LCS Laboratory Inc. 700 Collip Circle, Unit 218, London ON, N6G 4X8, (519) ,

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1 LCS Laboratory Inc. 700 Collip Circle, Unit 218, London ON, N6G 4X8, (519) ,

2 What is Acidity and Alkalinity? What does ph mean? How do we measure ph? How does a ph meter work? How to calibrate a ph meter? Why temperature is important? Hanna Instruments HI ph meter Standard Methods 4500-H + B. ph Value in Water by Potentiometry Using a Standard Hydrogen Electrode. References 2

3 H 2 SO 4 2H + + SO 2-4 Sulfuric acid dissociates producing proton (H + ) and anion NaOH Na + + OH - Sodium Hydroxide dissociates producing hydroxyl (OH - ). H + - is a chemical abbreviation for Hydrogen cation often used as a short symbol for acidity OH - - is a chemical abbreviation for Hydroxyl anion, is a symbol for alkalinity 3

4 HOH H + + OH - Water also dissociates producing H + and OH - In pure water concentration of H + is equal to concentration of OH -. Such water is called neutral If water is unbalanced and we have more H + than OH -, this water is acidic. 4

5 Concentration of H + in pure water is extremely low: g per metric tone. This concentration can be written as: (g)/1000(L)=10-7 g/l=10-7 mol/l We call such acidity ph=7. ph is the order of magnitude of H + concentration (activity) taken without - sign ph=-log 10 ([H+]*γ) γ activity coefficient is usually between 0.3 and 1.0 If solutions A (ph=4) and B (ph=5) differ by 1 ph unit, then A contains 10 fold more acid than B 5

6 Environmental comfort zone for most existing species is from ph 6 to 8 6

7 ph indicating paper ph test strips Dip a test strip into solution, read colour and compare with a ph chart. Pros: Excellent method. Instantaneous reading. Inexpensive. Good to have as fast way to check ph. Cons: Not very accurate. 7

8 Electrometric ph meters are the most accurate instruments for the task Pocket size testers (left). One point calibration Portable instruments with 1-2 point calibration and temperature probe Bench top models with temperature compensator and multi point calibration 8

9 Electrode(s) or probes Temperature probe Gain adjustment (calibration) Processor or analog device Indicator or screen for output ph meter, to some extend, is an electrochemical battery. Like every battery it has two electrodes. Glass electrode (left) generates potential which is strictly proportional to ph. Reference electrode (right) generates constant potential which doesn t change with ph. If ph around the electrodes changes, the voltage changes as well, and therefore the ph can be measured 9 Modern combination electrodes contain two electrodes and a temperature sensor inside a single probe. But this is still a twoelectrode device.

10 Modern ph meters have built-in calibration chart calculated for an ideal and perfect electrode. But this is not always a case. When we calibrate our instrument, we adjust slope and zero point on calibration chart. With one-point calibration: we correct zero point in a such way that ph meter reads ph=7.0 when voltage is zero. (Poor calibration practice) With two-point calibration we also adjust slope of the calibration curve (Good calibration practice) 10

11 Electrode response changes with temperature. Therefore temperature instability can cause significant error in ph measurement To adjust for effect of temperature, the ph meter may have an external temperature probe (right), or a temperature sensor built into the electrode (left). In both instruments, the microprocessor automatically adjusts gain to take changing temperature in account. 11

12 Handheld, waterproof portable ph and temperature meter. The large display shows readings in an extended range from to ph and simultaneously shows temperature from -5.0 to C or 23 to 221 F. Automatic Temperature Compensation Automatic one or two point calibration HI 1296 ph/temperature probe in titanium enclosure Compact, heavy-duty, and waterproof 12

13 Use small portion of buffer (20 ml) for calibration. Dispose the 20 ml sample after each use. Wash probe in DI water before placing into next solutions. Remove electrode from storage solution. Prepare calibration buffer solutions. You can use Hanna or any other quality brand. Please note that calibration solutions with ph=8 and more have limited shelf life. Use calibration standards with ph which is similar to one of your samples. For example: if your stream is acidic, you should use ph 4.00 and 7.01 for calibration. 13

14 From normal mode, press and hold ON/OFF/ MODE until OFF on the secondary display is replaced by CAL. Release the button. For a two-point calibration, place the probe in ph 7.01 buffer. After the calibration point has been accepted, the ph 4.01 USE message appears. If a valid buffer with ph 4.01 is detected, then the meter completes the calibration procedure. When the buffer is accepted, the LCD shows the accepted value with the OK 2 message on the secondary display. The meter then returns to the normal measuring mode. Note: When the calibration procedure is completed, the CAL tag is turned on. Check your calibration before you start your measurement. To check calibration, measure ph of a buffer with ph4 or ph10. Check calibration every 20 samples and at the end of the day. Keep record of instrument calibration 14

15 Calibrate the instrument Rinse the electrode in DI water Transfer small portion of sample (20 ml) into a beaker Immerse the electrode into sample Let the ph reading to stabilize Record reading down to 2 decimals Rinse the electrode in DI water Repeat measurement on the same sample. If both readings are within ±0.1 range, report average value rounded to 0.1. Error of Measurement. Measure ph of a buffer 7 times. Record results into Excel spreadsheet and calculate standard deviation. Multiply by The result is expected to be around 0.1. This is typical error of your test at 95% confidence interval. 15

16 The main steps: Check instrument functionality Check calibration using NIST traceable ph standards. Standard must come with a certificate, which is filed for future reference. Record calibration into logbook Test ph of the samples and record the results into logbook Typical QC requirements (some of them). Calibration has to be checked every time the instrument is used Calibration should cover the range of ph of the samples Calibration should be valid during the test (check before and after the test) Laboratory needs to know it error of measurements The error is used to accept or reject the test Results should be traceable (who did and how) 16

17 1. HANNA ph meter Manual 2. HANNA ph meter Video 3. Standard Methods 4500-H+B 17

18 We want to work for you For more training materials, or to learn about our services, please visit us at Collip Circle, Unit 218, London ON, N6G 4X8, (519)