Chemistry - Testing Water pH

What is pH and the pH scale? – A more complex approach.

In Chemistry, pH is the measure of the acidity or alkalinity of an aqueous solution. Pure water is said to be neutral, with its pH being approximately 7.0 at r.t.p. The pH scale ranges from 0.0 to 14.0. Aqueous solutions with a pH value from 7.0-14.0 are said to be alkaline or basic and solutions with pH from 0.0-7.0 are said to be acidic. pH measurements are important in many fields, including medicine, biology, chemistry, agriculture, forestry, food science, oceanography, civil engineering and many others.



The idea of pH is actually a mathematical concept as the pH scale is a logarithmic one. This means that each change of pH 1 in the scale corresponds to 10 times more or less acidity/alkalinity. For example, pH 4 is ten times more acidic than pH 5 and 100 times more acidic than pH 6. The same holds true for pH values above 7, as pH 11 is 10 times more alkaline than pH 10 and 100 times more alkaline than pH 9, for example.
As the pH measures the acidity/alkalinity in an aqueous solution, the idea behind this concept lies on the ionization of water. Water molecules exist in equilibrium with hydrogen and hydroxide ions as some of the molecules tend to dissociate into ions. The water equilibrium constant is written as:

Kw = [H+][OH-]


Experimentally, it has been found that [H+] = [OH-] = 10-7.
Therefore K=[10-7][ 10-7] = [10-14]

Now, as an acid is known to donate H+ ions (proton donor) and an alkali is known to accept H+ ions (proton acceptor), if an acid (which contains the H+ ion) is added to water, the equilibrium shifts to the left and the OH- ion concentration decreases. The opposite happens when a base (containing OH- ions) is added to the solution, where the concentration of H+ ions decreases. This follows the Water Equilibrium Principle: the product of H+ and OH- ion concentration must always be equal to 10-14.

The concentrations of hydrogen ions and, therefore, indirectly, of hydroxide ions are given by a pH number. 
pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is:
pH = -log [H+]
Thus, this means that H+ ion concentration and pH are inversely proportional whereas OH- ion concentration and pH are directly proportional.

pH in seawater
Due to its salinity (approx. 3.5% of the average seawater’s mass), seawater tends to be a little alkaline, being limited to the range of 7.0/7.5-8.4. However, impure seawater tend to leave this range and, as this is a global one, in specific locations, smaller ranges can be established which therefore means that smaller  variations may determine if the water is pure or impure.

Where our Project comes in
In our project, the group is going to measure the pH of two samples of water: one from the Lagoa Rodrigo de Freitas and one from the Ipanema beach. As it was previously mentioned, the Lagoa Rodrigo de Freitas is a lagoon connected to the Ipanema beach. Therefore, it contains salt water but its salinity is reduced compared to that of Ipanema beach as it receives fresh water from rainwater and rivers which flow into the lagoon, such as the Rio dos Macacos. By knowing the pH value of a sample for each location, we will be able to know the level of purity of each site ad consequently, the level of pollution.

Method

There are many ways to measure the pH of a substance. Some of them are by using pH indicator paper, Universal indicator solution, a probe and meter and litmus paper. Save for the probe and meter, which was not available to the group at the school, the other forms of measurement can turn out to be subjective to a certain extent. Therefore, the group reached a consensus of using two methods: the universal indicator solution and the pH indicator paper.

1.       After having followed all of the safety precautions necessary, place one pH indicator strip on a petri dish and, using a pipette, add a few drops of one of the samples of water (Lagoa or Ipanema) until the strip is covered in water. Wait a few seconds and record the reading according to the table on the pH indicator paper package.

Comparing colours obtained with those in the scale. (Ipanema - right, Lagoa, left)
2.       Repeat the same process with the other sample of water (Lagoa or Ipanema).

Gabriel comparing the results from each sample with the pH scale from the Indicator paper package.
3.       Place 50 cm3 of one of the samples (Lagoa or Ipanema) inside a large test tube. Using a pipette, add a few (3-4) drops of Universal Indicator solution to the sample and mix it. With a copy of a precise pH table beside, compare the colour seen with the one in the pH table and record the pH.

The difference in colour (and therefore pH) between the two samples (Ipanema - right, Lagoa -left).

4.       Repeat the same process with the other sample of water (Lagoa or Ipanema).

Results


pH Indicator Paper/ pH
Universal Indicator Solution/ pH
Ipanema Beach
7
7.5
Lagoa Rodrigo de Freitas
8.5
9

These results may be seen more clearly in the graph below.




Conclusion

As it was mentioned previously, the pH of seawater varies within the range of 7.0/7.5-8.4. It therefore may be seen that the water in Ipanema, through this experiment which analyses only the variable of pH, may be considered clean as it is near the neutral zone, which indicates that it has no additional impurities which may change its pH. The Lagoa's water, however, surpasses the acceptable range and can be considered significantly alkaline, especially due to the fact that its level of salinity is smaller than that of Ipanema and therefore, the change in pH is caused mainly by impurities due to pollution instead of dissolved minerals. A plausible reason for the high pH may be the high amount of heavy metals which studies have proven to have found in the Lagoa's water, as mentioned in the introduction to the project. These heavy metals can react with the water, forming metal hydroxide, which are basic substances which thus increases the water's pH.

Sources of Reference:


http://www.elmhurst.edu/~chm/vchembook/184ph.html
http://www.inmetro.gov.br/consumidor/produtos/praias.asp
http://www.gewater.com/handbook/Introduction/ch_1_sourcesimpurities.jsp

Nenhum comentário:

Postar um comentário