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Premium member Presentation Transcript Slide 1: pH Metry Mrs. Rani Ashok Assistant Professor of Zoology Lady Doak College, Madurai - 2 firstname.lastname@example.org Introduction : Introduction pH is a common way to express acidity and basicity pH = - log [H3O+] Definition : pH is the Unit of Measure used to express the degree of acidity of a substance. Slide 3: In pure water [H3O+] = [OH-] =1 x 10 -7M Therefore the pH of pure water is : pH = - log [H3O+] = - log (1 x 10 -7) pH = 7 Water : Water Most abundant compound in the body It accounts for about 60% of body mass Biochemical reactions occur in aqueous solution Membranes constitute a barrier between aqueous compartments in the cell However, many chemical reactions occur within membranes, a non-aqueous chemistry Slide 5: Van der Waals Water Solvent Properties of Water : Solvent Properties of Water Ions are always hydrated in water and carry around a "hydration shell" Water forms H-bonds with polar solutes Hydrophobic interactions – important in protein structure and nucleic acid structure Slide 7: Hydration of Ions Hydrophobic Interactions : Hydrophobic Interactions A nonpolar solute "organizes" water The H-bond network of water reorganizes to accommodate the nonpolar solute This is an increase in "order" of water This is a decrease in ENTROPY Hydrophobic interactions are one of the main stabilizing forces in determining protein structure Slide 9: Hydrophobic Group Organizes Water pH and the Henderson-Hasselbalch Equation : pH and the Henderson-Hasselbalch Equation Definition of pH (after Sorensen) : Definition of pH (after Sorensen) Acids and Bases : Acids and Bases According to Bronsted, acids are proton donors: HB H+ + B- HB is an acid B- is the conjugate base Strong acids are completely ionized, and the hydrogen ion concentration equals the concentration of the acid: HCl and HNO3 Weak acids are partially ionized: acetic acid, formic acid Bases are proton acceptors: B + H+ BH+ B is a base BH+ is the conjugate acid Strong bases are completely ionized, and the hydroxide ion concentration equals the concentration of the base: NaOH, KOH Weak bases are partially protonated and partially unprotonated: NH3 is an example of a weak base. It combines with a proton to produce NH4+ pH Calculations : pH Calculations What is the pH of 0.0001 M HCl? Strong acid is completely ionized [H]+ = 1 X 10-4 M pH = - log 1 X 10-4 = 4.0 What is the [OH]- of the above solution? [OH]- = 1 X 10-14 M /[H]+ [OH]- = 1 X 10-14 /1 X 10-4 = 1 X 10-10 M What is the pH of 0.001 M NaOH? [OH]- = 1 X 10-3 M [H]+ = 1 X 10-14 M /[OH]-/ = 1 X 10-14/1 X 10-3 M or 1 X 10-11 M The pH = -log 1 X 10-11 = - (-11) = 11 pH : pH For pure water, [H]+ is 10-7 M and the pH is 7 10-3 M [H]+ pH = 3 10-7 M [H]+ pH = 7 10-11 M [H]+ pH = 11 pH 7.4 corresponds to [H]+ = 10-7.4 pH and pOH = 14 : pH and pOH = 14 The [H]+ and the [OH]- are reciprocally related [H]+ X [OH]- = 1 X 10-14 [OH]- = 1 X 10-14/[H]+ If the pH is 5 ([H]+ = 1 X 10-5) The pOH is 9 ([OH]- = 1 X 10-9) Buffers : Buffers A buffer decreases the change in pH of a solution that would otherwise occur if acid or base were added Buffers are made up of weak acids and their salts, or weak bases and their acids Buffering capacity is greatest at the pKa : Buffering capacity is greatest at the pKa Good Buffers : Good Buffers Named after Dr. Good These are unreactive (poor nucleophiles) in contrast to Tris, phosphate, and imidazole (which are good nucleophiles) They have a high temperature coefficient (which means that the pH changes with temperature) Good Buffers : Good Buffers Enzymes Exhibit pH Profiles and Optima : Enzymes Exhibit pH Profiles and Optima The Phosphate Buffer SystemOne of the Main Physiological Buffers : The Phosphate Buffer SystemOne of the Main Physiological Buffers More on Phosphate : More on Phosphate The Carbon Dioxide/Bicarbonate Buffer System : The Carbon Dioxide/Bicarbonate Buffer System Slide 27: The pH scale acidic [H3O+] > [OH-] neutral basic [H3O+] < [OH-] It is used to show the acidity of a soln. [H3O+] = [OH-] Methods : Methods Measurement of pH INSTRUMENTATION : INSTRUMENTATION pH paper Paper in 0.5 increments preferred pH pen Jewelers screwdriver (for calibration) Two pH buffers (pH 7 and either pH 4 or pH 10) pH meter Three pH buffers (pH 4, 7, 10) Distilled water Slide 30: Instrument Placement Take water sample from the same location as the water temperature measurement. Again, once a week at the same time of day. Collect water in a clean container. Slide 31: Instrument Reading – pH paper Rinse a 50 mL or 100 mL beaker at least twice with sample water. Fill the beaker about halfway with water to be tested. Dip one strip of indicator paper into the sample water for 1 minute. Make sure all segments of the paper are immersed in sample water. Remove the paper from the water and compare the resultant four color segments with the chart on the back of the pH indicator paper box. Slide 32: Instrument Reading - pH paper (continued) If reading is unclear, place the paper back into the sample for an additional minute and check again. Continue until readings stabilize (i.e. 2 consecutive readings are the same). Do not exceed 10 minutes in water. Important: pH paper is not accurate in low conductivity water (<300 µS/cm). Slide 33: Instrument Calibration - pH pen Prepare two pH buffer solution following the manufacturer’s instructions, or use pre-made buffer solution. [One standard solution should be pH 7. The second standard should be either 4 or 10. Choose the second standard that will allow you to bracket your expected site values.] Condition the electrode as described by the manufacturer. Rinse the electrode twice with distilled water and blot dry. Turn pen on and immerse electrode in the pH 7.0 buffer solution. Gently stir and wait for the reading to stabilize. Slide 34: Instrument Calibration - pH pen (continued) Using a jewelry screwdriver adjust the small screw labeled “7” in the back of the pen until it reads 7.0, if necessary. Remove pen from buffer solution, rinse with distilled water, and blot dry. Immerse electrode in the pH 4.0 or pH 10.0 buffer solution. Gently stir and wait for the reading to stabilize. Using a jewelry screwdriver adjust the small screw labeled “4/10” in the back of the pen until it reads 4.0 or 10.0, if necessary. Remove pen from buffer solution, rinse with distilled water, and blot dry. Turn off instrument. pH Meters and Glass Electrodes : pH Meters and Glass Electrodes The glass electrode likes ions Don’t store the glass electrode in distilled water The glass electrode is very stable in acid (0.1 N HCl), and it can be cleaned with acid The glass electrode is dissolved by strong base (0.1 N NaOH) Proteins stick to glass electrodes Rinse for a few minutes in base Wash with acid Store in salt Slide 36: Instrument Calibration - pH meter Prepare pH buffer 4, 7, and 10 solutions following the manufacturer’s instructions, or use pre-made buffer solutions. Condition the electrode as described by the manufacturer. Rinse the electrode twice with distilled water and blot dry. Turn the meter on. Push the CAL button. Immerse the electrode in the pH 7.0 buffer solution. Gently stir the buffer solution and wait for the display to stabilize. After the reading has stabilized, press the HOLD/CON button to accept the value. Slide 37: Instrument Calibration - pH meter (continued) Remove the pH meter from the buffer 7.0 solution, rinse with distilled water, and blot dry. Repeat the above steps using the pH 4 and then the pH 10 buffers. After calibration has been completed for all 3 buffer solutions, rinse electrode with distilled water, blot dry, and turn meter off. Save pH buffer solutions in labeled bottles, as they can be reused for future calibrations (but not the original bottles if you purchased pre-made solutions). Slide 38: Instrument Reading – pH Pen and pH Meter Fill a clean, dry 100-mL beaker to the 50-mL line with the water to be tested. Immerse the electrode in this water so that the entire glass probe is immersed. Be sure not to immerse above the black line of the tester. Stir once, then let the display value stabilize. Once this value is stable, read the display value on the meter. Slide 39: Instrument Reading – pH Pen / Meter (continued) Record value on in the Hydrology Data Work Sheet as pH. Rinse electrode and repeat steps for another sample if needed. The two pH values should be within 0.2 of each other. Calibrations should be performed before each use. Important: pH meter is not accurate in low conductivity water (<100 µS/cm). Slide 40: Element Reporting Make measurements weekly, at the same time of day. Complete the Hydrology Data Work Sheet for Water pH. Take averages of the pH values measured by the student groups. If the recorded values are all within +/- 0.2 (pen, meter) or 1 (paper) of the average, report the average value to the GLOBE Student Data Server. Slide 41: pH meter Volt meter Stirrer THANK YOU : THANK YOU You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.