الاس الهيدروجينى والاكسجين المذاب/PH/DISSOLVED OXYGEN

TECHNOLAB EL-BAHAA GROUP

GENERAL.DR

BAHAA BADR

pH

is a measure of how acidic or alkaline a solution is.

In pure water at room temperature, a small fraction (about two out of every billion) of the water molecules (H2O, or really, H-O-H) splits, or dissociates, spontaneously, into one positively charged hydrogen ion (H+) and one negatively charged hydroxide ion (OH-) each.

There is an equal number of each ion, so the water is said to be "neutral".

Some materials, when dissolved in water, will produce an excess of (H+), either because they contain these ions and release them when they dissolve, or because they react with the water and cause it to produce the extra hydrogen ions.

Substances which do this are called acids.

Likewise, some chemicals, called bases or alkalis, produce an excess of hydroxide ions.

The scale which is used to describe the concentration of acid or base is known as pH, for power or potential of the Hydrogen ion.

A pH of 7 is neutral. pH's above 7 are alkaline (basic); below 7, acidic.

The scale runs from about zero, which is very acidic, to fourteen, which is highly alkaline.

The scale is logarithmic, meaning that each change of one unit of pH represents a factor of 10 change in concentration of hydrogen ion.

So a solution which has a pH of 3 contains 10 times as many (H+) ions as the same volume of a solution with a pH of 4, 100 times as many as one with a pH of 5, a thousand times as many as one of pH6, and so on.

Some common materials and their approximate pH's are: Acids--- carbonated beverages, 2 to 4; lemon juice, about 2.3; vinegar,about 3; Bases: baking soda, 8.4; milk of magnesia.10.5; ammonia,11.7;lye,14 to 15.

While the pH measures the concentration of hydrogen or hydroxide ions, it may not measure the total amount of acid or base in the solution.

This is because most acids and bases do not dissociate completely in water.

That is, they only release a portion of their hydrogen or hydroxide ions.

A strong acid, like hydrochloric acid, HCl, releases essentially all of its H+ in water.

The concentration of H+ is the same as the total concentration of the acid.

A weak acid, like acetic acid (the acid in vinegar), may release only a few percent of the hydrogen that it has available.

If you are trying to neutralize an acid by adding a base, like sodium hydroxide, the amount you would need to neutralize a strong acid could be calculated directly from the pH of the acid solution.

But for a weak acid, the pH does not tell the whole story; the total amount of base needed would be a lot more.

This is because as the OH- from the base reacts with the H+ in solution to form water, more H+ will break loose from the undissociated portion of the acid to take its place.

The neutralization will not be complete until all of the weak acid has dissociated.

To measure the total acidity, also called base-neutralizing capacity (BNC) of a water sample, it has to be titrated with base.

That is, a solution of a base whose concentration is known must be added to the water sample slowly until the neutralization is complete.

By measuring the volume of the base added, you can figure out the original concentration of acid.

In a similar way, the acid-neutralizing capacity (ANC), or alkalinity of a water sample has to be determined by titrating it with a solution of a strong acid of known concentration.

For a more technical explanation of pH and alkalinity, look at this "mini-tutorial", which includes formulas, reactions, examples, and titration curves.

Significance: Although there are some microorganisms which can function at extreme pH's, most living things require pH's close to neutrality.

Many enzymes and other proteins are denatured by pH's which differ much from pH7, which disrupts the functioning of the organism and may kill it.

Besides the harm to aquatic life in natural waters, pH imbalances can inhibit-- or completely wipe out-- biological processes in wastewater treatment plants, resulting in incomplete treatment and pollution of the receiving waters.

Low (acidic) pH's also cause corrosion in sewers systems and increase the release of toxic and foul-smelling hydrogen sulfide gas.

(This gas has been responsible for the deaths of numerous sewer workers.)

Low pH's also increase the release of metals, some toxic, from soils and sediments.

Alkalinity is an important parameter because it measures the water's ability to resist acidification, for instance, to acid rain.

In wastewater treatment, some processes produce acidity.

If there is not enough alkalinity to neutralize it, the pH of the process can drop and cause it to become inhibited. Alkalinity can be supplemented by chemical addition to avoid this.

Measurement:

There are indicator solutions which change color in different pH ranges, and these can be used for approximate estimation of pH in solutions which contain high enough concentrations of pH-determining ions.

"pH paper",

impregnated with such indicators, are commonplace in testing laboratories.

For accurate measurements and use in dilute solutions, electrochemical measurement (a "pH meter") is required.

Alkalinity and acidity are determined by titration with strong base or acid, respectively, using either indicators or a pH meter to mark the endpoint.
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Dissolved Oxygen (D.O.):

Like solids and liquids, gases can dissolve in water.

And, like solids and liquids, different gases vary greatly in their solubilities, i.e, how much can dissolve in water.

A solution containing the maximum concentration that the water can hold is said to be saturated. Oxygen gas, the element which exists in the form of O2 molecules, is not very water soluble.

A saturated solution at room temperature and normal pressure contains only about 9 parts per million of D.O. by weight ( 9 mg/L).

Lower temperatures or higher pressures increase the solubility, and visa versa.

Significance:

Dissolved oxygen is essential for fish to breathe.

Many microbial forms require it, as well.

The oxygen bound in the water molecule (H2O) is not available for this purpose, and is in the wrong "oxidation state", anyway.

The low solubility of oxygen in water means that it does not take much oxygen-consuming material to deplete the D.O.

As mentioned before, the biodegradation products of bacteria which do not require oxygen are foul-smelling, toxic, and/or flammable.

Sufficient D.O. is essential for the proper operation of many wastewater treatment processes. Activated sludge tanks often have their D.O. monitored continuously.

Low D.O.'s may be set to trigger an alarm or activate a control loop which will increase the supply of air to the tank.

Measurement:

D.O. can be measured by a fairly tricky wet chemical procedure known as the Winkler titration.

The D.O. is first trapped, or "fixed", as an orange-colored oxide of manganese.

This is then dissolved with sulfuric acid in the presence of iodide ion, which is converted to iodine by the oxidized manganese.

The iodine is titrated using standard sodium thiosulfate.

The original dissolved oxygen concentration is calculated from the volume of thiosulfate solution needed.

Measurements of D.O. can be made more conveniently with electrochemical instrumentation.

"D.O. meters" are subject to fewer interferences than the Winkler titration.

They are portable and can be calibrated directly by using the oxygen in the air.

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