Severn Trent Water Case Study – Chemical Testing Key Skills References How Science Works References 1a, 1b, 3b, 4a, 4b Vocational Implications: Methods Used by Severn Trent Water to Identify Ions Read the information. Make brief notes by answering the questions in full sentences. Severn Trent Water provides millions of homes in the midlands with their water. Water boards such as Severn Trent monitor ions in the water by carrying out chemical tests to identify impurities and other dissolved substances that affect the water used. Ions in water can affect the taste, and ‘hardness’ (hard water needs a lot more soap to make a lather than soft water) and our health. It is important the water board uses quantitative tests to determine the exact levels of these ions in the water. If the levels get too high the water board needs to treat the water to lower the ion levels. The water boards and consumers also use qualitative tests on water include colour – should be clear and bright odour- smell – sometimes it will smell of chlorine. At low levels chlorine is not harmful and disinfects water taste – if water has been standing in the pipes it may have a flat, stale, metallic taste. Questions 1. Why do Seven Trent need to analyse the ions present in water? 2. Why is it important that these tests are quantitative? 3. What qualitative tests are carried out on water, and why? Ions in Water Read the following information on ions and their effect on water and use it to complete the table in the next section. Calcium and magnesium ions cause water hardness and result from limestonetype materials in underground soil layers. Separate values are of minor concern but they are combined for calculating hardness. Hardness is the soap-consuming capacity of water; that is, the more soap required to produce lather, the harder the water. Hard water also causes greasy rings on bathtubs, film on dishes or hair after washing, limescale in kettles, and poor laundry results. Hardness is reported as calcium carbonate in milligrams per litre. Hard water may irritate eczema. Max calcium = 250 mg/l Hardness expressed as mg/l of CaCO3. 0-100 100-200 200-300 300-400 400-500 Severn Trent Water Case Study – Chemical Testing © Copyright of 3E’s Enterprises (Trading) Limited Water hardness Soft Moderately soft Moderately hard Hard Very hard Severn Trent Water Case Study – Chemical Testing Sodium ions in water may be of health significance to people on a low-sodium diet. Sodium can be reduced or removed by expensive treatment systems, Sodium ions occur in water if it has been ‘softened’. Max = 150 mg/l Potassium is an essential nutritional element, but its concentration in most drinking water is trivial and quantities seldom reach 10 mg/l. Max = 12 mg/l Carbonates and bicarbonates are the major contributors to the "total alkalinity" that may be determined in a routine water test. If the alkalinity is low problems could occur due to corrosion of metal in plumbing systems. Chloride ions concentrations in drinking water may be important to people on lowsalt diets. Most people will detect a salty taste in water containing more than 250 mg/l of chloride. Expensive treatment methods are needed to remove chloride from water. Max = 400 mg/l Sulphate ions may give water a bitter taste and have a laxative effect on people not adapted to the water. Max = 250 mg/l Nitrate ions are of health significance to pregnant women and infants under 6 months. Do not use high-nitrate water in infant formulas or other infant foods. Most adults tolerate considerably higher nitrate content. Max = 50 mg/l Iron ions are nuisance chemicals that cause troublesome stains and deposits on light-coloured clothes and plumbing fixtures. Iron causes yellow, red or reddishbrown stains and deposits. Excessive amounts also may cause dark discoloration in some food and beverages and cause an unpleasant taste. Max = 0.2 mg/l Copper ions will cause an undesirable metallic taste if concentrations are above the recommended limits. Max = 3 mg/l Comparing my Method to the Method Used by Severn Trent Water Use the information above and the background from your flame and chemical tests write up to complete the table. Severn Trent Water Case Study – Chemical Testing © Copyright of 3E’s Enterprises (Trading) Limited Severn Trent Water Case Study – Chemical Testing Ion Effect on water Iron How we detected it in the lab (both flame and chemical tests and results) Qualitative chemical test Added sodium hydroxide and a brown or green precipitate forms Calcium Copper Potassium Sodium Chloride Nitrate Calcium Carbonate (hardness) Sulphate How Severn Trent detect the ions in their lab Max amount (mg/l) in tap water Heat in nitric acid to dissolve metals then use spectrometry. To quantify results they compare results to standard solutions Chemical added to make iron III thiocyanate. Colourimetry sat at wavelength 480 nm. Concentration determined by comparing to standard solutions. Chemical added to make pink azo dye. Colourimetry used at wavelength 510 nm. Concentration determined by comparing to standard solutions. Calculation- Calcium result from above Result multiplied by 2.5 Chemical added to make a barium sulphate precipitate. The amount of which is determined turbidimetrically. - Extension for top grades (distinction) Read the 3 quantitative methods used by Severn Trent, make BRIEF notes on each method and answer the questions comparing the Severn Trent Methods with our lab methods…. Severn Trent Water Case Study – Chemical Testing © Copyright of 3E’s Enterprises (Trading) Limited Severn Trent Water Case Study – Chemical Testing 1) Atomic (Flame) Emission Spectrometry is Used to Analyse of Metals and Metallic Ions. Flame emission spectrometry techniques are often quantitatively used to measure the concentration of chemicals in a sample. The sample to be tested is dissolved in water and the resulting solution is then aspirated into a flame. The flame excites the atom and releases light of a certain wavelength (colour). The intensity of the light given out is measured so the amount of that ion can be worked out quantitatively. This method is very accurate and is used in a wide variety of areas including the Olympic drugtesting program and in environmental studies such as water testing. Comparing Atomic Spectrometry to Our Flame Test Method. This method is only good for ions, which are easily excited – similar to flame tests. For a mass spectrometer to work the ions must be placed in a vacuum. Otherwise interference from air stops the ions reaching the detector. Did your flame tests and chemical tests need a vacuum? To reduce contamination from light produced by other chemicals in the sample, a wavelength selector is used. In our flame tests why did we sometimes see contaminatiuon in the flame? The detectors measure the exact wavelength of light. Is this more accurate than subjectively ‘guessing’ the colour. For quick analysis the detector is linked to a computer which states the result. In our flame tests we had to look at our background table to work out what colours corresponded to what ion. Which method gives a quicker result? In order to make quantitative measurements, an instrument has to first be calibrated. Instruments generally record some signal (e.g. the intensity of light). If incorrectly calibrated the results will be inaccurate. Did our flame tests need calibration? 2) Colourimetry The ions are reacted with certain chemicals to produce a coloured solution. The more ions present, the deeper the colour of the solution. Light is passed through a filter of a certain wavelength (colour) and the amount of light absorbed (% absorbance) by the solution corresponds to how much of the ion is present. The darker the solution, the more light absorbed, the more ion present. This method also needs calibrating and the solution must be coloured. Did you have any coloured solutions in your chemical tests? Which ions made coloured solutions? Would more concentrated copper sulphate be darker or lighter than week copper sulphate? Was our chemical test methods qualitative? Severn Trent Water Case Study – Chemical Testing © Copyright of 3E’s Enterprises (Trading) Limited Severn Trent Water Case Study – Chemical Testing Look back to the table at the start. How do Thames water carry out colourimetry for different ions? 3) Turbidity. Turbidity is a measure of how cloudy a solution is by measuring the amount of precipitate made. Name the precipitates made in your chemical tests experiment and compare this to the ones made using Severn Trent’s methods. Are any of them the same? What effect would increasing the ions in the chloride, sulphate, copper, iron II and Iron III solutions have on the amount of precipitate? Did you measure the amount of coloured precipitate? How do Thames water measure the amount of precipitate? Severn Trent Water Case Study – Chemical Testing © Copyright of 3E’s Enterprises (Trading) Limited