Analysis of Inorganic Substances

Inquiry question: How are the ions present in the environment identified and measured?

Why do we need to monitor the environment chemically?

Take a look at the video above where an algal bloom threatened to cancel the sailing events in the 2008 Beijing Olympic Games in China. It covered one third of the sea off the city of Qindao. Also look at the consequences of algal blooms.

Which cations can we identify?

Cations include metals, such as Ba2+, Ca2+, Mg2+, Pb2+, Ag+, Cu2+, Fe2+ and Fe3+, and ammonium ion Ba2+ and hydronmium ion Ba2+.

We use precipitation reactions, flame tests and complexation reactions to identify the presence of specific cations.

Below is a diagram showing seven examples of flame tests. Limitations of the flame test:

• it relies on the experimenter's subjective idenitification of the colour

• it does not easily detect small concentrations of some elements

• some cations, such as sodium ions, often contaminate samples and the bright yellow of its flame can easily mask the presence of other ions

• some cations do not produce a visible flame colour.

Which anions can we identify?

Fertilisers containing phosphate PO43- and nitrate NO3- promote root and leaf growth in plants, and are used extensively for agricultural crops. These are washed into creeks and streams by runoff after storms. Especially in summer months, sunlight will cause algae to spread and become an algal bloom, which spreads over the surface of the waterway. As a result, sunlight is blocked from reaching underwater plants, which then die. The populations of any aquatic organisms that feed on these submerged plants is also reduced or become zero. This leads to death of living organisms and high levels of pollution in the waterway.

A precipitation reaction is commonly used to identify anions in solution.

An indicator, such as litmus paper, can be used to identify the presence of carbonate PO32-, hydroxide HO- and acetate CH3COO- ions. Acetate ions do not form precipitates with metal ions at dilute concentrations, so its presence is deduced by the ansence of results.

To identify the halide ions Cl-, Br-  and I-, silver nitrate is used.

How can we measure the concentration of cations or anions?

Precipitation titrations can be used to determine the quantity of a particular ion present in a solution.  Three methods used are:

• Mohr's method - direct titration using a known concentration of silver nitrate to determine the amount of chloride, bromide or cyanide in a sample; requires pH between 6 and 9; is not suitable for I-; blank titration required

• Volhard's method - back titration using an acidic solution to determine the amount of anions in a particular solution; can only be used for low pH levels and if precipitate needs to be removed, this introduces possible transfer errors

• Fajan's method - direct titration using a specific absorption indicator to determine the amount of chloride, bromide or iodide in a sample; cannot be used if ion concentration is too low or other ions too high

We can also use atomic absorption spectroscopy (AAS) to find the amount of an element that is present (quantitative analysis).

Analysis of Organic Substances

Inquiry question: How is information about the reactivity and structure of organic compounds obtained?

Chemical Synthesis and Design

Inquiry question: What are the implications for society of chemical synthesis and design?