pH and pH meters Der Unfassbare (2006), “Ph-Meter” [photograph]. Retrieved from

Acids and Bases (Alkali) We all have heard about the words ‘acid’ and ‘alkaline’ (which is also known as ‘base’) Acids include hydrochloric acid, sulfuric acid, nitric acid, acetic acid (vinegar), citric acid (lemon juice), carbonic acid (soda water), etc. Alkaline substances include lye, slaked lime, ammonia, milk of magnesia, baking soda, etc. Pure water is neither acidic or alkaline, but becomes acidic or alkaline by addition of acids or alkaline substances (basic substance or base)

How Acidic or Basic? pH All acids/bases are not created equal! Some are toxic or caustic, e.g., hydrochloric acid, sulfuric acid, lye, ammonia. They become less harmful as they are diluted in water Others occur naturally and can be consumed: acetic acid (vinegar), citric acid (lemon juice), wine/beer, etc. Acidity of basicity of aqueous solutions of acids and bases is defined by a quantity called pH. pH = the “power of hydrogen”

What is pH? Water and pH The Water Molecule. All substances are made up of millions of tiny atoms. These atoms form small groups called molecules. In water, for example, each molecule is made up of two hydrogen atoms and one oxygen atom. The formula for a molecule of water is H2O. "H" means hydrogen, "2" means 2 hydrogen atoms, and the "O" means oxygen. Acids and bases in water. When an acid is poured into water, it gives up H (hydrogen) to the water. When a base is poured into water, it gives up OH (hydroxide) to the water. Find out more about atoms, ions, acids, and bases. A pH graph helps demonstrate this idea. A pH table lists the pH of common substances.

pH Scale * Edward Stevens (2010). “pH scale” [image]. Retrieved from

pH of Body Fluids Arterial blood: pH 7.35 – 7.45 Venous blood: pH Gastric juice: pH Small intestine juice: pH 8. 0 Urine*: pH 4.5 (*based on diet and metabolic state)

Why is pH Important? Consequence of Blood pH Imbalance Virginia Reid (2015), “pH imbalance” [image].

How is pH maintained? Buffering Solutions of weak acids or bases along with their salts added to water resist pH changes Such solutions are called buffers as they hold the pH constant despite the addition of small amounts of acids and bases Blood is highly buffered with proteins and bicarbonate/CO 2

pH Maintenance in the Body The body has 3 ways of maintaining a normal pH range : -chemical buffer system (acts within seconds) a) carbonic acid / bicarbonate b) phosphate buffer c) protein buffer -respiratory controls a) acts within minutes b) important in compensating for metabolic acidosis or alkalosis c) permits elimination of the volatile acid ( carbonic acid as CO2) -renal mechanisms a) acts within hours or days b) compensate for respiratory acidosis or alkalosis c) eliminate fixed acids from the body (metabolic acids generated in the body that are eliminated only in the urine ).

Measurement of pH Chemicals that change color with pH –Litmus paper –pH indicator dyes –pH paper Electrochemical measurement –pH meter

Litmus/pH Paper Litmus paper is useful to determine the pH of a solution when you do not have a pH meter available Proper storage of the litmus paper is essential to maintain quality control Michael Krahe (2005), “pH measurement with indicator paper” [image]. Retrieved from

pH meters Electrochemical instruments for measuring pH of solutions Contain two electrodes - a reference electrode and a measuring electrode- most times combined A voltmeter measures the electrical voltage difference between the two electrodes Electrodes are always kept wet and calibrated with standard commercial buffer solutions before use Electrodes are expensive and need to be maintained carefully World Health Organization (2008), “pH Meter”. Retrieved from library.ewh.org

Principle of a pH Meter Electrochemical Potential When a metal wire (e.g. copper) is immersed in a solution of its salt (copper sulfate) electrons move between the metal and the salt solution which creates a voltage at the metal wire characteristic of the metal/salt combination Such an arrangement is called a half cell. If a voltmeter is connected between two half cells of different metals, the voltage difference between them can be measured pH is measured as the difference in the voltage between two electrodes which are half cells. The voltage of the reference electrode is constant and the voltage at the measuring electrode changes with the pH of the solution

Two Half Cells A Galvanic cell Gringer (2008), Galvanic Cell [drawing]. Retrieved from

pH Meter as a Galvanic Cell Virginia Reid (2015), pH Meter as a Galvanic Cell [image].

World Health Organization (2008), “pH Meter”. Retrieved from library.ewh.org

Schematic Diagram of a pH Meter World Health Organization (2008), “pH Meter”. Retrieved from library.ewh.org

pH Meter Models Many Designs Der Unfassbare (2006), “Ph-Meter” [photograph]. Retrieved from Datamax (2008). “A pH meter” [photograph]. Retrieved from File:PH_Meter.jpg KoiQuestion (2007), “pH meter” [photograph]. Retrieved from 10/

pH Meter – Calibration and Use

Commercial pH Calibration Buffers World Health Organization (2008), “pH Meter”. Retrieved from library.ewh.org BUFFER SOLUTION

Calibrating a pH Meter Make sure the meter is in pH mode For a 3 point calibration, use high pH (10.0), pH 7.0 and low (4.0) solution Before calibration, rinse probe thoroughly with de- ionized water or a rinse solution Immerse the end of the probe completely in the calibration solution Stir the probe gently to create a homogenous sample

Measuring the pH of a Solution Always rinse electrodes with de-ionized water prior to placing in a solution for pH measurement Allow meter to stabilize for 30 seconds or a minute, then read Remove electrodes and rinse with de-ionized water The pH bulb should always be stored wet preferably in pH 4.0 buffer with 1/100 KCl Other buffers or tap water can be used for storage World Health Organization (2008), “pH Meter”. Retrieved from library.ewh.org

Maintenance of pH Meter Wipe off exterior of pH meter with a damp cloth after useWipe off exterior of pH meter with a damp cloth after use The outside of the meter is made of polyester and is not affected by most solvents but is affected by some organic solventsThe outside of the meter is made of polyester and is not affected by most solvents but is affected by some organic solvents

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