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pH the measurement of acidity or alkalinity in aqueous solutions

pH is the scale of measurement of acidity or alkalinity in aqueous (water based) solutions. A neutral solution such as pure water has a pH of 7. Solutions with a lower pH are termed acidic and solutions with a higher pH are termed alkaline. pH ranges from highly acidic pH 0 to highly alkaline pH 14. The pH of the human eye is about 7.5.

pH was introduced by a Danish biochemist Soren Peter Lauritz Sorensen in 1909 to measure the acidity of water in the brewing of beer. The letters pH are an abbreviate for pondus hydrogenii (translated as potential hydrogen) meaning hydrogen power as acidity is caused by a predominance of hydrogen ions (H+).

Water is H₂O, this is two hydrogen atoms and one oxygen atom. Water is a polar molecular shaped like a boomerang with the oxygen atom in the middle and the hydrogen atoms at the tips. This shape comes about because of the way the 8 negatively charged electrons in oxygen pack themselves in close to the 8 positively charged nucleus of the oxygen atom. The nucleus of an atom is very small like a sparrow in a cloud (of electrons). The Hydrogen atoms at the tips have more positive charge than negative charge and the oxygen atom has more negative charge than positive charge. This causes the molecule to be polar, that is to have an electrical field difference along it. The tips of the boomerang are positively charged and the middle bend is negatively charged.

Water dissociates (breaks up) into H⁺ and OH^- ( H₂O < == > OH^- + H⁺). The energy of the atoms bumping into each other cause the water molecules to split up. Because water is a polar molecule the H⁺ will attract the negatively charged Oxygen atoms in other water molecules and will “float” away. The OH^- will attract the Hydrogen in other water molecules and float away on its own. Eventually a H⁺ and a OH^-  will bump into each other and join up to become water again. This process of dissociation and association is dynamic.

In pure water, for every one H⁺ There are 10,000,000 normal water molecules. This is 1 in10⁷ and the pH of pure water is 7 meaning for every 10⁷ molecules of water there is a H⁺ ion. Said in another way the H⁺ concentration is 10^-7 parts (pH of 7), that is one part in 10Million parts. Likewise with the OH^-. If the amount of H⁺ increases by adding acidic compounds (hydrochloric acid HCl < == > H⁺ and Cl^-) then there are more H⁺'s to combine with OH^-'s so the concentration of OH^- is reduced. Likewise adding OH^- by adding an alkaline compound (Caustic Soda NaOH < == > Na⁺ and OH^-) reduces the concentration of H⁺. A Chemical rule is that the concentration of H⁺ times the concentration of OH^- always equals 10^-14.

Low pH (acidic) conditions cause water to be corrosive. Acids will cause pitting of concrete, dissolve metals, wrinkle vinyl,  irritate skin and eye and stains pool and spa walls and pipes. 

High pH (caustic) conditions cause scaling - minerals precipitate out of the water (calcium, copper, iron etc. and those minerals will block filters and pipes. 

The numerical value of the pH is the negative of the exponent of the molar concentration. A mole is defined as the weight in grams that corresponds to the summed atomic weights of all the atoms of a molecule - its molecular mass. In the case of H+ the molecular mass equals 1, and a mole of H+ ions would weigh 1 gram. In the case of water (H2O) the molecular mass equals 34, and a mole of water ions would weigh 18 grams (The sum of the atomic weights).

Thus low pH values indicate high concentrations of H+ ions (acid), and high pH values indicate low concentrations. Each pH unit downward represents, therefore, a tenfold increase in the H+ concentration. A pH of 3, for example, indicates a 10-3 molar concentration of hydrogen ions. To put it simply the pH scale is logarithmic, meaning that every pH unit means "10 times". Therefore a pH of 6 is 10 times more acidic than a pH of 7, and a pH of 3 is 1000 times more acidic than 6 and 10,000 times more acidic than 7.

To increase pH use sodium carbonate, caustic soda (sodium hydroxide) or baking soda (sodium bicarbonate) - any alkaline compound. These compounds add to the water OH^- ions.

To decrease your pH use muriatic acid (HCl - hydrochloric acid), or by using dry acid (sodium bisulfate) or any acidic compound.

Combining an acidic compound with an alkaline compound produces a salt. Sodium Hydroxide plus Hydrochloric acid when mixed will become very hot and result in a salt - Sodium Chloride. Salts by themselves can increase the rate of corrosion and mineral precipitates.

Some salts give water the ability to resist changes in pH or buffer the water from wild pH swings. In water that contains no buffering ability, the pH can wander dramatically with the addition of small amounts of acids or bases (alkali), or other pH altering agents like chlorine or bromine. With a buffer the pH will hold steady and pH bounce will be eliminated for a while until so much acid or alkaline has been added that the buffering effect is overwhelmed.




Measuring pH

Two parts are involved in a pH electrode system. The first is a pH sensor, which consists of a special pH sensitive glass and whose voltage output is proportional to the pH. The second is a reference sensor which provides a stable and constant reference point. Electrical contact is made with the solution using a saturated salt solution (usually KCl) which leaks slowly out of a porous junction. These two sensors are usually built into the same housing, hence the term "combination" pH electrode.

A pH meter is simply a device which measures the voltage from the electrodes and converts it to a pH reading on a display.

In theory, the output of a pH electrode is zero millivolts (mV) at pH7. At 25 degrees Celsius the output of the electrode changes by 59 mV per pH. At pH4 the output is +177mV, and at pH 10 the output is -177mV.

In practice, the output varies from electrode to electrode. The "zero" output (called the "asymmetry") may be slightly away from pH7, and the mV change per pH (called the "slope" or "span") may be slightly lower than 59 (especially as the probe ages). Therefore, pH meters must have two calibration controls, which are adjusted when the pH electrode is immersed in pH buffers (solutions of a known pH which are resistant to pH change).

The first control, usually labelled CALIBRATE (also ISO, ZERO, ASYMMETRY), is used to calibrate the zero point of the electrode. This is done in a pH buffer at or near pH7. Instruments Australia supplies pH6.88 buffer. This calibration MUST be done FIRST.

The second calibration point calibrates the slope of the electrode. This is usually labelled SLOPE (also SPAN). In order for this calibration step to be effective, it must be done at least 2 to 3 pH away from 7. Instruments Australia supplies pH4.00 buffer. This calibration MUST be done SECOND.

Micro-processor based instruments calculate the asymmetry and slope regardless of which buffer they are calibrated in, so it is not necessary to calibrate at 7 and then at 4.

The frequency of calibration varies from application to application. As a guideline, calibration should be checked at least weekly.

Factors that affect the output of pH electrodes include temperature changes, a blocked reference junction, and coatings on the pH glass.

Temperature effects can be compensated for in two ways. Firstly, the pH meter can have a manual control which can be set to the temperature of the solution. Secondly, the pH meter can be fitted with an Automatic Temperature Compensation ("ATC") probe, which, as the name suggests, measures the temperature and automatically compensates for the temperature effect on pH electrode performance.

Prepared by Trevor Croll,

42 Pearse Street, Keperra, Queensland Australia, 4054. Phone 61 7 3855 1115,

Email........... tcroll@bigpond.net.au