1. Safety Before starting work, students must familiarise themselves with the safety and handling information provided with the procedures. Laboratory coat and safety glasses must be worn at all times when in the laboratory. Any additional personal protective equipment provided, such as gloves, must be used. If you are in any doubt of how to proceed - consult the lecturer or demonstrator

2. Assessment This is split equally between the reports, the notebook and the quality of the results.

3. The KS Practicals The KS Labs are analyses performed on the same white crystalline, water soluble, solid. The solid contains potassium (between 5 and 15%) and sulfate (between 15 and 40%). In KS1 you will determine K, in KS4 you will determine SO 4 2-. The solid is NOT pure potassium sulphate.

4. KS1 Determination of Potassium Using standard additions with Flame Emission Spectroscopy (FES)

5. Theory Electrons are found around atomic nuclei in discrete energy levels as described by quantum theory. Each energy level can hold a defined maximum number of electrons, and the lower energy levels are filled preferentially.

6. Thus, in the ground state, no electron is found in a particular energy level if a vacancy exists at a lower level. It is possible to promote an electron to a higher energy level, creating a temporary vacancy. The atom is then said to be in an excited state. Production of the excited state requires energy, in this case from a flame The excited state produced is very short lived and the electron returns to the lower energy level. The energy is re-emitted as a photon of EM radiation producing an emission spectrum. As the energy of a particular level is defined by quantum theory, so the differences in energy between levels are also defined, and depend upon the element involved.

7. In this experiment you will Observe the emission spectrum of sodium. Use a flame to raise potassium atoms to an excited state, and then measure the intensity of emitted light in order to determine the concentration of potassium in a sample. The apparatus, fitted with a potassium filter, responds to the concentration of potassium ions in the test solution. An electronic sensing device gives a linear indication of the extent to which a non-luminous flame is coloured by the emission of photons by gaseous potassium atoms. The Standard Additions technique is used to overcome possible interference from other components of the sample.

8. Observation of Emission spectrum Use the spectrometer to observe (and draw) the emission spectrum of sodium (sodium lamp used for refractive index determination)

9. Determination of [K + ]. Prepare the standard solution.You are supplied with anhydrous KCl. Use this to prepare a 10 ppm (10 mg dm -3 ) solution of potassium ions (not 10 ppm potassium chloride!). Prepare the test solutions. Bear in mind that FES works best with potassium solutions in the ppm range Prepare a dilute solution of the unknown solid.

10. Pipette 10 ml portions of the prepared test solution into five 25 ml volumetric flasks and make the following standard additions flask 1 no addition flask 2 1.0 ml standard (10ppm K + ion) flask 3 2.0 ml standard flask 4 5.0 ml standard flask 5 10.0 ml standard Make up to volume with deionised water. Calculate the concentration of added standard in each flask.

11. Read each solution on the flame photometer using the technique described on the supplied sheet. Plot the results on graph paper as they are obtained. Be careful about what is plotted in the X axis. Evaluate the results. If the solution you have prepared was too dilute, or too concentrated, you will not obtain reliable results. This should be obvious from the plot obtained above. If necessary prepare a fresh solution of the unknown and repeat the analysis

13. Accuracy and Precision Accuracy Systematic errors Use standards and blanks Precision Random errors Estimate by replication Standard Deviation

14. Confidence Limits Given by

15. Standard Additions Y=m[X]+c if c = 0 Y=m[X] Y/[X]= m Y o /[X] o = m

17. Calculate the concentration of potassium ions in the solid sample. Quote an average value with 95% confidence limits.