General
Nitrogen exists in four viz. 1) Ammonia nitrogen, 2) Organic nitrogen, 3) Nitrite nitrogen & 4) Nitrate nitrogen. All these forms are important to environmental engineers. Ammonia nitrogen is that which exist as ammonium ion or in the equilibrium stated below
NH4 + 0 NH3 + H+
This type of nitrogen can be determined either by Nesslerzation for small concentration of NH3, by distillation for high concentration of NH3.
Since Nesslerization procedure is subjected serious errors from extraneous colour and turbidity, distillation procedure is generally adopted.
Ammonium ion exists in equilibrium with ammonia and hydrogen ion as shown above in the equation at pH level above 7. If the pH is too alkaline there is every possibility of release of nitrogen from organic sources at boiling temperatures. Hence, a buffer is added to maintain the pH at 7.5.
A) Distillation Method
Reagents
1. Phosphate buffer: Dissolve 14.3g KH2PO4 and 90.2g of K2HPO4 in ammonia free water and make upto 1L.
2. Ammonia free water: Prepare by re-distillation of water after theaddition of a few drops of diluted sulphuric acid.
3. Boric acid: Dissolve 20g H2BO3 in distilled water, add 10ml mixed indicator and dilute to 1000ml.
4. Mixed indicator: Dissolve 200mg methyl red in 100ml 95% ethylalcohol, 100mg methylene blue in 50 ml 0.5% ethyl alcohol. Mix the two solutions.
Procedure
Distill 100ml of solution and 100ml distilled water at pH 7.5 and collect the distillate in 50ml boric acid (indicating mixture) till the solution in the conical flask becomes triple in volume i.e. 150 ml.
See the colour changes in the indication mixture. Original purple colour will be converted to green indicating NH3 is reacting with the boric acid. Blue coloured indicating mixture (Boric acid) is than titrated with standard 0.02 N sulphuric acid till it turns red. Purple colour indicates the reaction of NH3 with sulphuric acid.
Calculation
100ml 0.02N H2SO4 = 0.28 mg Nitrogen
ml of 0.02 N H2SO4 x 0.28 x 1000 /
ml of sample distilled
= mg/L NH3-N
B) Nesslerization Method
Principle
Ammonia produces a yellow coloured compound when reacted with alkaline Nessler reagent provided sample is clarified properly. Pretreatment with ZnSO4 and NaOH precipitates Ca, Fe, Mg and sulfides and removes turbidity and apparent colour.
Addition of EDTA (Before reagent) or Rochelle salt solution prevents precipitation of residual Ca and Mg in the presence of alkaline Nessler reagent.
Interferences
Colour, turbidity, Ca, Mg, salts and Fe in the sample constitute the prime sources of interferences.
Apparatus
1. Spectrophotometer having range of 300 to 700nm.
2. Nessler’s tube or 100ml capacity volumetric flasks.
Reagents
1. Zink sulfate: Dissolve 10g ZnSO4 7H2O in distilled water and dilute to 100ml.
2. Sodium hydroxide: Dissolve 24g NaOH in distilled and dilute to 100ml (6N)
3. EDTA reagent: Dissolve 50g EDTA in 60ml distilled water containing 10g NaOH, cool and dilute to 100ml.
4. Rochelle salt solution: Dissolve 50g potassium sodium tartarate tetrahydrate in 100ml distilled water. Remove ammonia by boiling off 30ml solution. Cool and dilute to 100ml
5. Nessler reagent: Mix well 100g HgI2 and 70g KI Dissolve in small quantity of distilled water. Add this mixture to a cooled solution of 160g NaOH in 500ml water. Dilute to 1000ml. Keep overnight store supernatant in coloured bottle.
6. Standard ammonia solution: Dissolve 3.819g NH4Cl dried at 100oC in distilled water and dilute to 100ml. Dilute 10ml of the solution to 1000ml1ml = 10g N or 12.2 g NH3
Procedure
1. Take 100ml of sample. Add 1ml ZnSO4 solution and 0.4 to 0.5 ml NaOH to obtain a pH of 10.5. Allow settling. Filter the supernatant thought Whatman filter paper No. 42.
2. Take a suitable aliquot of sample and dilute to 50ml
3. Add 3 drops of Rochelle salt solution or 1 drop of EDTA, mix well.
4. Add 3ml Nessler reagent if EDTA is used or 1 ml if Rochelle salt solution is used. Make to 100ml.
5. Mix well and read percentage transmission after 10 min at 410nm, using a blank prepared in the same way by taking distilled water instead of sample.
6. Prepare a calibration curve using suitable aliquots of standard solution in the range of 5 to 120 g/100ml for reference following the same procedure as 1 to 5 above using the standard solution in place of sample.
REACTION
1. 2K2HgI4 + NH3 + 3KOH (NH3Hg2IO) + 7KI +2H2O
Yellow brown coloured complex