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Digital pH Meters- Table Model (Acm-34091-R)

digital pH meter manufacturers

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 (H 2 O, 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. (Some of these figures are from the Handbook of Chemistry and Physics, 56th ed., CRC Press,1976).

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.

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