United Christian College

AL Chemistry Project

Presented By Class 6B

•Leung Wei Ching

•Cheng ka Lun

•Tang Ka Yen

•Fai

To compare the degree of pollutions in different places in HK.

Our Aim

After a long journey, 5 places of water samples are separately filled in the 5 bottles.

average seawater has a salinity of ~35‰

additional minor solutes found in seawater

nutrients (compounds of nitrogen, phosphorus, and silica)

dissolved gases (molecular oxygen and carbon dioxide)

trace elements (typically metals)

organic compounds (fats, proteins, carbohydrates)

chloride (Cl-) and sodium (Na+) ions comprise over 85% of the seawater solutes

Composition of seawater

As everyone knows, except water, the major component in sea water is Chloride ions.That’s why sea water tasted briny.

The reasons why we measured the chloride concentration are that it can indicate pollution by sewage or industrial wastes. A high chloride content has a corrosive effect on metal pipes and structures, also harmful to most trees and plants.

Composition of seawater

Salinity Heavy metal Acidity Hardness

as indication.

Outline

To determine the degree of pollutions in the sea water, we based on

Outline

1. Silver Nitrate titration method – to determine the salinity…

2. Heavy metal determination – to determine the heavy metal ions

3. pH meter – to determine the acidity

4. Traditional detective method of hardness of water – to determine how hard…

In these water samples, we have carried out a series of experiments, and they are…

of each water sample

Silver Nitrate Titration

Silver Nitrate has a special properties that its silver cat ions react with chloride ions to form a white precipitate called Silver Chloride. So that it can be used to determine the amount of chloride (salinity) of the water sample.

Cautions: Silver nitrate can stain the hands

Ag+ + Cl- → AgCl

Silver Nitrate Titration

Indicator

Silver chloride is quantitatively precipitated before red silver chromate is formed. This time, potassium chromate acts as an indicator, the colour changed at end-point from yellow to red.

Silver Nitrate Titration

Silver Nitrate Titration

First: Ag+ + Cl- → AgClSecond: Ag+ + CrO4

2- → Ag2CrO4

Result

Silver Nitrate Titration

深井＞赤柱＞尖沙嘴＞官塘＞銀線灣

After the complicated procedure, we found that the salinity sequences of places are:

Indicated that 深井 has a heavier chloride pollution!

Heavy metal determination

Heavy metal determination

Metals occur naturally in the environment and are present in rocks, soil, plants, and animals. Metals occur in different forms: as ions dissolved in water, as vapors, or as salts or minerals in rock, sand, and soil. They can also be bound in organic or inorganic molecules, or attached to particles in the air. Both natural and anthropogenic processes and sources emit metals into air and water.

Heavy metal determination

Ethylene-Diamine-Tetraacetic acid disodium (EDTA)

This is the method that determine the % of heavy metal ions in sea water sample.

Since it reacts with heavy metal ions to form a equilibrium mixture, using Eriochrome Black T as an indicator.

Heavy metal determination

However, this indicator only works in alkaline medium. Hence a alkaline buffer is required.

Indicator

This alkaline buffer is the mixture of concentrated ammonia and ammonium chloride

Caution: Ammonia is toxic and corrosive, giving an unpleasant smell.

Buffer preparation

Heavy metal determination

Ammonia Buffer

Result comparisons

5 places were tested and gave the following

result…

Heavy metal determination

Hardness of water

Hardness of water

This means the water sample contained large amount of calcium and Magnesium ions.

Scum forming

We added some soapy detergent into the water sample. There is a layer of thick, sticky, insoluble scum is formed.

Hardness of water

To increase the efficiency, the soap was taken to dissolve in the distilled water.

Hardness of water

The resultant solution is then filtered to separate the solid and liquid content.

The resultant solution is then filtered to separate the solid and liquid content. The wet filter paper and the remained residue are taken to oven.This can rapidly vaporize the water out.

Hardness of water

The dried samples are taken to weigh by electronic balance.

The following results are obtained 銀線灣＞ TST ＞深井＞Stanley ＞官塘

Hardness of water

General hardness is primarily the measure of calcium (Ca++) and magnesium (Mg++) ions in the water. Other ions can contribute to General hardness but their effects are usually insignificant and the other ions are difficult to measure. General hardness will not directly affect pH although "hard" water is generally alkaline due to some interaction of general hardness and carbonate hardness. In common situation, carbonate hardness can be reduced by boiling the water impractical for all but the smallest aquariums; let it cool before adding to the tank or by peat filtering.

Affect our environment

If the local general hardness is too low, it can be raised by adding calcium sulfate and/or magnesium sulfate. This has the drawback of introducing sulfates (SO4--) into the water, so care should be exercised. Calcium carbonate can be used, but it will also raise the carbonate hardness (this is ideal for the lucky few who have naturally soft water). Various combinations can be used to produce the desired results.

Affect our environment

General hardness is the more important of the two in biological processes. When a fish or plant is said to prefer "hard" or "soft" water, this is referring to general hardness . Incorrect general hardness will affect the transfer of nutrients and waste products through cell membranes and can affect egg fertility, proper functioning of internal organs such as kidneys and growth. Within reason, most fish and plants can successfully adapt to local general hardness conditions, although breeding may be impaired.

Acidity of water

Final StageAcidity

pH meter inserted into the water sample to measure its pH

Acidity of water

The pH values are obtained as followings

Places: 官塘＞銀線灣＞深井＞ 尖沙嘴＞ 赤柱 pH 6.7 7.0 7.1 7.3 7.4

Acidity of water

Acidity of water

In simple terms, pH is determined by the negative log of the concentration of free hydrogen ions (H+) in the water. If you add a strong acid such as nitric acid to water, it completely dissociates into hydrogen ions (H+) and its "conjugate base" or "salt", NO3- or nitrate. The hydrogen ions freed in the reaction then increase the concentration of hydrogen ions and reduce the pH. Since nitric acid is the end product of the nitrogen cycle, this explains why aquarium pH tends to decrease and nitrates tend to increase over time

Affect the environment

It is well known that dissolving CO2 in sea water will decrease

the pH of the water, due to the formation of bicarbonate and carbonate ions. It is also known that marine animals are, in general, intolerant to changes in the pH of the water in which they live.

But how severe would the pH changes be from CO2

sequestration, and what would be the impact on the ecology?

Affect the environment

It is no doubted that when the carbon dioxide dissolved in the seawater, it will lower the pH value in the sea. Also the micro-organism, bacteria in the sea will give out carbon dioxide.

The organisms in the sea may suffer this, and some may be died. As the concentration of hydrogen ions in the seawater is high, it will affect the organism’s metabolism.