⚗️ Ammonium nitrogen in water and wastewater — distillation with magnesium oxide into boric acid and titration with hydrochloric acid

Physicochemistry PN-ISO 5664

Overview

ISO 5664:1984 specifies the distillation and titration method for determining ammonium nitrogen in raw water, drinking water and wastewater. Up to 10 mg of ammonium nitrogen can be determined in the test portion — with a portion of 10 ml this corresponds to a concentration of up to 1000 mg/l; the limit of detection determined in practice (4 degrees of freedom) with a portion of 250 ml is 0.2 mg/l; sensitivity: with a portion of 100 ml, 1.0 ml of hydrochloric acid 0.02 mol/l corresponds to 2.8 mg/l N. The main interferences: urea, which under the conditions of the determination distils as ammonia and gives a high result, volatile amines that distil and react with the acid, and chloramines in chlorinated waters (determined together with the ammonia). The 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) lists PN-ISO 5664 (the volumetric, titrimetric method) among the reference methods for ammonium nitrogen in wastewater. In Poland the standard is in force as PN-ISO 5664:2002.

The same principle — distilling off the ammonia from an alkaline solution into boric acid with a mixed indicator and titrating with standardised hydrochloric acid back to the initial colour — is used in the determination of nitrogen by the Kjeldahl method after digestion (the laboratory course of Politechnika Gdańska [Gdańsk University of Technology]: steam distillation after the addition of alkali, a receiver with 25 cm³ of saturated boric acid with methyl red and bromocresol green, titration with 0.01 M HCl). The result can be expressed as nitrogen (N), as ammonia (NH₃) or as the ammonium ion (NH₄⁺) in mg/l, or as a molar concentration; 1 mg/l NH₄⁺ corresponds to 0.777 mg/l N (1 mg/l N = 1.216 mg/l NH₃ = 1.288 mg/l NH₄⁺ = 71.4 µmol/l).

The year in the national number tells when the document entered the PN collection, not how old the method is: PN-ISO 5664:2002 adopts ISO 5664:1984 — eighteen years lie between them.

Method principle

The pH of the test portion is brought into the range 6.0–7.4, magnesium oxide is added to obtain slightly alkaline conditions, the liberated ammonia is distilled and collected in a receiver containing boric acid solution; the ammonium ion in the distillate is titrated with a standardised acid solution against the indicator contained in the boric acid solution. Reagents: ammonia-free water (from a column of strongly acidic cation exchanger in the H form, with about 10 g of resin per litre added for storage, or redistilled with 0.10 ml of H₂SO₄ per 1000 ml, the first 50 ml being discarded); standardised hydrochloric acid 0.10 mol/l and 0.02 mol/l; boric acid solution with an indicator (20 g of H₃BO₃ in warm water, 10 ml of a 0.5 g/l methyl red solution and 20 ml of a 1.5 g/l methylene blue solution, made up to 1 l); bromothymol blue 0.5 g/l; hydrochloric acid 1 % (V/V) and sodium hydroxide 1 mol/l for the pH adjustment; light magnesium oxide free of carbonates (ignited at 500 °C); anti-bumping granules; an antifoaming agent (e.g. paraffin shavings) for certain types of wastewater. Apparatus: a distillation assembly with a flask of 800–1000 ml, a splash head and a vertical condenser whose outlet is immersed in the absorbing solution; before use after a longer break the apparatus is cleaned by distilling about 350 ml of ammonia-free water (at least 100 ml of distillate is discarded). Samples are taken into polyethylene or glass bottles and are analysed as soon as possible, being stored at 2–5 °C; they may be preserved with sulfuric acid to pH < 2, avoiding the absorption of ammonia from the air. The volume of the portion is chosen according to the table: up to 10 mg/l N — 250 ml, 10–20 mg/l — 100 ml, 20–50 mg/l — 50 ml, 50–100 mg/l — 25 ml (with titration by HCl 0.10 mol/l). Result: ρN = (V₁ − V₂)/V₀ · c · 14.01 · 1000, where V₀ is the volume of the portion (ml), V₁ and V₂ the volumes of HCl used for the sample and for the blank (ml) and c the concentration of the HCl (mol/l).

Applications

• Raw and treated wastewater — the reference method for ammonium nitrogen (titrimetric method) under Dz.U. 2019 poz. 1311
• Raw water and drinking water — range from 0.2 mg/l N (portion of 250 ml)
• Coloured, turbid or saline samples, for which the spectrophotometric methods require prior distillation
• High concentrations (up to 1000 mg/l N with a portion of 10 ml) — industrial wastewater, leachate
• Control of nitrification and of ammonia stripping in treatment plants

Key parameters

Standard

Standard number

PN-ISO 5664:2002 (ISO 5664:1984)

Title (PL)

Jakość wody — Oznaczanie azotu amonowego — Metoda destylacyjna z miareczkowaniem

Title (EN)

Water quality — Determination of ammonium — Distillation and titration method

Step-by-step procedure

1. Sampling and storage

   Take the sample into a polyethylene or glass bottle; analyse it as soon as possible or store it at 2–5 °C (it may be preserved with H₂SO₄ to pH < 2, protected from ammonia in the air).

2. Preparation of the apparatus

   After a longer break clean the assembly: distil about 350 ml of ammonia-free water with granules, discard at least 100 ml of the distillate and the residue.

3. Receiver and test portion

   Pour 50 ± 5 ml of boric acid solution with the indicator into the receiver and immerse the tip of the condenser below the surface. Measure out the portion according to the table (250 ml up to 10 mg/l N; 100 ml for 10–20; 50 ml for 20–50; 25 ml for 50–100 mg/l) into the distillation flask; where chlorine is present add a few crystals of sodium thiosulfate.

4. Adjustment of the pH

   Add a few drops of bromothymol blue and, if necessary, bring the pH to between 6.0 (yellow) and 7.4 (blue) with NaOH 1 mol/l or HCl 1 %; make up with ammonia-free water to about 350 ml.

5. Distillation

   Add 0.25 ± 0.05 g of light magnesium oxide and a few granules (an antifoaming agent for wastewater that foams) and connect the flask at once. Heat so that the distillate collects at about 10 ml/min; stop once about 200 ml has been collected.

6. Titration

   Titrate the distillate with hydrochloric acid 0.02 mol/l (0.10 mol/l at high concentrations) to a purple end point; record the volume.

7. Blank and calculation

   Run a blank with about 250 ml of ammonia-free water. Calculate ρN = (V₁ − V₂)/V₀ · c · 14.01 · 1000 [mg/l]; convert if necessary into NH₃ or NH₄⁺ (1 mg/l N = 1.216 mg/l NH₃ = 1.288 mg/l NH₄⁺).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The hot distillation assembly and the foaming of wastewater — shields, control of the heating, boiling granules
• Hydrochloric acid and sodium hydroxide — goggles, gloves
• Ammonia in the vapours if the assembly is not tight — work in a fume hood

Frequently asked questions