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pH Meter

pH Meter. pH Meter Keypad Functions. ON/OFF Powers the unit on and shuts the unit off. Meter directly enters measurement mode when you turn it on. HOLD Freezes the measured readings. To activate, press HOLD while in pH or mV mode. To release, press HOLD again.

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pH Meter

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  1. pH Meter

  2. pH Meter Keypad Functions ON/OFF Powers the unit on and shuts the unit off. Meter directly enters measurement mode when you turn it on. HOLD Freezes the measured readings. To activate, press HOLD while in pH or mV mode. To release, press HOLD again. MODE Selects the parameter of measurement. pH is the default mode. Pressing MODE lets you scroll first to temperature mode, then to mV mode, then back to pH mode again. • NOTE: Temperature mode is used for temperature calibration; temperature readings are always displayed on the secondary display. CAL/MEAS Enters CAL mode for the calibration function. You can access CAL only in the pH or temperature modes. pH buffer values are 4.01, 7.00, and 10.01. To end calibration and return to measurement mode, press CAL/MEAS again. Press OFF to end all functions. To confirm calibration, press the CON key. ▲Use to scroll up the buffer calibration values provided by the calibration program. It will scroll one value up from the present calibration value displayed on the LCD with each key press. When the highest calibration value is reached, it will scroll around to the lowest value. ▼Similar to ▲, except with each key press, the value scrolls one step down. CON Press CON to confirm the calibration point. “CON” will flash on the screen.

  3. Calibration of pH Meter • Turn meter on (On/Off button) • Rinse electrode with deionized water • Do not wipe the electrode; this causes a build-up of electrostatic charge on the glass surface. • Dip the electrode into the calibration buffer, pH 7.0. • The glass bulb of the electrode must be completely immersed into the sample. • Stir the electrode gently to create a homogeneous sample. • Press CAL/MEAS. • The display will show CAL—see Figure H. • The primary display will show the measured reading while the smaller secondary display will indicate the pH standard buffer solution. • If using a buffer other than pH 7, press the ▲or ▼keys to scroll up or down until the secondary display value is the same as the pH buffer value • Wait for the measured pH value to stabilize. • The READY indicator will display when the reading stabilizes. • Press HOLD/ENTER to confirm calibration • A confirming indicator flashes for one second and disappears • The meter is now calibrated at the buffer indicated in the primary display.

  4. Calibration of pH Meter (cont’d) The secondary display automatically scrolls to the next buffer calibration option. 7) Press the ▲or ▼key to select the second buffer value you want to calibrate. - The buffer values are shown on the secondary display (pH 4.01, 7.00, and 10.01). See Figure J. 8) Follow steps 2 through 6 for additional calibration points (pH 4.01, 10.01 - up to 3 values). • Do not press CAL/MEAS until all calibration is completed. 9) Press CAL/MEAS to return to measurement mode. 10) When measuring the pH (see Figure L), wait until READY shows in the display

  5. Exp 16A – Equilibria with Weak Acids and Bases • Strong acids (HCl, H2SO4) • dissociate completely in water • donate all their H+ to H2O to form H3O+ HCl(aq) + H2O  Cl-(aq) + H3O+(aq) They are completely ionized • Weak acids (acetic acid, vinegar (CH3COOH)) • dissociate partly in water • donate a small portion of H+ to H2O to form H3O+ HAc(aq) + H2O Ac-(aq) + H3O+(aq) (acetic acid) (acetate ion) They are partially ionized • Solution of 0.1 mol acetic acid in 1 liter of water • 0.0004 mol H+ • 0.0004 mol acetate ions • 0.0996 mol acetic acid

  6. Exp 16A – Equilibria with Weak Acids and Bases • Strong bases (NaOH, KOH) • dissociate completely in water • release all their OH- in solution NaOH(s) Na+(aq) + OH-(aq) They are completely ionized • Weak bases (NH3) • ionizes partly in water • remove a proton (H+) from water, resulting in increase in OH- in aqueous solution NH3(aq) + H2O NH4+(aq) + OH-(aq) They are partially ionized • Solution of 0.1 mol ammonia in 1 liter of water • 0.001 mol NH4+ • 0.001 mol OH- • 0.099 mol NH3

  7. Exp 16A – Equilibria with Weak Acids and Bases • Study the effect of • dilutions • mixing acids and bases • mixing acids • mixing acids or bases with the salt of the acid or base • CH3COONa = sodium acetate, the sodium salt of acetic acid • NH4NO3 = ammonium nitrate, the ammonium salt of nitric acid, or the nitrate salt of ammonia • Na3PO4 = sodium phosphate, the sodium salt of phosphoric acid • Equilibria of weak acids and bases is affected by • dilution • common ions • solution containing sodium acetate and acetic acid • salts

  8. Exp 16A – Equilibria with Weak Acids and Bases • Weak acids (acetic acid, vinegar (CH3COOH)) • dissociate partly in water • donate a small portion of H+ to H2O to form H3O+ HAc(aq) + H2O Ac-(aq) + H3O+(aq) (acetic acid) (acetate ion) • What happens if you add a (large) amount of acetate ions (e.g Na-acetate)? • “common ion effect” • the equilibrium is disturbed: “Le Chatelier’s Principle” • The reaction equilibrium shifts away from forming more of the ion that is already present • Reaction above shifts to the left • What about adding H+ ions?

  9. Exp 16A – Equilibria with Weak Acids and Bases Experimental • Purpose • Determine or estimate the pH of a series of solutions • Measure pH of solutions with pH meter • Getting started • Prepare solutions as described in the table in your lab manual (see also next slide) • Mix solutions in flasks, beakers or test tubes numbered 1-18 • Mix solutions thoroughly

  10. Exp 16A – Equilibria with Weak Acids and Bases

  11. Exp 16A – Equilibria with Weak Acids and Bases

  12. Exp 16A – Equilibria with Weak Acids and Bases Experimental • The Experiments • Put about 5 mL of each solution in a test tube • Measure pH with a calibrated pH meter • Record your results in your lab manual and on the board

  13. Exp 16A – Equilibria with Weak Acids and Bases Experimental II: pH with pH meter

  14. Exp 16A – Equilibria with Weak Acids and Bases Experimental II: pH with pH meter

  15. Exp 16A – Equilibria with Weak Acids and Bases Experimental Calculations HA(aq) + H2O(l) A-(aq)+ H3O+(aq) • Use of the pH formula, the acid dissociation constant Ka, and the Henderson-Hasselbalch equation • Acid dissociation constant Ka = [H3O+]*[A-]/[HA] = [H3O+]*[base]/[acid] • pH =-log[H3O+] • pH = pKa + log ([base]/[acid])

  16. Exp 16A: Equilibria with Weak Acids and Weak Bases Turn in Result Sheet Answer Postlab Questions Conclusions for each sample Explain the pH that you observed/measured Why does the pH change when you make a dilution? Why does the pH change when you add a salt/common ion What is the effect of adding a strong acid or base to the sample you are investigating? Exp 16B: An Acid-Base Titration Curve Prelab preparations Goal Procedure Properties of potassium hydrogen phthalate and sodium hydroxide Prelab exercises 1a-e, 2, 3 Next week

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