the addition of the sodium salicylate solution, Step 6.2. Subtract the absorbance of this solution from that obtained for the sample to which salicylate solution has been added.
CALCULATION

Select the calibration graph for the size of cell used and read- off the weight of nitrate present in the solution.

Nitrate content

g NO3 * d/ v mg/l

Where v = sample volume (ml) used and
d = dilution where appropriate.


For nitrate concentrations up to 10 mg/l, report to nearest 0.1 mg/l, above 10 mg/l, report to nearest mg/l.