Determination of pH PDF - Menoufia Faculty of Medicine

Instrumentation used to measure pH Faculty of medicine menoufia university Department of medical biochemistry and molecular biology Intended Learning Outcomes (ILOs): Definition of pH of a solution. Methods of PH determination. Composition of PH meter and i...

Instrumentation used to measure pH Faculty of medicine menoufia university Department of medical biochemistry and molecular biology Intended Learning Outcomes (ILOs): Definition of pH of a solution. Methods of PH determination. Composition of PH meter and its principle. Need for knowing pH values. ❑ pH of a solution is defined as the negative logarithm of hydrogen ion concentration. ❑ pH value is a number on a scale 0 to 14 indicating hydrogen ion concentration. ❑ If value is below 7, the solution is acidic and if above 7 the solution is alkaline. ❑ The pH value at 7 means neutral i.e. neither acidic nor alkaline.  How to measure the pH of a sample There are two methods : 1. Paper Strip Method 2. Electrometric Method (pH meter) (i) Paper Strip method: Readymade strips are available which change color at different pH values. The change in color of strips as we put it in the solution indicates the pH value of the solution. This method gives only a general estimation of pH and not the precise pH. (ii) Electrometric Method (pH meter) : It gives accurate pH value of the solutions like body fluids and laboratory reagent. PH meter is electronic device composed simply by voltmeter connected to PH electrode & scaled to read PH value. Composition and principle of PH meter  Principle: It measure the potential difference between reference electrode and the PH electrode that’s sensitive to hydrogen ion concentration. When dipped into a solution of unknown hydrogen ion concentration, a potential difference dependent upon the difference in [H+] is established across the glass membrane. The difference in the potential is amplified and converted into direct PH reading. Composition: It is glass electrode. Inside the glass is a silver wire covered with AgCl paste dipped in 0.1 NHCl and it serve as the internal reference electrode.  Measurement of pH ▪ A pH measurement system consists of of three parts: a pH measuring electrode, a reference electrode, and a high input meter. ▪ A sample is placed in a cup and the glass probe at the end of the retractable arm is placed in it. ▪ There are two electrodes inside the probe that measure voltage.  Measurement of pH ▪ The pH measuring electrode (glass electrode) measures the acidity of the sample through the amount of H+ ions and has a hydrogen ion sensitive glass bulb. ▪ The reference (calomel) electrode does not vary with the activity of the hydrogen ion and is contained in liquid with fixed pH. ▪ Before taking a pH measurement the meter must be calibrated using a solution of known pH.  PROCEDURE Turn on the pH meter. Rinse off the electrode with distilled water. If the meter has a temperature setting knob, set it to room temperature. Place the electrode in the pH 7 buffer standard. While swirling the solution, adjust the calibration knob until the display reads “7.”  PROCEDURE Rinse the electrode with distilled water, being very careful of the very delicate tip. Place the electrode into the solution to be tested. Swirl the solution until the pH display stops changing. Read the pH value. If the solution is a strong acid or base, use a pH 4 buffer (for the strong acid) or a pH 10 buffer (for the strong base) to slope the meter after it has been calibrated to pH 7. Need for knowing pH values: 1) All enzymatic actions in animals and plants including microorganisms take place at a specific pH value. 2) PH value of soil is important for the growth of crops. 3) PH value of blood is kept within the normal range through homeostasis. PH determination is essential in the diagnosis and monitoring of acid-base balance disorder. 4) PH value is used, in industries (leather and food processing). Thank you