SECTION 5: Many factors affect water quality. In this section we shall study the following points:

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1 SECTION 5: HUMAN ACTIVITY ALSO IMPACTS ON WATERWAYS. CHEMICAL MONITORING AND MANAGEMENT ASSISTS IN PROVIDING SAFE WATER FOR HUMAN USE AND TO PROTECT THE HABITATS OF OTHER ORGANISMS. Many factors affect water quality. In this section we shall study the following points: Concentrations of common ions AAS is used to measure the concentration of common ions in water, such as Fe3+, Na+, and F-. Gravimetric analysis (by precipitating ions out of the sample) can also be used to determine ion concentration. Total dissolved solids (TDS) A conductivity meter can be used to measure the total dissolved solids because most solutes are ionic. In the lab TDS can be measured by slowly evaporating a known volume of filtered water. TDS measures the total salinity of water. Hardness water hardness is caused by dissolved calcium and magnesium ions. Water hardness is measured in mol/l of CaCO 3. A titration using EDTA is used to determine the hardness of a water sample. However, hardness can also be assessed qualitatively using height of lather head technique. Turbidity turbidity can be measured by pouring water in to a turbidity tube which allows turbidity to be measured in NTU. Potable water needs to have a reading lower than 3 NTU. Acidity Acidity can be measured using a ph probe or universal indicator. Potable water needs to be in the ph range of Dissolved oxygen (DO) dissolved oxygen levels are important because aquatic organism require oxygen to survive. DO can be measured using a calibrated oxygen sensor electrode, or special tablets that dissolve in water and change colour depending on the DO level. Biochemical oxygen demand (BOD) Biochemical oxygen demand is a measure of the rate of oxygen use of microscopic organisms. It is also an indirect measure of organic waste present in the water. One sample is measured for DO as soon as possible while the other sample is kept in a dark place for 5 days and then tested for DO. The BOD is calculated by subtracting the DO value after 5 days from the initial DO value. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 54

2 WATER HARDNESS CALCULATIONS EXAMPLE Calculate the hardness of a sample of water containing 1.5x10-4 mol/l Mg 2+ and 2.0x10-4 Ca 2+. Step 1: 1.0x x10-4 = 3.5x10-4 mol/l of total hardness. Step 2: 3.5x10-4 x (MM of CaCO 3 ) = 3.5x10-2g g/l Step 3: 3.5x10-2g g/l = 3.5 mg/l = 35 ppm QUESTION 49 Calculate the hardness of a sample containing 3.8x10-4 mol/l Mg 2+ and 0.8x10-4 mol/l Ca 2+. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 55

3 QUESTION 50 (5 marks [a:2 marks] [b:3 marks]) (HSC 2002:26) Water can be described as either hard or soft. (a) Describe a test you have used to determine whether a given sample of water is hard or soft. (b) A sample of hard water contains 6 x 10-4 mol L -1 of magnesium carbonate. Calculate the mass, in mg, of magnesium carbonate in 150 ml of this sample. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 56

4 QUESTION 51 (4 marks [a:1 mark] [b:2 marks] [c:1 mark]) (HSC 2006:27) One of the most common methods for determining the concentration of metal ions in water samples involves titration with a reagent called EDTA. In alkaline solution EDTA is present as an anion with a 4- charge. In this form it reacts with metal ions such as calcium and magnesium in a 1 : 1 ratio: Ca EDTA Ca( EDTA) When the reaction between the metal ions and EDTA4- is complete, and indicator also present in the solution changes colour. A student used the following procedure to determine the concentration of calcium in a sample of water. (a) 50.0 ml of water sample was pipetted into a conical flask. 5.0 ml of ammonia/ammonium ion buffer and two drops of indicator were added. Sample was titrated with mol L -1 EDTA4- until indicator changed colour. The above procedure was repeated a further three times. The average volume of EDTA4- used in the four titrations was 24.0 ml. What is the average number of moles of EDTA4- added to reach the end point? (b) The student used the answer to part (a) to calculate the concentration of Ca 2+ in the water sample in mg L -1. What concentration was obtained? (c) The concentration of Ca 2+ in the water sample was also determined by atomic absorption spectroscopy and found to be 16% lower than the value obtained by titration with EDTA4-. Suggest a reason why the concentration of Ca 2+ determined by EDTA titration was higher. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 57

5 QUESTION 52 (HSC 2005:15) The Winkler method is used to determine the amount of dissolved oxygen in a sample. In this procedure, oxygen reacts with Mn 2+ under the alkaline conditions to produce a precipitate of MnO( OH ) 2. 2Mn O 4OH 2 MnO( OH ) 2 ( aq) 2( aq) ( aq) 2( s) The precipitate is then dissolved in acid and reacted with iodide, forming iodine and Mn 2+. MnO( OH ) 2I 4H I Mn 3H O 2 2( s) ( aq) ( aq) 2( aq) ( aq) 2 ( aq) Finally, the amount of iodine produced is determined by reaction with thiosulfate. I 2S O 2I S O 2 2 2( aq) 2 3( aq) ( aq) 4 6( aq) When a sample of water was analysed using the Winkler method, a total of 0.60 mol of thiosulfate was used in the reaction. How many moles of oxygen were present in the original sample? A 0.15 B 0.30 C 0.60 D 1.20 QUESTION 53 (5 marks) (HSC 2010:25) What is the relationship between dissolved oxygen and biochemical oxygen demand and why is it important to monitor both in natural waterways? The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 58

6 Identify factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans. Agricultural run-off (eg. increasing phosphate and nitrates). Industrial wastes and runoff (eg. releasing heavy metals). Sewage discharge into water (eg. increasing phosphates and nitrates). Rainfall in (increasing minerals washed from soils and rocks, can become very significant in land clearing/building/mining). ph of rain acidic rain is able to better leach certain ions from soils. EXAMPLE (2009 HSC Q25) (7 marks) An analytical chemist determined the phosphate concentration of water samples from three local streams. (a) Using the absorbance values in the table and graph, determine the mean absorbance and mean phosphate concentration for each stream and complete the table. Stream Absorbances measured Mean absorbance , 0.092, , 0.511, , 0.237, Mean phosphate concentration (mg/l) The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 59

7 (b) The recommended maximum level of phosphate in streams is mg/l With reference to the recommended level of phosphate for stream water, explain why there are differences between the three streams. (3 marks) (a) Why is phosphate concentration a water quality issue? (2 marks) The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 60

8 Monitor possible eutrophication of waterways. Eutrophication is when the presence of nutrients enriches waterways so that aquatic plants grow abundantly. Eutrophication can lead to algal blooms. To monitor eutrophication the concentrations of nitrogen and phosphorus needs to be determined. To avoid eutrophication nutrients must be diverted from waterways or absorbed by plants before they reach waterways. Also, fertilisers should not be applied before heavy rain. QUESTION 54 (4 marks) (HSC 2003:26) Describe the process of eutrophication and assess the suitability of water quality tests used to monitor it. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 61

9 Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies. Water is purified and sanitised in three steps: screening, flocculation, filtration and chlorination. 1. Flocculation is the addition of a flocculent such as Iron chloride to coagulate suspended particulates allowing them to settle in tanks. 2. Filtration is then performed through sand and anthracite (a type of coal) filters to remove all undissolved particulates. 3. Water is then chlorinated to kill any microorganisms. NB It is Cl 2 that is added not Cl -. Also, some authorities add Fluoride (F - not F 2!) The water treatment is very effective but not perfect. Chlorination is highly. An example of failure, however, was when Cryptosporidium and Giardia entered Sydney s drinking water in the late 1990 s. Careful, regular monitoring is essential. QUESTION 55 (HSC 2001:11) Why is chlorine used to treat local water supplies? A B C D To make water suitable for swimming. To kill micro-organisms living in the water. To promote sedimentation of finely suspended solids. To precipitate heavy metal ions such as lead and mercury. Describe the design and composition of microscopic membrane filters and explain how they purify contaminated water. A membrane filter is a thin film of a synthetic polymer through which there are microscopic pores. With the aid of high pressure on the unclean side of the membrane, water can be forced through the membrane leaving impurities behind. This process is thus termed reverse osmosis as it involves water moving high to low concentration (as opposed to low to high concentration as is the definition of osmosis). These membranes can be used in desalination. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 62

10 QUESTION 56 (HSC 2001:14) Which diagram represents the most effective design for a microscopic membrane filter to purify contaminated water? QUESTION 57 (HSC 2006:13) Why are microscopic membrane filters useful for water purification? A They can kill bacteria. B They adjust the ph of water to 7. C They are composed of biodegradable polymers. D They can remove very small particles from water. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 63

11 QUESTION 58 (7 marks) (HSC 2005:26) The map shows the catchment for a town water supply. Describe TWO possible sources of contamination in this catchment, and assess methods that could be used for purifying the water before it reaches the town. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 64

12 OTHER WAYS OF EXPRESSING CONCENTRATIONS This section is added to these notes as expressing concentration in units such as %w/v, 5%w/w and %v/v is an expectation of the NSW HSC course. However, it is not clearly noted in the syllabus and is not covered effectively. Hopefully these examples may fill the gap. PERCENTAGES Percentage by mass (w/w) describes the mass of solute (g) in 100 g of solution. For example: Concentrat ion 0.8% w/w indicates that there are 0.8 g of solute dissolved in 100 g of solution. mass of solute ( g) ( w / w) 100 mass of solution ( g) QUESTION 59 Twenty grams of a salt solution contains 4.0 grams of salt. What is the concentration (w/w) of salt in this solution? The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 65

13 QUESTION 60 How much salt is required to prepare 500 g of a 10% w/w salt solution? Percentage by volume (v/v) describes the volume of solute (ml) in 100 ml of solution. For example: Concentrat ion 11% alcohol v/v indicates that there are 11 ml of alcohol dissolved in 100 ml of solution. volume of solute ( ml) ( v / v) 100 volume of solution ( ml) QUESTION 61 A 170 ml glass of fruit drink contains 15% (v/v) of pure orange juice. What volume of pure orange juice is present in this solution? Percentage mass per volume (w/v) describes the mass of solute (g) dissolved in 100 ml of solution. For example: Concentrat ion 2% (w/v) indicates that there are 2% of solute dissolved in 100 ml of solution. mass of solute ( g) ( w / v) 100 volume of solution ( ml) QUESTION 62 The concentration of Mg 2+ in water is 10% (w/v). Calculate the mass of Mg 2+ in 2.00 L. The School For Excellence 2011 Trial Exam Preparation Lectures Chemistry Book 3 Page 66