Friends at sea, please help!

This post was last edited by zhaolijun on 2016-5-4 at 16:56. Water quality – Determination of ammonia nitrogen – Distillation-neutralization titration method, HJ 537—2009. 1 Scope: This standard specifies the distillation-neutralization titration method for determining ammonia nitrogen in water. This standard is applicable to the determination of ammonia nitrogen in domestic wastewater and industrial wastewater. When the sample volume is 250 ml, the detection limit of the method is 0.05 mg/l (all expressed as N). 2 Method Principle: Adjust the pH of the water sample to 6.0–7.4, add light magnesium oxide to give it a slightly alkaline character, and absorb the ammonia released during distillation using a boric acid solution. Using methyl red-methylene blue as an indicator, an acid standard solution is used to titrate the ammonia nitrogen in the distillate (expressed as N). 3 Interferences and Elimination: Any substance that can be distilled under the conditions specified in this standard and that reacts with acids interferes with the determination. For example, urea, volatile amines, and chloramines in chlorinated samples, etc. 4 Reagents and Materials Unless otherwise specified, the reagents used in the analysis are analytical-grade chemical reagents that meet **standards, and the water used in experiments is the water prepared according to 4.1. 4.1 Without ammonia water, prepare in an ammonia-free environment using one of the following methods (for the check of absence of ammonia water, see 10.1). 4.1.1 Ion exchange method: Distilled water is passed through a column of strongly acidic cation exchange resin (hydrogen type), and the effluent is collected in a glass bottle with a ground glass stopper. Add 10 g of the same resin per liter of the effluent to facilitate storage. 4.1.2 Distillation Method: Add 0.1 ml of sulfuric acid (4.2) to 1,000 ml of distilled water, and redistill in a glass distillation apparatus. Discard the first 50 ml of the distillate, then collect approximately 800 ml of the distillate in a glass bottle equipped with a ground glass stopper. Add 10 g of strongly acidic cation exchange resin (hydrogen type) per liter of distillate. 4.1.3 Purifier method: Prepared directly using a commercially available water purifier. 4.2 Sulfuric acid, ρ(H2SO4) = 1.84 g/ml. 4.3 Salt acid, ρ = 1.19 g/ml. 4.4 Anhydrous ethanol, ρ = 0.79 g/ml. 4.5 Anhydrous sodium carbonate (Na2CO3), primary reagent. 4.6 Light magnesium oxide (MgO), free of carbonates. Heating at 500°C is carried out to remove carbonates. 4.7 Sodium hydroxide solution, c(NaOH)=1 mol/l. Weigh 20 g of sodium hydroxide (NaOH), dissolve it in about 200 ml of water, cool to room temperature, and dilute to 500 ml. 4.8 Sulfuric acid solution, c(1/2H2SO4)=1 mol/l. Measure 2.8 ml of sulfuric acid (4.2) and add it slowly to 100 ml of water. 4.9 Boric acid (H3BO3) absorption solution, ρ = 20 g/L. Weigh 20 g of boric acid and dissolve it in water, then dilute to 1000 ml with methyl red indicator, with a density of 0.5 g/L. Weigh 50 mg of methyl red and dissolve it in 100 ml of ethanol (4.4). 4.11 Bromothymol blue indicator, ρ = 1 g/L. Weigh 0.10 g of bromothymol blue, dissolve it in 50 ml of water, add 20 ml of ethanol (4.4), and dilute to 100 ml with water. 4.12 Mixed indicator: Weigh 200 mg of methyl red and dissolve it in 100 ml of ethanol (4.4) ； Additionally, 100 mg of methylene blue was dissolved in 100 ml of ethanol (4.4). Prepare by mixing two portions of methyl red solution with one portion of methylene blue solution; this solution can remain stable for 1 month. 4.13 Sodium carbonate standard solution, c(1/2Na2CO3)=0.020 0 mol/l. Weigh 0.530 0 g of anhydrous sodium carbonate (4.5), which has been dried at 180°C for 2 hours, dissolve it in freshly boiled and cooled water, transfer the solution to a 500 ml volumetric flask, and dilute to the mark. 4.14 Salt acid standard titration solution C(HCl)=0.02 mol/l. Measure 1.7 ml of salt acid (4.3) and transfer it to a 1,000 ml volumetric flask, then dilute to the mark with water. Calibration method: Transfer 25.00 ml of the sodium carbonate standard solution (4.13) into a 150 ml conical flask, add 25 ml of water, add 1 drop of methyl red indicator (4.10), and titrate with the hydrochloric acid standard solution (4.14) until a pale red color is reached. Record the volume consumed. Calculate the concentration of the hydrochloric acid solution (4.14) using equation (1): C(HCl) = C1 × V1 / V2. Here, C is the concentration of the standard hydrochloric acid titrant solution (4.14), in mol/l ； C1 —— concentration of the sodium carbonate standard solution (4.13), mol/l ； V1 — volume of the sodium carbonate standard solution (4.13), in ml ； V2 —— Volume of the acid standard titration solution (4.14) consumed, in ml. 4.15 Glass beads 4.16 Antifoam agents, such as paraffin fragments. 5 Instruments and Equipment 5.1 Ammonia nitrogen distillation apparatus: It consists of a 500 ml Kjeldahl flask, a nitrogen bulb, a straight condenser tube, and tubes; a dropper of appropriate length can be attached to the end of the condenser tube so that its tip is submerged below the surface of the absorbing solution. A distillation flask can also be used. 5.2 Acidic burette: 50 ml. 6 Samples 6.1 Sample storage: Water samples should be collected in polyethylene or glass bottles and analyzed as soon as possible. To preserve it, sulfuric acid should be added to acidify the water sample to a pH of <2; it can be stored for 7 days at 2–5°C. 6.2 Sample pre-distillation: Transfer 50 ml of the boric acid absorption solution (4.9) into the receiving flask, making sure that the outlet of the condenser is below the level of the boric acid solution. Transfer 250 ml of the water sample into a flask (if the ammonia nitrogen content is high, a smaller amount of sample can be used and water added to bring the volume to 250 ml). Add 2 drops of bromothymol blue indicator (4.11); if necessary, adjust the pH to 6.0 (when the indicator is yellow)–7.4 (when the indicator is blue) using sodium hydroxide solution (4.7) or sulfuric acid solution (4.8). Add 0.25 g of light magnesium oxide (4.6) as well as several glass beads; if needed, add an antifoam agent (4.16). Immediately connect a nitrogen ball and a condenser for heating and distillation, ensuring that the rate of distillation is about 10 ml/min. Stop the distillation once 200 ml of distillate have been collected. 7 Analysis Steps 7.1 Sample Analysis Transfer all the distillate to a conical flask, add 2 drops of the mixed indicator (4.12), and titrate with the hydrochloric acid standard solution (4.14) until the color of the distillate changes from green to light purple; record the volume Vs of the hydrochloric acid standard solution used in the titration. 7.2 Blank test: Replace the water sample with 250 ml of water, perform pre-distillation as described in 6.2, carry out titration as specified in 7.1, and record the volume Vb of the saline standard titrant consumed. 8 Results The concentration of ammonia nitrogen in the water sample is calculated using equation (2): Vs – Vb/V × C × 14.01 × 1000 (2) Where: ρN —— the concentration of ammonia nitrogen in the water sample (expressed in N), mg/l ； V — Volume of the sample, ml ； Vs —— Volume of the salt acid standard titrant solution consumed in titrating the sample, in ml ； Vb —— Volume of the salt acid standard titrant consumed for the titration blank, in ml ； C —— Titrant salt; concentration of the acid standard solution, in mol/l ； 14.01 — Atomic weight of nitrogen, g/mol. Table 1 Accuracy and Precision of Standard and Actual Samples Sample Table 1 Accuracy and Precision of Standard and Actual Samples Ammonia nitrogen content/(mg/l) Repeatability limit r/(mg/l) Reproducibility limit R/(mg/l) Relative error/% Standard sample 1 2.76 0.106 0.146 0.73 Standard sample 2 23.8 0.641 1.39 -0.42 Surface water 6.60 0.109 0.515 — Domestic wastewater 21.4 0.694 3.09 — Note: Validation was carried out by 5 laboratories, with each laboratory performing 6 repeated measurements for each sample. Note: Five laboratories participated in the verification, with each laboratory performing 6 repeated measurements for each sample.