3-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية النظرى(قياس الاس الهيدروجينى)

physical parameters of water

pH

pH measures hydrogen concentration in water and is presented on a scale from 0 to 14. A solution with a pH value of 7 is neutral; a solution with a pH value less than 7 is acidic; a solution with a pH value greater than 7 is basic.

Natural waters usually have a pH between 6 and 9.

The scale is negatively logarithmic, so each whole number (reading downward) is ten times the preceding one (for example, pH 5.5 is 100 times as acidic as pH 7.5).

The pH of natural waters can be made acidic or basic by human activities such as acid mine drainage and emissions from coal-burning power plants and heavy automobile traffic.

pH

pH represents the effective concentration (activity) of hydrogen ions (H+) in water.

This concentration could be expressed in the same kind of units as other dissolved species, but H+ concentrations are much smaller than other species in most waters.

The activity of hydrogen ions can be expressed most conveniently in logarithmic units.

pH is defined as the negative logarithm of the activity of H+ ions:

pH = -log [H+]

where [H+] is the concentration of H+ ions in moles per liter (a mole is a unit of measurement, equal to 6.022 x 1023 atoms).

Because H+ ions associate with water molecules to form hydronium (H3O+) ions, pH is often expressed in terms of the concentration of hydronium ions.

In pure water at 22 C (72 F), H3O+ and hydroxyl (OH-) ions exist in equal quantities; the concentration of each is 1.0 x 10-7 moles per liter (mol/L).

Therefore, pH of pure water = -log (1.0 x 10-7) = -(-7.00) = 7.00. Because pH is defined as –log [H+], pH decreases as [H+] increases (which will happen if acid is added to the water).

Since pH is a log scale based on 10, the pH changes by 1 for every power of 10 change in [H+].

A solution of pH 3 has an H+ concentration 10 times that of a solution of pH 4. The pH scale ranges from 0 to 14. However, pH values less than 0 and greater than 14 have been observed in very rare concentrated solutions.

• Water Quality Standard - Values outside the standard (pH 6.5 - 8.3) indicate poor water quality

Measurement of pH

The pH of water can be measured with a pH meter, which is an electronic device with a probe.

The probe contains an acidic aqueous solution enclosed by a glassmembrane that allows migration of H+ ions.

The electrical potential of the glass electrode depends on the difference in [H+] between the reference solution and the solution into which the electrode is dipped.

pH can also be measured with pH paper or by adding a reagent (indicator solution) to the water sample and recording the color change.

Factors Affecting pH:

The concentration of carbon dioxide in the water

Carbon dioxide (CO2) enters a water body from a variety of sources, including the atmosphere, runoff from land, release from bacteria in the water, and respiration by aquatic organisms.

This dissolved CO2 forms a weak acid. Natural, unpolluted rainwater can be as acidic as pH 5.6, because it absorbs CO2 as it falls through the air.

Because plants take in CO2 during the day and release it during the night, pH levels in water can change from daytime to night.

Geology and Soils of the watershed

Acidic and alkaline compounds can be released into water from different types of rock and soil.

When calcite (CaCO3) is present, carbonates (HCO3, CO3-2) can be released, increasing the alkalinity of the water, which raises the pH.

When sulfide minerals, such as pyrite, or "fool’s gold," (FeS2) are present, water and oxygen interact with the minerals to form sulfuric acid (H2SO4).

This can significantly drop the pH of the water. Drainage water from forests and marshes is often slightly acidic, due to the presence of organic acids produced by decaying vegetation.

Drainage from Mine Sites

Mining for gold, silver, and other metals often involves the removal of sulfide minerals buried in the ground.

When water flows over or through sulfidic waste rock or tailings exposed at a mine site, this water can become acidic from the formation of sulfuric acid.

In the absence of buffering material, such as calcareous rocks, streams that receive drainage from mine sites can have low pH levels.

Air Pollution

Air pollution from car exhaust and power plant emissions increases the concentrations of nitrogen oxides (NO2, NO3) and sulfur dioxide (SO2) in the air.
These pollutants can travel far from their place of origin, and react in the atmosphere to form nitric acid (HNO3) and sulfuric acid (H2SO4).

These acids can affect the pH of streams by combining with moisture in the air and falling to the earth as acid rain or snow.

Water Quality Standards and Other Criteria Regarding pH

The U.S. Environmental Protection Agency (U.S. EPA) sets a secondary standard for pH levels in drinking water:

the water should be between pH 6.5 and 8.5 Secondary standards are unenforceable, but recommended, guidelines.

waters to be used for domestic water supply should have pH values between 5.0 and 9.0

Mathematical definition
pH is defined as a negative decimal logarithm of the hydrogen ion activity in a solution.

where aH is the activity of hydrogen ions in units of Mol/L (molar concentration).
Activity has a sense of concentration, however activity is always less than the concentration and is defined as a concentration (Mol/L) of an ion multiplied by activity coefficient.

The activity coefficient is a number between 0 and 1 and it depends on many parameters of a solution, such as nature of ion, ion force, temperature etc.

Activity can be measured experimentally by means of an ion-selective electrode which responds, according to the Nernst equation, to hydrogen ion activity.

pH is commonly measured by means of a glass electrode connected to a milli-voltmeter with very high input impedance which measures the potential difference, orelectromotive force, E, between an electrode sensitive to the hydrogen ion activity and a reference electrode, such as a calomel electrode or a silver chloride electrode.

Quite often glass electrode is combined with the reference electrode and a temperature sensor in one body.

The glass electrode relatively good (95 - 99.9%) follows the Nernst equation:

Where

E is a measured potential

E0 is the standard electrode potential, that is, the electrode potential for the standard state in which the activity is one.

R is the gas constant.

T is the temperature in kelvins,

F is the Faraday constant

n is the number of electrons transferred (ion charge).

The electrode potential, E, is proportional to the logarithm of the hydrogen ion activity.

the hydrogen ion activity is the product of the concentration and an activity coefficient