NH3-selective electrodes Manual 8.109.
Metrohm AG CH-9100 Herisau Switzerland Phone +41 71 353 85 85 Fax +41 71 353 89 01 info@metrohm.com www.metrohm.com NH3-selective electrodes Manual 8.109.8031EN 11.
Teachware Metrohm AG CH-9100 Herisau teachware@metrohm.com This documentation is protected by copyright. All rights reserved. Although all the information given in this documentation has been checked with great care, errors cannot be entirely excluded. Should you notice any mistakes please send us your comments using the address given above. Documentation in additional languages can be found on http://documents.metrohm.com.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ Table of contents Table of contents 1 Introduction 1 2 Mode of operation 2 3 Selecting the electrode 4 4 Assembling the electrode 5 5 Storing the electrode 8 6 Performing measurements 9 6.1 Measured quantities ............................................................. 9 6.2 General notes ........................................................................ 9 6.3 Performing direct measurements ..................................... 10 6.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 1 Introduction 1 Introduction The NH3-selective gas membrane electrodes are combined electrodes, i.e. they contain one measuring electrode and one reference electrode. They enable the rapid, simple, inexpensive and precise determination of dissolved ammonia (NH3) in aqueous systems, e.g. in natural water, sewage, boiler feed water, beer, etc.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 2 Mode of operation Ammonia (NH3) reacts in water with the formation of ammonium ions (NH4+): (1) The equilibrium of this reaction, i.e. the relative proportion of ammonia and ammonium ions, is determined by the pH value of the solution. In acidic solutions, ammonia reacts almost completely to ammonium ions. At a pH value of 9.2, the ratio of ammonia to ammonium ions is approximately 1 : 1.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 2 Mode of operation (3) UN is the Nernst potential (slope) of the electrode. Because of the fact that the hydroxide concentration is proportional to the ammonia concentration (equation 4), the electrode obeys the Nernst equation for the ammonia concentration as well: (4) (5) Using equation 5, either the ammonia concentration or the ammonium concentration can be determined from the measured data. U‘0 is determined essentially by means of the internal reference electrode.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 3 Selecting the electrode In the following you will find a list of criteria that is intended to make it easier for you to select the correct electrode: 6.0506.100, 6.1255.000 6.0506.150, 6.1255.050 5 · 10–6…10–2 mol/L 10–4…1 mol/L Clean sample (e.g.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 4 Assembling the electrode 4 Assembling the electrode The way that you assemble your electrode depends on the type of the electrode. ￭ ￭ The electrode 6.0506.100 is supplied with complete membrane modules. The electrode 6.0506.150 is supplied with a membrane module with separate membranes. The membrane must first be mounted on the membrane module. It can be replaced as needed. Mounting the membrane on the membrane module Electrode 6.0506.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 4 Place the membrane on the screw thread of the membrane module and use your thumb to hold it in place. 5 Place the membrane carefully lengthwise over the opening of the screw thread. NOTE The part of the membrane which lies above the opening of the screw thread may not come into contact with fingers or other objects under any circumstances. The water-repellent effect of the membrane would otherwise be reduced as a result.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 4 Assembling the electrode 8 Check once again to ensure that the visible part of the membrane lies taut and wrinkle-free above the opening. Preparing the electrode for use CAUTION The membrane of the membrane module may not come into contact with fingers or with other objects under any circumstances. The waterrepellent effect of the membrane would otherwise be reduced as a result. Electrode 6.0506.100 and 6.0506.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 5 Storing the electrode The type of storage depends on the storage duration. Observe the following notes: Storage between measurements ￭ Store electrode in distilled water. Storage duration between 1 and 5 days ￭ Store the electrode in measuring electrolyte (6.2316.030). Storage duration of more than 5 days 1 Unscrew membrane module. 2 Rinse the membrane module thoroughly with distilled water inside and outside and place it in dry storage.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 6 Performing measurements 6 Performing measurements 6.1 Measured quantities Ammonia concentrations are usually specified in one of the following units: ￭ ￭ ￭ mol/L ppm (NH3) ppm (N) Conversion factors for various concentration specifications Table 2 6.2 Conc. in mol/L N content in ppm NH3 content in ppm 10–4 1.4 1.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 6.3 Performing direct measurements ￭ ￭ ￭ ￭ 6.3 The electrode must be conditioned in distilled water for at least 10 minutes between the measurements. If measurements are being performed at very high concentrations, then the electrode should be conditioned for 30 minutes. If a standard or sample solution is measured repeatedly, then it must be stored in a sealed or covered vessel between the measurements. This ensures that ammonia will not escape.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 6.4 6 Performing measurements Performing standard addition Standard addition is a considerably more convenient method for the measurement of ion concentrations with an ion-selective electrode (ISE). If you need to measure only a few samples, then this method is very simple, because you do not need to incorporate a calibration curve. Furthermore, all matrix effects are eliminated, because the electrode is calibrated in the sample solution.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 6.5 Performing measurements with very low concentrations Buret volume 10 mL Sample size 10 mL ISA/TISAB 10 mL Total volume 20 mL Factor cStandard solution/cSample 20 This results in a sample concentration in the measuring solution of 2.5 mg/L. The optimum concentration of the standard solution is thus 2.5 mg/L · 20 = 50 mg/L. Please note that this is merely to be considered a guideline for standard additions.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 6.6 6 Performing measurements Influence of dissolved particles on the measurement Water vapor is a potential interference factor. It can enter through the membrane and change the concentration of ammonium chloride in the measuring electrolyte. This leads to a potential drift. The introduction of water should not be a problem, however, if the following conditions are met: ￭ ￭ The total concentration of the dissolved particles is approx. 0.1 mol/L (osmotic pressure).
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 7.1 Problems and their solutions 7 Troubleshooting 7.1 Problems and their solutions Problem Cause Remedy The electrode slope is too large (> 62 mV). The electrode was insufficiently prepared or conditioned for too short a time in distilled water. Refill the electrode and condition for at least 30 minutes in distilled water. The electrode slope is too small. The membrane has a tear. Replace the membrane or the membrane module.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 7 Troubleshooting Problem Cause Remedy Meter, 867 pH Module or a Titrando. Replace the measuring electrode if required. The measuring signal is noisy, the measured values are unstable. There is not enough measuring electrolyte in the membrane module. Unscrew the membrane module, empty it and refill it with 2 mL measuring electrolyte. The pH glass electrode (measuring electrode) is possibly defective.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 8 Technical specifications Measuring range 6.0506.100 5 · 10–6…10–2 mol/L (NH3) 0.1…170 ppm (NH3) 6.0506.150 10–4…1 mol/L (NH3) 10…17000 ppm (NH3) Temperature range 0…50 °C pH range (measuring electrode) pH 0…14 Minimum immersion depth 2 mm Shaft length 133 mm Shaft diameter 12 mm Shaft material PEEK Module material 6.0506.100 POM black 6.0506.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 9 Accessories 9 Accessories NOTE Subject to change without notice. 9.1 Scope of delivery 6.0506.100 NH3-selective electrode Order no. Description 1 – Measuring electrode with Metrohm plug-in head G and certificate 3 – Membrane modules with certificates 1 6.2316.030 Measuring electrolyte (50 mL) 1 8.109.8031ML Manual for NH3-selective electrodes, German/English Qty. 6.0506.150 NH3-selective electrode Qty. Order no.
9.2 Optional accessories 9.2 ￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ Optional accessories 6.0506.100 NH3-selective electrode Order no. Description 6.1255.000 Replacement module kit for NH3-selective electrode 6.0506.100 Comprised of: 3 replacement membrane modules with certificates, 1 measuring electrolyte (50 mL). 6.0506.150 NH3-selective electrode Order no. Description 6.1255.050 Replacement module kit for NH3-selective electrode 6.0506.
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ Index Index A Accessories ............................... 17 Alkali sample .............................. 9 Application Bulletin 133 ........... 10 C Concentration Converting ........................... 9 High ................................... 13 Low .................................... 12 Conversion factor Concentration ...................... 9 D Difficulties Nonaqueous system ............. 1 Surface-active substance ....... 1 Direct measurement .................
