PH ph Ph pH

Introduction

• What do the p and H in pH stand for?
• The p stands for potential or power
• The H stands for the Hydrogen atom
• pH is the correct written version
• pH refers to the amount of Hydrogen ions (H+) in a solution
• pH is calculated from the formula:

pH = -log10[H+]

Fundamentals of pH

pH Determination

pH Scale

• Each whole value below 7 is 10 times more acidic than the previous number
• For example, pH 3 is 10 times more acidic than pH 4 and is 100 times more acidic than pH 5
• For every number decrease we have to multiply by 10
• 10 x 10 = 100
• The opposite is also true
• Each pH value above 7 is 10 times more alkaline than the previous number
• pH 10 is 10 times more basic (alkaline) than pH 9 and is 100 times more basic (alkaline) than pH 8

How Do We Measure pH?

How Does a pH Meter Work?

Measuring Tools

• Litmus paper is used as a general acid - base indicator
• pH tape/paper is used for approximations
• Liquid indicators are used for approximation
• Pocket pH meters are used for field trending +/- 0.2
• pH meter is used for accuracy, reproducibility, precise and continuous measurement +/- 0.1 / 0.01 / 0.001 (user decides)

How Does a pH Meter Work?

• The difference between the electrical potential of the solutions inside and outside the thin glass membrane creates an electromotive force in proportion to the difference in pH.
• This relationship is linear with pH.
• Measured in mV which is converted to values on the pH scale.

Electrodes

pH Meters

• HACH
• Thermo Scientific Orion
• Oakton
• Hanna
• Others
• Need TLC, read the manual first

Sample Collection

• Sample should be collected in glass or plastic containers (minimum of 100 mL)
• Sample should be collected without agitation, as too much agitation can cause a loss of CO2 and increase pH of sample
• Sample should be analyzed as soon as possible after collection

Bench pH Measuring

Our Procedure

1. Calibrate lab pH meter
2. Sample collection throughout plant/distribution system
3. Between measurements, rinse electrodes with distilled water and then with the next sample to be measured
4. Pour sample in beaker and immerse electrode
5. Add magnet, turn on stir plate
6. Record pH and temperature when meter says ready/hold
7. When finished ensure electrode is placed in storage solution

How Do We Know Our pH Meters Are Reading Correctly?

• Follow our SOP pH procedure
• Calibrate meters daily
• Record standard results for quality control
• Participate in external proficiency testing, CALA (Canadian Association of Laboratory Accreditation)

Calibration

Electrode Do's and Don'ts

 DO's DON'Ts Do store electrode in electrode storage solution Do not store electrode in distilled water Do calibrate meter daily Do not leave exposed to air Do keep electrode moist Do not wipe electrode but blot with lint free tissue

• Sluggish
• Won't calibrate properly
• Keep a spare electrode on hand

Factors Affecting pH

• Temperature
• Need an Automatic Temperature Compensator (ATC probe)
• Make sure samples and buffers are at the same temperature
• Heat from magnetic stirrer transfers to sample
• Exposure to air - CO2 loss - "drifting"

Inline vs Bench pH Measurement

 Inline pH Analyzers Bench Top Meters continuous contact with the sample influenced by exposure to atmosphere so you may experience "drifting" conditioned to the process stream gain or loss of CO2 may result in different pH readings does need calibration and maintenance does need calibration and maintenance daily should be checked periodically against bench top meter

*If comparing the two, +/- 0.1 is ideal but +/- 0.2 is realistic

pH Facts

• pH - Hydrogen ion concentration in water
• The pH scale goes from 0 - 14
• 0 being acidic and 14 being the most base or alkaline
• Water with a pH below 7 is considered acidic
• Water with a pH above 7 is considered base or alkaline
• pH stands for the hydrogen ion
• Water should be stable, neither corrosive or scale forming
• Low pH causes corrosion
• High pH causes scale
• The lower the pH the faster the disinfection
• A higher pH requires higher chlorine residual
• Higher the pH, the faster the rate of oxidation of iron to insoluble ferric hydroxide
• Water with a pH of 7 is considered pure water
• Calcium and magnesium become less soluble as pH increases
• Natural waters usually have a pH of between 6.5 and 8.5
• Free chlorine is mostly hypochlorite at pH levels above 7.5 and is a less powerful disinfectant
• If pH is above 9, the water will cause scale to form
• Recarbonation will lower pH to between 8.8 and 8.4 and the langelier index will be positive and there will be little or no corrosion
• Hydrolysis is at a minimum at a pH of about 4.7
• Respiration by algae results in an increase in carbon dioxide in water (lower of pH)
• Photosynthesis decreases carbon dioxide in water (increasing pH)
• Respiration by algae resultsin an increase in carbon dioxide in water (lower of pH)

Chemicals That Raise pH

• Lime
• Soda ash
• Hypochlorination
• Sodium bicarbonate
• Caustic soda
• Calcium hypochlorite
• Sodium hypochlorite

Chemicals That Lower pH

• Gas chlorine - lowers pH and strips alkalinity
• Sulfuric acid
• Carbon dioxide - causes corrosion (used to recarbonate water and lower pH)
• Sodium hydroxide
• Alum
• Ferric chloride
• Hydrofluosilicic
• Soda ash