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【技術資料】pH測定セオリーガイド(インプロセス分析)

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pH 測定実践と応用のガイド (第1章のみダウンロード可能)

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ドキュメント名 【技術資料】pH測定セオリーガイド(インプロセス分析)
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Mettler-Toledo GmbH Process Analytics Industry Environment A Guide to pH Measurement Theory and Practice of pH Applications pH Theory Guide
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Copyright © 2016 by Mettler-Toledo GmbH CH-8902 Urdorf/Switzerland
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A Guide to pH Measurement - the Theory and Practice of pH Applications

A Guide to pH Measurement Theory and Practice of pH Applications
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Content

Content Preface 8 1 Introduction to pH 9 1.1 Acidic or alkaline? 9 1.2 Why are pH values measured? 11 1.3 The tools for pH measurements 12 1.3.1 The pH electrode 13 1.3.2 Reference electrodes 15 1.3.3 Combination electrodes 16 1.4 What is a pH measuring system? 17 2 Practical considerations 18 2.1 The pH m easuring system 18 2.2 Obtaining an accurate pH measurement 19 2.2.1 General principles of pH measurement 19 2.2.2 Industrial pH measurement 21 2.2.3 Signal processing and environmental influences 24 2.2.4 Calibration 28 2.2.5 Buffer solutions 29 2.3 How to maintain a reliable signal 30 2.3.1 Maintenance of the electrode function 30 2.3.2 Storage 33 2.3.3 Temperature compensation 33 2.4 Troubleshooting 37 2.4.1 Instructions and comments for the trouble- shooting diagram 37 3 Intelligent Sensor Management 42 3.1 Signal integrity 42 3.2 Pre-calibration 43 3.3 Predictive diagnostics 43 3.4 Asset management software 45 3.4.1 Electronic documentation 45 3.4.2 Sensor management 46 pH Theory Guide 3 METTLER TOLEDO
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4 Electrode selection and handling 48 4.1 Different kinds of junction 48 4.1.1 Ceramic junctions 48 4.1.2 PTFE annular diaphragm 50 4.1.3 Open junctions 51 4.1.4 Dual-membrane without junction 51 4.2 Reference systems and electrolytes 52 4.3 Types of membrane glass and membrane shape 56 4.4 pH e lectrodes for specific applications 58 4.4.1 Highly accurate problem solver 58 4.4.2 Complex samples or such of unknown c omposition 59 4.4.3 Semi-solid or solid samples 60 4.4.4 At the toughest applications in chemical process  industries 61 4.4.5 Prepressurized electrolyte pH electrodes 62 4.4.6 Dual-membrane pH electrodes 63 4.4.7 pH measurements in high purity water samples 64 4.4.8 Installation in an upside-down position 65 4.4.9 Non-Glass (ISFET) pH electrodes 66 4.4.10 For low maintenance and simple installation 67 4.5 Electrode maintenance 68 4.6 Electrode storage 68 4.6.1 Short term storage 68 4.6.2 Long term storage 69 4.7 Electrode cleaning 69 4.7.1 Blockage with silver sulfide (Ag2S) 69 4.7.2 Blockage with silver chloride (AgCl) 70 4.7.3 Blockage with proteins 70 4.7.4 Other junction blockages 70 4.8 Electrode r egeneration and lifetime 70 4 pH Theory Guide METTLER TOLEDO
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5 Comprehensive pH theory 71 5.1 Definition of the pH value 71 5.2 Correlation of concentration and activity 72 5.3 Buffer solutions 75 5.3.1 Buffer capacity (ß) 77 5.3.2 Dilution value (∆pH) 78 5.3.3 Temperature effect (∆pH / ∆T) 78 5.4 The measurement chain in the pH measurement setup 78 5.4.1 pH electrode 80 5.4.2 Reference electrode 81 5.5 Calibration / adjustment of the pH measurement setup 84 5.6 The influence of temperature on pH measurements 85 5.6.1 Temperature dependence of the electrode 85 5.6.2 Isothermal intersection 86 5.6.3 Further temperature phenomena 87 5.6.4 Temperature dependence of the measured sample 88 5.7 Phenomena in the case of special measuring solutions 89 5.7.1 Alkaline error 89 5.7.2 Acid error 90 5.7.3 Reactions with the reference electrolyte 90 5.7.4 Organic media 91 5.8 Signal processing 93 6 Mathematical parameters 99 pH Theory Guide 5 METTLER TOLEDO
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Figures

Figures Figure 1 The reaction of an acid and a base forms water 9 Figure 2 Dissociation of acetic acid 9 Figure 3 The formula for calculating the pH value from the concentration of hydronium ions 9 Figure 4 pH values for some chemicals and everyday products 10 Figure 5 The reaction of ammonia with water 10 Figure 6 The relationship between the amount of acid in solution and the output potential of a pH electrode 12 Figure 7 The measurement assembly of pH and reference electrode 13 Figure 8 Cross sections through the glass membrane 14 Figure 9 pH electrode with pH-sensitive membrane 14 Figure 10 Reference electrode with reference electrolyte, reference element and junction 15 Figure 11 Typical combination pH electrode with inner pH sensor and outer reference element 16 Figure 12 pH measurement system 17 Figure 13 InTrac 776 e 22 Figure 14 Industrial measuring sites 23 Figure 15 Signal transformation 24 Figure 16 Complete measurement system 27 Figure 17 Electrode with built-in electrolyte bridge 32 Figure 18 Calibration line and isothermal intersection points 35 Figure 19 Symmetrical structure of an Equithal®-system in comparison with a conventional electrode 36 Figure 20 Troubleshooting diagram 38 Figure 21 Electrode with ceramic junction 49 Figure 22 Example of electrode with PTFE diaphragm 50 Figure 23 Example of electrode with open junction 51 Figure 24 Dual-membrane pH electrode 52 Figure 25 Schematic drawing of the ARGENTHAL™ reference system 53 Figure 26 Differently shaped pH membranes 56 Figure 27 InPro 200x (i) 58 Figure 28 InPro 426x (i) 59 6 pH Theory Guide METTLER TOLEDO
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Figure 29 Puncture pH electrode 60 Figure 30 InPro 480x (i) 61 Figure 31 InPro 325x (i) 62 Figure 32 InPro 4850 i 63 Figure 33 pHure Sensor™ 64 Figure 34 InPro 3100 (i) 65 Figure 35 InPro 3300 (ISFET pH sensor) 66 Figure 36 InPro 4501 67 Figure 37 InPro 4550 67 Figure 38 Buffering capacity of acetic acid 77 Figure 39 Temperature dependence for the pH electrode slope factor 79 Figure 40 Different sources of potential in a combination electrode 79 Figure 41 Ion mobility and diffusion of ions through a junction 82 Figure 42 Left: offset adjustment of a pH electrode in the pH meter, right: slope adjustment of a pH electrode. Solid lines show ideal behavior, dashed lines show real behavior 85 Figure 43 Isothermal intersection, theory and practice 87 Figure 44 Illustration of alkaline and acid error electrode behavior 90 Figure 45 pH scale for different solvents 92 Figure 46 Typical process control loop 93 Figure 47 Intersection of the process control system and sensor / activator s ystem 94 pH Theory Guide 7 METTLER TOLEDO
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Preface

Preface The aim of this book is to give a representative description of pH meas- urement in the process industries. The actual sensor, the pH electrode, is therefore the main focus of the text. Correct sensor use is fundamen- tal for a meaningful pH measurement. Accordingly, both practical and theoretical requirements are dis cussed in depth so that the measuring principle is understood and an accurate measurement made possible. The first section (practical considerations) of the book describes the sensor, and the other elements that constitute a pH measurement sys- tem. Together with a troubleshooting diagram, this section gives the in- formation needed in order to ensure the correct working of the pH elec- trodes for long periods of time. The second, application orientated section gives solutions to different measuring tasks, giving examples from the lab and from industry. The last, theoretical part explains the basis of the pH measurement and completes, by further explanation, the information given in the first section. In addition, this book is outlined to be a useful tool in solving different measuri ng tasks. Thereby it can be read either in its totality or in parts. Urdorf, Switzerland, January 2013 8 pH Theory Guide METTLER TOLEDO pH Theory Guide
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1 Introduction to pH、1.1 Acidic or alkaline?、Figure 1 The reaction of an acid and a base forms water.、Figure 2 Dissociation of acetic acid.、Figure 3 The formula for calculating the pH value from the concentration of hydronium ions.

1 Introduction to pH 1.1 Acidic or alkaline? Why do we classify an everyday liquid like vinegar as being acidic? The reason is that vinegar contains an excess of hydronium ions (H3O +) and this excess of hydronium ions in a solution makes it acidic. An excess of hydroxyl ions (OH–) on the other hand makes something basic or alkaline. In pure water the hydroniumn ions are neutralized by hydroxyl ions, therefore this solution has a neutral pH value. H3O + + OH– ↔ 2 H2O Figure 1 The reaction of an acid and a base forms water. If the molecules of a substance release hydrogen ions or protons through dissociation we call this substance an acid and the solution becomes acidic. Some of the most well-known acids are hydrochloric acid, sulfuric acid and acetic acid or vinegar. The dissociation of acetic acid is shown below: CH3COOH + H – + 2O ↔ CH3COO + H3O Figure 2 Dissociation of acetic acid. Not every acid is equally strong. Exactly how acidic something is, is determined by the total number of hydrogen ions in the solution. The pH value is then defined as the negative logarithm of the hydrogen ion concentration. (To be precise, it is determined by the activity of the hy- drogen ions. See “5.2 Correlation of concentration and activity“ on page 72 for more information on the activity of hydrogen ions). pH = – log [aH+] Figure 3 The formula for calculating the pH value from the concentration of hydronium ions. pH Theory Guide 9 METTLER TOLEDO
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Figure 4 pH values for some chemicals and everyday products.、Figure 5 The reaction of ammonia with water.

The quantitative difference between acidic and alkaline substances can be determined by performing pH value measurements. A few examples of pH values of everyday substances and chemicals are given in Figure 4 below. Food & Beverages / Household products Orange juice Egg white Coca Cola Cheese Water Antacid Mg(CH)2 Lemon juice Beer Milk Borax 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sulfuric Acetic acid Caustic acid 4.9 % 0.6 % (0.1M) soda 4 % (1M) Calcium Hydrochloric acid Hydrocyanic acid carbonate (sat) 0.37 % (0.1M) 0.27 % (0.1M) Ammonia sol. 1.7 % (1M) Ammonia sol. 0.017 % (0.01M) Potassium acetate 0.98 % (0.1M) Sodium hydrogen carbonate 0.84 % (0.1M) Chemicals Figure 4 pH values for some chemicals and everyday products. The alkaline end of the scale is between pH 7 and 14. At this end of the scale the hydroxyl or OH– ions are present in excess. Solutions with these pH values are created by dissolving a base in an aqueous solu- tion. The base dissociates to release hydroxyl ions and these make the solution alkaline. Among the most common bases are sodium hydrox- ide, ammonia, and carbonate. NH3 + H2O ↔ NH + 4 + OH – Figure 5 The reaction of ammonia with water. The whole scale of pH values in aqueous solutions includes both the acidic and alkaline ranges. The values can vary from 0 to 14, where pH values from 0 to 7 are called acidic and pH values from 7 to 14 are termed alkaline. The pH value of 7 is neutral. 10 pH Theory Guide METTLER TOLEDO pH Theory Guide
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1.2 Why are pH values measured?

1.2 Why are pH values measured? We measure pH for a lot of different reasons, such as: • to produce products with defined properties – During production it is important to control the pH to ensure that the end product con- forms with the desired specifications. The pH can dramatically alter the properties of an end product such as appearance or taste. • to lower production costs – This is related to the above mentioned reason. If the yield of a certain production process is higher at a given pH, it follows that the costs of production are lower at this pH. • to avoid doing harm to people, materials and the environment – Some products can be harmful at a specific pH. We have to be care- ful not to release these products into the environment where they can be a danger to people or damage equipment. To be able to determine whether such a substance is dangerous we first have to measure its pH value. • to fulfill regulatory requirements – As seen above, some products can be harmful. Governments therefore put regulatory requirements in place to protect the population from any damage caused by dan- gerous materials. • to protect equipment – Production equipment that comes into con- tact with reactants during the production process can be corroded by the reactants if the pH value is not within certain limits. Corrosion shortens the lifetime of the production line, therefore monitoring pH values is important to protect the production line from unnecessary damage. • for research and development – The pH value is also an important parameter for research purposes such as the study of biochemical processes. These examples describe the importance of pH in a wide range of a pplications demonstrating why it is so often determined. pH Theory Guide 11 METTLER TOLEDO
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1.3 The tools for pH measurements、Figure 6 The relationship between the amount of acid in solution and the output potential of a pH electrode.

1.3 The tools for pH measurements To be able to measure pH you need to have a measurement tool which is sensitive to the hydrogen ions that define the pH value. The principle of the measurement is that you take a sensor with a glass membrane which is sensitive to hydrogen ions and observe the reaction between it and a sample solution. However, the observed potential of the pH-sen- sitive electrode alone does not provide enough information and so we need a second sensor. This is the sensor that supplies the reference signal or potential for the pH sensor. It is necessary to use the differ- ence between both these electrodes in order to determine the pH value of the measured solution. The response of the pH-sensitive electrode is dependent on the H+ ion concentration and therefore gives a signal that is determined by how acidic / alkaline the solution is. The reference electrode on the other hand is not responsive to the H+ ion concentration in the sample solution and will therefore always pro- duce the same, constant potential against which the pH sensor poten- tial is measured. The potential between the two electrodes is therefore a measure of the number of hydrogen ions in the solution, which by definition gives the pH value of the solution. This potential is a linear function of the hydrogen concentration in the solution, which allows quantitative measurements to be made. The formula for this function is given in Figure 6 below: RT E = E0 + 2.3  log [a nF H +] Figure 6 The relationship between the amount of acid in solution and the output potential of a pH electrode. E = measured potential E0 = constant R = gas constant T = temperature in degrees Kelvin n = ionic charge F = Faraday constant 12 pH Theory Guide METTLER TOLEDO pH Theory Guide
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1.3.1 The pH electrode、Figure 7 The measurement assembly of pH and reference electrode.

High impedance pH meter Glass Reference electrode electrode Figure 7 The measurement assembly of pH and reference electrode. In Figure 7 a pH measurement setup with two separate electrodes, a pH electrode and a reference electrode, is shown. Nowadays, a merger of the two separate electrodes into one sensor is very common and this combination of reference and pH electrodes is called the combina- tion pH electrode. Each of these three electrodes is different and has its own important features and properties. 1.3.1 The pH electrode The pH electrode is the part that actually senses the pH in the solution. It consists of a glass shaft with a thin glass membrane at the end, s ensitive to H+ ions. The outside of this membrane glass forms a gel layer when the membrane comes into contact with an aqueous solu- tion. A similar gel layer is also formed on the inside of the membrane glass, since the electrode is filled with an inner aqueous electrolyte solution. An example of this gel layer is shown in Figure 8 below: pH Theory Guide 13 METTLER TOLEDO
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Figure 8 Cross sections through the glass membrane.、Figure 9 pH electrode with pH-sensitive membrane.

Inner buffer Inner Internal Glass H+= constant gel layer buffer membrane SiO3 Li+ ▼ ▼ SiO ▼3 Li+ SiO internal buffer Li+ Positive Negative3 Li+ SiO3 charge charge SiO3 SiO3 Li + Li+ Li+ SiO SiO3 3 SiO3 SiO3 Li+ SiO Li + ▲ 3 H+ Measured Outer + H+ Hgel layer H+ Acidic solution Alkaline solutionsolution Glass membrane (0.2–0.5 mm) Gel layer ca. 1000 A (10-4 mm) Figure 8 Cross sections through the glass membrane. The H+ ions in and around the gel layer can either diffuse into or out of this layer, depending on the pH value and therefore the H+ ion con- centration of the measured solution. If the solution is alkaline the H+ ions diffuse out of the layer and a negative charge is established on the outer side of the membrane. If the solution is acidic the reverse happens, H+ ions diffuse into the layer and a positive charge builds-up on the outer side of the membrane. Since the glass electrode has an internal buffer with a constant pH value, the potential on the inner sur- face of the membrane remains constant during the measurement. The pH electrode potential is therefore the difference between the inner and outer charge of the membrane. A drawing of a standard pH electrode is shown in Figure 9 below. Inner buffer Lead-off Membrane element Shield Socket Figure 9 pH electrode with pH-sensitive membrane. 14 pH Theory Guide METTLER TOLEDO pH Theory Guide ▲ ▼ ▼ ▲ ▲
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1.3.2 Reference electrodes、Figure 10 Reference electrode with reference electrolyte, reference element and junction.

1.3.2 Reference electrodes The purpose of the reference electrode is to provide a defined stable r eference potential for the pH sensor potential to be measured against. To be able to do this the reference electrode needs to be made of a glass which is not sensitive to the H+ ions in the solution. It must also be open to the sample environment into which it is dipped. To achieve this, an opening or junction is made in the shaft of the reference elec- trode through which the inner solution or reference electrolyte is in con- tact with the sample. The reference electrode and pH half-cell have to be in the same solution for correct measurements. A picture of a typical reference electrode is shown in Figure 10 below: Refill Reference opening Junction Element Electrolyte (option) Figure 10 Reference electrode with reference electrolyte, reference element and junction. The construction of the electrode is such that the internal reference element is immersed in a defined reference buffer and is indirectly in contact with the sample solution via the junction. This contact chain ensures a stable potential. There are several reference systems available, but the one used almost exclusively today is the silver / silver chloride system. The potential of this reference system is defined by the reference electrolyte and the s ilver / silver chloride reference element. It is important that the reference electrolyte has a high ion concentration which results in a low electri- cal resistance (see “5.4 The measurement chain in the pH measure- ment setup“ on page 78 for more details). pH Theory Guide 15 METTLER TOLEDO
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1.3.3 Combination electrodes、Figure 11 Typical combination pH electrode with inner pH sensor and outer reference element.

Since the reference electrolyte flows into the sample solution during measurement, you should be aware of any possible reactions between the reference electrolyte and the sample solution as this can affect the electrode and measurement. 1.3.3 Combination electrodes Combination electrodes (see Figure 11 below) are much easier to han- dle than two separate electrodes and are very commonly used today. In the combination electrode the pH-sensitive glass electrode is con- centrically surrounded by the reference electrode filled with reference electrolyte. The separate pH and reference parts of the combination electrode have the same properties as the separate electrodes; the only difference is that they are combined into one electrode for ease of use. Only when the two components of the combination electrode are expected to have very different life expectancies is the use of individual pH and reference electrodes recommended rather than a single combined electrode. To further simplify pH measurements, it is possible to house a tem- perature sensor in the same body as the pH and reference elements. This allows temperature compensated measurements to be made. Such electrodes are also called 3-in-1 electrodes. Inner Membrane buffer Reference Lead-off Junction electrolyte element Socket Reference element Figure 11 Typical combination pH electrode with inner pH sensor and outer reference element. 16 pH Theory Guide METTLER TOLEDO pH Theory Guide
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1.4 What is a pH measuring system?、Figure 12 pH measurement system.

1.4 What is a pH measuring system? An electrode housing is necessary in order to protect and securely hold the pH electrode in a continuous industrial process. The function of a pH transmitter is to present the signals of the elec- trode in a suitable way; for instance with the help of a pH display or an output for a recording device. The different components of the pH mea- suring system can be summarized as follows: pH transmitter Cable Measuring system Combination pH electrode pH electrode assembly Electrode housing Figure 12 pH measurement system. pH Theory Guide 17 METTLER TOLEDO