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pH Measurement Guide by Erich K. Springer 3 pH Measurement Guide Table of Conte

pH Measurement Guide by Erich K. Springer 3 pH Measurement Guide Table of Contents Table of Contents 1 Introduction.......................................................................................................4 1.1 Booklet Overview...................................................................................................... 5 2 The pH Scale.....................................................................................................6 3 The pH Measurement.......................................................................................9 3.1 The NERNST Equation.............................................................................................. 9 3.2 The pH Measuring System.......................................................................................12 3.2.1 The Measuring Electrode. ..............................................................................12 3.2.2 The Reference Electrode...............................................................................15 3.2.3 The Combination (Single Rod) Electrode.......................................................16 3.2.4 Combination Electrode filled with KCl-Gel as a Reference Electrolyte...........16 3.2.5 Reference Electrode with POLISOLVE Electrolyte (Polymer) . ........................17 3.2.6 The Measurement of the Potential Difference................................................18 3.3 The Characteristics of a pH Measuring Chain .........................................................19 3.3.1 The Different Potentials of a Combination Electrode. .....................................19 3.3.2 The Zero point of an Electrode Assembly..................................................... 20 3.3.3 The Asymmetry Potential.............................................................................. 21 3.3.4 The Slope or Sensitivity of a pH Electrode Assembly................................... 22 3.3.5 The Isotherm Intersection Point.................................................................... 23 3.4 The Diaphragm. ........................................................................................................24 3.4.1 The Diffusion Potential.................................................................................. 27 3.4.2 Diaphragm Contamination through Chemical Reaction. ............................... 28 3.5 Alkaline and Acid Error............................................................................................ 30 3.5.1 Alkaline Error. ................................................................................................ 30 3.5.2 Acid Error...................................................................................................... 31 3.6 Temperature Influence and Temperature Compensation........................................ 31 3.7 Various Electrode Shapes....................................................................................... 33 3.8 Ageing..................................................................................................................... 35 3.8.1 The Ageing of a Measuring Electrode........................................................... 35 3.8.2 The Ageing of the Reference Electrode ....................................................... 36 3.9 Calibration............................................................................................................... 37 3.9.1 pH Standard and Technical Buffer Solutions................................................ 37 3.9.2 The Calibration Procedure............................................................................ 40 3.9.3 Re-calibration. ............................................................................................... 44 3.9.4 Response Time. ............................................................................................ 45 3.9.5 Reasons for calibration problems. .....................................................................45 3.10 The Accuracy to be Expected................................................................................. 46 4 Electrode handling..........................................................................................47 4.1 Storage. ....................................................................................................................47 4.1.1 The Storage of Measuring Electrodes. ...........................................................47 4.1.2 The Storage of Reference Electrodes........................................................... 48 4.1.3 The Storage of Combination Electrodes....................................................... 49 4.2 Cleaning of Electrode Assemblies........................................................................... 49 4.3 Refilling or Replacing the Reference Electrolyte...................................................... 50 5 The connection of the pH Electrodes to the pH measuring instrument. ..............................................................................51 5.1 Measuring Electrode Connection Cable. ..................................................................51 5.2 Reference Electrode Connection Cable...................................................................51 5.3 Combination Electrode Connection Cable...............................................................51 5.4 Cable Preparation and Cable Routing..................................................................... 52 5.5 Plug or Cable?......................................................................................................... 52 6 Appendix: the hydrogen Ion concentration..................................................54 6.1 The Atom Structure................................................................................................. 54 6.2 The Bohr Atom........................................................................................................ 55 6.3 The Periodic Table of Elements............................................................................... 56 6.4 The Molecule........................................................................................................... 56 6.5 The Ion. .................................................................................................................... 58 6.6 Dissociation............................................................................................................. 58 6.7 Acids – Bases – Salts.............................................................................................. 60 6.8 The Mole. ................................................................................................................. 61 6.9 The Hydrogen Ion Concentration in Aqueous Solutions. ......................................... 62 Table of Contents 5 pH Measurement Guide Introduction Introduction 1 Introduction Since the creation of man, he could differentiate between sour and bitter when eating liquid or solid foods. We say that vinegar and lemon juice taste sour and that soap water tastes bitter. This characteristic of these liquids and the marked degree of sour or bitter is indicated by the pH value. The significance of the pH value is today known to a great portion of our population alone through the fact that it is accepted that the pH value of swimming-pool water has to be kept within certain limits. The food we eat contains a lot of water and our drinks are nothing else than coloured and flavoured water, and this water can be either sour or bitter; or in other words, it will have a certain pH value. In general we can say that water is the most important substance on earth. Without water there will be no life on our planet. People living in areas of severe drought have experienced the importance of water for their existence. Continuous droughts do not only affect the farmers, they are a disaster for the entire economy of a nation. It is therefore of utmost importance that we treat this precious liquid with care, use it responsibly and try to save every drop of it. Water is everywhere! Three quarters of the surface of the earth is covered with water in the form of oceans, rivers, lakes and swamps. Brandy, whiskey, beer, wine, cool-drinks, fruit juices, coffee and tea consist of water containing certain additives which change the property and the taste of the water. Blood, the vital liquid of our body is mainly water, in fact our entire body consists of approximately 65% water. Water is used in industry as cooling water, wash water, boiler water, steam, condensate and solvent. Dirty water is usually disposed of as effluent. Notably the chemical industry especially makes use of this universal solvent, because most substances are somewhat soluble in water. Through its usage water undergoes changes, the most substantial being the change of its hydrogen ion concentration. All the above mentioned liquids differ in their hydrogen ion concen- trations, which makes them either acids or bases, sour or bitter. A measure for the hydrogen ion concentration of aqueous solutions is the pH value. It can be said that the human tongue is the first pH measuring instrument, but only the poten- tiometric pH measurement provides an accurate and reproducible result, when the acidity or alkalinity of an aqueous solution has to be determined. The pH measurement is therefore of vital significance in such diverse fields as biology, medicine, food technology, drinking water treatment, agriculture, mineralogy, surface treatment in metal processing, paper and textile manufacturing, chemical and petrochemical industry as well as waste water and sewage treatment for environmental protection. On a daily basis the importance of measuring the pH value in industry becomes more significant and with it new problems in process control appear and have to be solved. Even in laboratories is the demand for a highly accurate and repeatable pH measurement a predominating factor. The purpose of this booklet is to supply the basic knowledge of the pH measurement in a brief and understandable form. If it helps to eliminate the myths and mistakes arising from conceptual misunderstanding and if it leads to a successful application of pH measurement the author has fulfilled his task. 1.1 Booklet Overview Without a basic knowledge of a few chemical principles as well as of potentiometry and the functionality of a pH electrode a successful pH measurement is practically impossible. Therefore certain chemical and physical principles are described in the appendix of the booklet. These explanations will lead to the definition of the hydrogen ion concentration. The book starts with the definition of the pH scale, explaining the NERNST equation and de- scribing the fundamentals of the glass and reference electrode. pH related terminology will be dealt with in detail, the knowledge of this will lead to the successful application of the various Hamilton pH electrodes. One cannot achieve a better measuring accuracy than the accuracy of the buffer solution one uses for calibration. Hamiltons buffer solutions are described, after which the calibration of the pH measuring system is explained. Connecting the pH electrode to the measuring instrument and the “do’s and don’ts” of electrode handling will conclude the subject of pH measurement. 7 pH Measurement Guide The pH scale The pH scale Acknowledgements Many thanks go to Dr. Hannes Bühler of Hamilton who helped me in this venture. He read my first lecture book, advised me on certain subjects and corrected the odd mistakes. I also want to thank Andreas Brügger and Dirk Tillich, both of Hamilton, for providing me with special information and material. Finally I want to thank my loving wife for her patience and understanding. 2 The pH scale If we express the hydrogen ion concentration of an aqueous solution in relation to its molecular value we derive a scale of 1 (100) via 10-7 to 10-14 mole/litre. This scale is impractical but if written as a function of its negative logarithm a real and simple scale of 0 – 14 has been created: the pH scale. pH is the abbreviation of pondus hydrogenii and means the weight of hydrogen. This term was introduced in 1909 by the Danish biochemist S. P. L. Sørensen (1868 – 1939). The pH scale covers the active concentration of the H+ ions and OH¯ ions and therefore the pH value is defined as the negative common logarithm of the active hydrogen ion concentration in an aqueous solution. pH = log 1 hydrogen ion concentration (mole/litre) If the H+ ion concentration changes by a factor of ten, the pH value changes by one unit. This illustrates how important it is to be able to measure the pH value to a tenth of a unit or even a hundredth of a unit in special applications. The pH definition refers to the active hydrogen ion concentration and not just to the hydrogen ion concentration. It is important to understand this difference. Only in dilute solutions are all anions and all cations so far apart that they are able to produce the maximum of the chemical energy, i.e. the H+ ion concentration and the H+ ion activity are identical. For instance 0,01 mole hydrochloric acid is still classified as a dilute solution which dissociates completely and therefore concentration equals activity. 0,01 mole HCI: concentration = activity If the HCl concentration increases, the cation (H+) and the anion (Cl¯) obstruct each other increasingly as the space between them gets smaller and smaller. In this case the ion activity is slowed down and does not correspond any longer uploads/Geographie/ ph-measurement-guide-by-erich-k-springer.pdf