18 pH Meter.pptx

biochem

1. pH, pH meter calibration, pH adjustment and measurement

2. pH A pH meter provides a value as how acidic or alkaline a liquid is. The basic principle of the pH meter is to measure the concentration of hydrogen ions. A pH value of 7 indicates a neutral solution. Pure water should have a pH value of 7. pH values less than 7 indicate an acidic solution while a pH value greater than 7 will indicate an alkaline solution. So solution with pH value of 1 is highly acidic and a solution of pH value of 14 is highly alkaline

3. pH The pH is defined as the negative log 10  of the hydrogen  ion  concentration expressed in mol/L. A negative logarithmic scale is used because the numbers are all less than 1, and vary over a wide range. Since the pH is the negative logarithm of the hydrogen ion concentration, low pH numbers, indicate relatively high hydrogen ion concentrations, i.e. an acidic solution. High pH numbers, represent lower hydrogen ion concentrations, i.e. alkaline solutions. Because the pH scale is logarithmic to the base 10, a 1-unit change in pH represents a 10-fold change in hydrogen ion concentration. pH is defined as the negative logarithm of hydrogen ion concentration. pH= -log [ H+ ] p = power H = hydrogen [H+ ] = hydrogen ion concentration

4. So, It is important to remember that the pH varies inversely with hydrogen ion concentration. When hydrogen concentration in the blood increases, pH decreases, and the animal develops an  acidosis . When the hydrogen ion concentration in the blood decreases, the pH rises, and the animal develops an  alkalosis The pH scale from 0 to 14 covers all the hydrogen ion concentrations found in dilute aqueous solutions and biological systems. Pure water has a pH of 7 which is considered to be neutrality. When pH < 7, the solution is acidic and when pH > 7, the solution is basic or alkaline.

5. pH Determination pH can be measured by two techniques: colorimetric and potentiometric. The  colorimetric method  involves adding a suitable indicator to a solution and matching the color of the solution to a standard solution containing the same indicator. A more accurate technique and the one most frequently employed, the potentiometric method, uses a pH meter to determine hydrogen  ion  concentration. The two electrodes of the meter – a  calomel  reference electrode and a glass indicator electrode – are immersed in the solution, of known temperature, whose pH is to be measured. The electrode potential of the indicator electrode is linearly related to changes in hydrogen ion concentration and therefore indicating the pH.

6. Introduction The first commercial pH meter was built around 1936 by Radiometer in Denmark and by Arnold Orville Beckman in the United States. A typical pH meter consists of special measuring probes (a glass electrode and a reference electrode) connected to an electronic meter that measures and displays the pH reading

7. The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride ( calomel ) electrode Although a silver-silver chloride electrode is sometimes used. When the two electrodes are immersed in a solution, they act as a  battery . The glass electrode develops an  electric potential  (charge) that is directly related to the hydrogen-ion activity in the solution And the voltmeter measures the potential difference between the glass and reference electrodes. The meter may have either a digital or an  analog  (scale and deflected needle) readout. Digital readouts have the advantage of exactness, while analog readouts give better indications of rates of change

8. Both electrodes are hollow bulbs containing a potassium chloride solution with a silver chloride wire suspended into it. The glass sensing electrode has a bulb made up of a very special glass coated with silica and metal salts. This glass sensing electrode measures the pH as the concentration of hydrogen ions surrounding the tip of the thin walled glass bulb. The reference electrode has a bulb made up of a non-conductive glass or plastic

9. A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions. To complete the electrical circuit, also a reference electrode is needed

10. Working of a pH meter The pH meter measures the potential difference and its changes across the glass membrane. The potential difference must be obtained between two points; one is the electrode contacting the internal solution. second point is obtained by connecting to a reference electrode, immersed in the studied solution. Often, this reference electrode is built in the glass electrode (a combination electrode), in a concentric double barrel body of the device

11. Calibration Calibration is measurement technology. It is the process of configuring an instrument to provide a result for a sample within an acceptable range. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode. For pH meters to work efficiently and to be accurate, they have to be properly calibrated (the meter is accurately translating voltage measurements into pH measurements), so they usually need testing. A-pH calibration solutions, also called pH buffers, are a high-grade buffers and are used to calibrate pH meter before each use is to get the most accurate results every time. Buffer solutions are used to calibrate pH meters because they resist changes in pH on addition of small amounts of acid or base. They are easily prepared for a given pH. They are stable for long periods of time

12.   Calibration of pH meter Rinse the electrode with deionized water and blot dry using a piece of tissue. Place the electrode in the solution of pH 7 buffer, allow the display to stabilize and, then, set the display to read 7 by adjusting calibration knob. Remove the electrode from the buffer. Rinse and blot dry as before and put the electrodes in the buffer of either 4 or 10 pH whichever is near to the desired pH of the solution to be used for experiment. Allow the display to stabilize and, then, set the display to read 4 or 10 by adjusting calibration knob. Adjustment of pH of experimental solution: Rinse and blot dry the electrode and put it in experimental solution, allow the display to stabilize and adjust the pH of the solution by adding buffer components.

13. The  salt bridge  or liquid junction is an integral part of the reference  electrode . It provides the electrical connection between the reference  electrode  and the sample being measured. KCl is used as salt bridge because it provides positive K+ ions and negative Cl- ions. The salt bridge is required to maintain the neutrality in the system by providing enough negative ions equal to the positive ions during oxidation

14. Without the salt bridge, the solution in the anode compartment would become positively charged and the solution in the cathode compartment would become negatively charged, because of the charge imbalance, the electrode reaction would quickly come to a halt, therefore the purpose of a salt bridge is, to maintain charge balance

15. How to maintain pH meter Always keep pH electrode moist. It is better to store electrode in a solution of 4 M KCl . If 4 M KCl is not available, use a pH 4 or 7 buffer solution. DO NOT store electrode in distilled or deionized water—this will cause ions to leach out of the glass bulb and render electrode useless.

16. Uses pH meters are used for soil measurements in agriculture water quality for municipal water supplies and swimming pools Environmental remediation Brewing of wine or beer Manufacturing, healthcare and clinical applications such as blood chemistry, and many other applications. Advances in the instrumentation and in detection have expanded the number of applications in which pH measurements can be conducted. The devices have been miniaturized, enabling direct measurement of pH inside of living cells. In addition to measuring the pH of liquids, specially designed electrodes are available to measure the pH of semi-solid substances, such as foods. These have tips suitable for piercing semi-solids, have electrode materials compatible with ingredients in food.