WATER QUALITY: Analytical Methods - Determination of Ammonia Nitrogen (Method 1523-73), Electrode Procedure December 12, 1973 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 74.07 Subject: WATER QUALITY: Analytical Methods - Determination of Ammonia Nitrogen (Method 1523-73), Electrode Procedure Attached is a provisional method for the determiniation of dissolved ammonia (ammonium ion) by a gas-detecting electrode (Orion Model No. 95-10). The method should be of use to those Districts where on the spot ammonia determinations are necessary. The electrode should not be used in place of laboratory analyses, and data collected by the ammonia electrode method should not be entered into the WRD Water-Quality Data File. All comments pertaining to the method or the electrode should be addressed to the Chief, Quality of Water Branch (Mail Stop 412, Reston, Va.). The method will retain provisional status until Division personnel have become familiarized with the procedure and until reliable laboratory precision data can be obtained. J. K. Culbertson Acting Chief, Quality of Water Branch Attachment WRD Distribution: A, FO-LS, PO Water Nitrogen, ammonia (1523-73) Ammonia-electrode method 1. Summary of method 1.1 Ammonia is determined potentiometrically in an alkaline medium by using an ammonia gas-detecting electrode. The internal solution of the electrode is separated from the sample solution by a gas- permeable membrane. Dissolved ammonia in the sample diffuses through the membrane until the partial pressure of ammonia is the same on both sides. The change in partial pressure is proportional to the ammonia concentration and is measured as a potential with an internal chloride-sensing reference electrode and a glass electrode. 2. Application 2.1 This method may be used to determine ammonia in fresh waters and brines. Samples containing at least 0.10 mg/l ammonia nitrogen may be analyzed directly. Samples containing less than 0.10 mg/l ammonia nitrogen must be analyzed by the single standard addition technique (paragraph 6.6). 3. Interferences 3.1 Color and turbidity do not affect the measurements. Significant quantities of inorganic cations and anions cannot penetrate the non-wetable gas-permeable membrane and do not interfere directly. However, as the salinity of the sample increases, there is an increase in the apparent ammonia concentration. Samples containing dissolved substances at a total concentration greater than lM should be analyzed either by standard addition, or, if the ammonia concentration is sufficiently high, after appropriate dilution. 2 Nitrogen, ammonia 3.2 Other gases do present a potential interference; however, common gases such as C02, HCN, SO2, and Cl2, do not interfere (Orion Instruction Manual, 1971). 4. Apparatus 4.1 Ammonia electrode, Orion Model No. 95-10, or equivalent. 4.2 pH/mv meter, with expanded scale. 4.3 Stirrer, magnetic, battery operated, with teflon-coated stirring bar. 5. Reagents 5.1 Ammonium chloride standard solution I, 1.00 ml = 1.00 mg N: Dissolve 3.819 g ammonium chloride (NH4Cl), dried overnight over sulfuric acid, in ammonia-free water and dilute to 1,000 ml. 5.2 Ammonium chloride standard solution II, 1.00 ml = 0.010 mg N: Dilute 10.0 ml of ammonium chloride standard solution I to 1,000 ml with ammonia-free water. Prepare fresh daily. 5.3 Ammonium chloride.standard working solutions: Prepare a series of three standard solutions containing 0.1, 1.0, and 5.0 mg/l NH3-N by appropriate dilution of either ammonium chloride standard solution I or II with ammonia-free water. 5.4 Sodium hydroxide solution, lON. Dissolve 400 g sodium hydroxide (NaOH) in ammonia-free water and dilute to 1 liter. 6. Procedure 6.1 Adjust the ph/mv meter according to manufacturer's instructions. 6.2 Pipet 50.0 ml each of ammonium chloride standard working solutions (0.1, 1.0, and 5.0 mg/l NH3-N respectively) into lOO-ml beakers. 6.3 Pipet 50.0 ml of sample into a lOO-ml beaker. 6.4 Place cach standard and sample consecutively on a magnetic Nitrogen, ammonia 3 stirrer (Note 1), immerse the electrode, and then add 0.5 ml lON NaOH solution. Start the stirrer and record the potential (mv) reading after it has stabilized (2 to 5 min.) between samples, rinse the electrode thoroughly with distilled water and blot with a damp tissue. NOTE 1. The top of the stirrer must be insulated with an asbestos sheet and an air space to avoid raising the temperature of the solution. 6.5 If the mv reading from the analytical curve (paragraph 7.1) indicates a concentration of less than 0.10 mg/l, determine NH3-N in the sample by single standard addition (paragraph 6.6). 6.6 Add 0.5 ml of ammonium chloride standard solution II to the sample in paragraph 6.4 (equivalent to an increase in concentration of 0.10 mg/l NH3-N), and record the new potential. Approximately 5 minutes is required for the potential to stabilize. 7. Calculations 7.1 Construct an analytical curve of potential (mv) versus concentration of standards on semilog paper, with concentrations plotted on the logarithmic axis. Determine the mg/l NH3-N ln each sample from the analytical curve. If standard addition is used to determine NH3-N (paragraph 6.6), subtract 0.10 mg/l NH3-N from the concentration obtained from the analytical curve. 8. Report Report ammonia-nitrogen concentrations as follows: Less than 1.0 mg/l, two decimals; 1.0 mg/l and above, two significant figures. References Orion Research Inc., Cambridge, Mass., 1971, Instruction Manual, Ammonia Electrode, Model 95-10.