Effect of temperature on pH of water samples

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     Lab on effect of temperature on pH of water          

Lab type: DCP & CE/DCE.

Theory:

Water contains both hydrogen ions, H+, and hydroxide ions, OH–. The relative concentrations of these two ions determine the pH  value.1 Water with a pH  of 7 has equal concentrations of these two ions and is considered to be a neutral solution. If a solution is acidic, the concentration of H+ ions exceeds that of the OH– ions. In a basic solution, the concentration of OH– ions exceeds that of the H+ ions. On a pH  scale of 0 to 14, a value of 0 is the most acidic, and 14 the most basic. A change from pH  7 to pH  8 in a lake or stream represents a ten-fold increase in the OH– ion concentration. Rainfall generally has a pH  value

Between 5 and 6.5. It is acidic because of dissolved carbon dioxide and air pollutants, such as sulfur dioxide or nitrogen oxides. If the rainwater flows over soil containing hard-water minerals, its pH  usually increases. Bicarbonate ions, HCO3–, resulting from limestone deposits

react with the water to produce OH– ions, according to the equation:

HCO3 + H2O → H2CO3 + OH

As a result, streams and lakes are often basic, with pH  values between 7 and 8, sometimes as high as 8.5. The measure of the pH  of a body of

water is very important as an indication of water quality, because of the sensitivity of aquatic

organisms to the pH  of their environment. Small changes in pH  can endanger many kinds of plants

and animals; for example, trout and various kinds of nympH s can only survive in waters between pH  7 and  pH  9. If the pH  of the waters in which they live is outside of that range, they may not survive or reproduce.

Changes in pH  can also be caused by algal blooms (more basic), industrial processes resulting in a release of bases or acids (raising or lowering pH ), or the oxidation of sulfide-containing sediments (more acidic). Acidic rainfall may have very little effect on the pH  of a stream or lake if the region is rich in minerals that result in high alkalinity values. Other regions may have low concentrations of alkalinity ions to reduce the effects of acids in the rainfall. In the Northeastern United States and Eastern Canada, fish populations in some lakes have been significantly lowered due to the acidity of the water caused by acidic rainfall. If the water is very acidic, heavy metals may be released into the water and can accumulate on the gills of fish or cause deformities that reduce the likelihood of survival. In some cases, older fish will continue to live, but will be unable to reproduce because of the sensitivity of the reproductive portion of the growth cycle.

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Expected Levels

The pH  value of streams and lakes is usually between pH  7 and 8. Levels between 6.5 and 8.5 pH  are acceptable for most drinking water standards. Areas with higher levels of water hardness often have water with higher pH  values (between 7.5 and 8.5).

A word of warning!

If the pH  falls as temperature increases, does this mean that water becomes more acidic at higher temperatures? NO!

A solution is acidic if there is an excess of hydrogen ions over hydroxide ions. In the case of pure water, there are always the same number of ...

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