Basic principles of pH measurement

Basic Principles of pH Measurement Perhaps the most familiar and oldest zero-current measurement method used to determine the course of a chemical reaction is pH measurement. What is pH and what should you know about pH measurement? In general, pH measurement is used to determine the acidity or alkalinity of a solution. Adding acid to water increases the acidity of the water and decreases the pH. Adding alkali to water will increase the alkalinity of the water, and the pH value is a unit used to express acidity and alkalinity. When we say milk is 'cold' or acid is 'weak' we don't necessarily mean the state of things, it's because we don't say the unit of measure and the value of the measurement. And when we say that the temperature of milk is 10°C, it is an exact concept. Similarly, when we say that the pH value of a weak acid is 5.2, this is also an exact concept. There are various acids and bases in the world with different acid and base strengths. Example: Hydrochloric acid is a very strong acid, while boric acid is very weak (useful for rinsing eyes and wounds). The strength of an acid depends mainly on the dissociation of hydrogen ions in the solution. Hydrogen ions dissociate widely in strong acids, but rarely in weak acids. Hydrochloric acid is a strong acid because the chlorine has almost completely dissociated the hydrogen ions. Boric acid is a weak acid because only a few hydrogen ions are dissociated. Even chemically pure water is dissociated in a small amount: Strictly speaking, hydrogen nuclei do not exist in a free state until they cooperate with water molecules. H2O+H2O=H3O－+OH－Because the concentration of hydronium ion (H3O) can be regarded as the same as that of hydrogen ion (H), the above formula can be simplified into the following commonly used form: H2O=H++OH－here The hydrogen ions are expressed in chemistry as 'H+ ions' or 'hydrogen nuclei'. Hydronium nuclei are denoted 'hydronium ions'. Negative hydroxide ions are called 'hydroxide ions'. Using the law of mass action, an equilibrium constant can be found for the dissociation of pure water: since only a small amount of water is dissociated, the molar concentration of water is actually a constant, and there is an equilibrium constant K to obtain the ion product of water KW. KW=K×H2O KW = H3O+·OH－=10－7·10－7=10mol/l(25°C) That is to say, for a liter of pure water at 25°C there are 10－7 moles of H3O－ ions and 10 -7 moles of OH- ions. In order to avoid calculations with the negative exponent of this molar concentration, the biologist Soernsen suggested in 1909 to replace this inconvenient value with a logarithm and define it as 'pH value'. Mathematical definition of pH is the negative value of the common logarithm of the hydrogen ion concentration. That is: pH=-logH+ Strictly speaking, this formula ignores the interaction between hydrogen ions (H+) and hydroxide ions (OH-), because between ions, the action of electric field force makes the mobility of ions significantly reduced. That is to say: the active concentration (ie activity) of the hydrogen ion is also related to all other ions being dissolved. For example: when the hydrogen ion concentration is 10-1 mol/1, the theoretical pH value should be 1.0, but we only measured the pH value as 1.08. This means that the degree coefficient f≠1 is 0.823. That is to say, the exact definition of pH value should be: temperature coefficient of pH measurement solution: due to the strong dependence of ion product on temperature, the neutral point of pure water has the following distribution: 0°C = pH25°C = pH75°C = pH100°C = pH Acids and bases are diluted with water, and there must be the above-mentioned pH dependence on temperature. For strong acids, the effect of self-dissociation of water is zero, and the pH value is determined only by the dissociation of the acid: 0°C, 25°C, 50°C, 0.001nHCL 3.00 pH 3.00 pH 3.00 pH0.1n HCL 1.08 pH 1.08 pH 1.08pH for Alkaline solution, the above-mentioned influence is just very big. Since the activity of hydrogen ions decreases at this time, the self-dissociation of water takes advantage at the same time. At 0°C, at 25°C, at 50°C, 0.001nNAOH 11.94pH 11.00pH 10.26pH Saturated lime water ┄┄ 12.4 pH 11.68pH In practice, there are the following conclusions: For the pH value of process control, at the same time know the temperature characteristics of the solution ; Only when the measured medium is at the same temperature can its pH value be compared. How to measure pH Almost everyone knows how to use litmus paper or the property of changing color with pH. For example: litmus paper turns dark red or light red in acid solution, and dark blue or light blue in alkaline solution. However, this method will have obvious errors in weak buffer solutions, or solutions containing metal ions or organic compound solutions (≤2PH value). In order to obtain and reproducible pH values, it is necessary to use the potentiometric method for pH measurement.