Hsc Chemistry Water Notes: Gravimetric Analysis

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Gravimetric analysis can be used to determine the quantities of both cations and anions, e. g. or chloride ions precipitated and weighed as silver chloride. Common anion concentrations measured Include chloride, sulfate, bicarbonate and fluoride Ions. Total dissolved solids (TDS): Total dissolved solids (TDS) are determined by evaporation to dryness of a known volume of a filtered sample. The value is converted to parts per million (ppm) and expressed in mass per volume units, ppm (m/v). Since most of the dissolved solids are Ionic, their presence can be determined by data loggers that have the electrical conductivity probe attached. They can be set up to record continuously If needed.

Hardness: Hardness is due to the presence of calcium and magnesium ions in the water. These orm insoluble compounds with soap ions, resulting in a scum on the water surface and around sink basins. This removal of soap ions from solution reduces the ability of soap to lather. The test for hardness involves precipitating the calcium and carbonate (of known concentration), followed by filtering and drying of the precipitate. Most of the insoluble salt is assumed to be calcium and the concentration of calcium ions is calculated and reported in parts per million (ppm).

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Turbidity: This can be one measure of the ability of the water to support life. Turbidity results from the presence of suspended solids in the water. Water with a high turbidity reduces penetration of light and decreases photosynthesis, which in turn reduces the oxygen concentration. The test for turbidity is conducted using a turbidity tube standing on a white tile. The tube has a black cross marked on the base. The water sample is poured into the tube until the cross Just disappears when looking from above. The use of a turbidity tube is less reliable than a transmittance of light test done with a colorimeter.

The lower reliability is due to variations in human eyesight and the intensity of background light during testing. Acidity (pH): A pH reading below 7 would be expected where there are acid sulfate soils or where there is acid produced by decomposition of organic matter in stagnant situations. The test can be conducted with a data logger and pH probe, universal indicator solution or paper, or a pH meter. If using the universal indicator, comparison with a coloured pH scale provides the pH value. If it is less than 7, the solution is acidic. Dissolved oxygen (DO): There are several tests for determining the DO in a water sample.

The Winkler methodfixes the amount of dissolved oxygen, which is later determined by titration. The amount of manganese dioxide produced by adding manganese(ll) ions and hydroxide ions is a measure of the DO. Acidified iodide ions are added to cause the manganese dioxide to produce a yellow iodine solution. This is then titrated against a standard sodium thiosulfate solution using starch as the indicator. The indicator turns a blue colour with the iodine and the blue disappears at the endpoint.

The overall reaction can be represented by the equation: This shows that, for each mole of thiosulfate (S203 2-) used in the titration, there was 0. 25 moles of dissolved oxygen in the original sample. Biochemical oxygen demand (SOD): BOD measures the amount of oxygen used by bacteria and other micro-organisms away from the bank. 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 reading is given as milligrams per litre (mg L-1) identify factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans Factors include:

  • the frequency of rainfall (floods and droughts),
  • water temperature
  • evaporation rates
  • soil type
  • pollution sources, such as the presence of animal faeces and fertiliser usage (leading to eutrophication)
  • land use.

Farming practices such as the removal of native vegetation or irrigation can increase the salt (NaCl) concentration in rivers. Earthmoving associated with waterfront developments can expose layers containing sulfide to air. The sulfides are oxidised by xygen to form sulfates and sulfuric acid, which can kill fish and other living things. High evaporation rates in the Dead Sea have increased the dissolved salt concentration. Run-off of water from agricultural land in Queensland, near the Great Barrier Reef, contains ions, such as phosphate, which can affect the growth of describe and assess the effectiveness of methods used to purify and sanitise mass water supplies * There are several methods used to purify mass water supplies.

Most are variations on the following process:

  • Water is collected in dams and pumped to a treatment site where the larger olids such as pieces of plastic are removed by screens.
  • Fine particles suspended in water have electric charges on their surface that keep the particles from Joining together.

This stops particles from becoming large enough to settle as sediment. Separation of fine particulate matter suspended in water involves the addition of coagulants, such as iron(lll) chloride, FeC13, to the water. The added FeC13 neutralises these surface charges so the particles come together.

Iron(lll) hydroxide is formed by reaction of FeC13 with water and precipitates out as a floc (flocculant). The floc collects the neutralised particles into large masses that are more easily filtered.  The chloride ions added with the FeC13 remain in the water. They do not have a harmful impact on the water quality as determined by the Drinking water quality guidelines (1996), National Health and Medical Research Council (NHMRC). In some instances, after flocculation the particle size of suspended solids is too fine for filtration through sand beds and so membrane filters are used.

They are membrane filtration is more costly than sand bed filtration. * Sanitising mass water supplies involves disinfecting with chlorine gas, C12, liquid odium hypochlorite solution, NaOCl (aq), or solid calcium hypochlorite, Ca(OCl)2. Sedimentation and filtration removes some harmful organisms, such as bacteria, viruses, cryptosporidium and giardia, but disinfection is needed to ensure concentrations are acceptably low. In some parts of the water distribution system, ammonia, NH3, is added to form monochloramine, CINH2, by reaction with chlorine.

This is a less reactive disinfectant, but lasts much longer in the distribution system.  Assessment of the effectiveness of sanitisation methods should involve: microbiological testing of water samples throughout the distribution system, in articular before and after sanitisation processes public health surveys and reporting by medical doctors of incidences of illnesses that are possibly transmitted through water supplies.  A few years ago, some water supplies in NSW were contaminated by the micro- organisms, cryptosporidium and giardia.

As a result, water supplies are monitored daily at water treatment plants and throughout each catchment during storms or other events that cause a rise in stream water levels and could influence water quality. This is considered effective, as the cost of treatment of all water supplies with embrane filters would make treatment very expensive. describe the design and composition of microscopic membrane filters and explain how they purify contaminated water. Microscopic membrane filters have microscopic pores and the use of appropriate sized filters can avoid the need to chemically treat the water.

The filters can be classified as microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes depending on the size of the pore. These filters are often used in conjunction with other purification techniques. Pretreatment of water is often required for membrane filters to remove ontaminants that will damage or degrade the membrane.  The membrane is made from synthetic polymers dissolved in a mixture of solvents. Water-soluble powders of a particular size are added. The mixture is spread out over a plate and left for the solvent to dry.

The polymer membrane formed, containing particles of water-soluble powder, is then placed in water. Remaining solvent and the powder particles dissolve, leaving a very thin polymer sheet with definite sized microscopic pores where the water-soluble particles were located.  Semi-permeable membranes used in reverse osmosis are either made of ellulose acetate or a layer of polyamide attached to another polymer. Under pressure these polymers allow the passage of water molecules but not that of most atoms, ions or other molecules.

Removal of contiminants is facilitated by charge or size. Charged particles or larger molecules are more likely to be rejected by the membrane.  Water is made to flow across the membrane not through it. This reduces the blockage of the pores and contaminants are carried away as waste. The membrane is housed in a pressure vessel and is either made as a wound spiral or hollow fibres. Microfiltration removes protozoans, bacteria, colloids, some colouration and some through the membrane.

The finer the pore size the smaller the particles trapped and the more expensive the membrane perform first-hand investigations to use qualitative and quantitative tests to analyse and compare the quality of water samples.  A qualitative test shows what (element, compound or ion) is present.  A quantitative test shows how much (element, compound or ion) is present. Gather, process and present information on the range and chemistry of the tests used to: identify heavy metal pollution of water monitor possible eutrophication of waterways Heavvy metals: Heavvy metal pollution of water may be caused by the presence of unacceptable levels of the ions of arsenic, cadmium, copper, chromium, mercury, nickel, lead and zinc.

Simple techniques to identify the presence of heavy metals include precipitation and flame testing. Techniques to quantitatively determine heavy metals include volumetric and gravimetric analyses, colorimetry, chromatography and spectroscopy, particularly atomic absorption spectroscopy (AAS) and mass spectroscopy. Summary: A metal with a relative density of 5. or higher.  Heavvy metals, such as mercury and lead, can cause various health problems. The heavy metal pollution of water can be tested using:  Atomic absorption spectroscopy, using a light source specific to the heavy metal being tested.

Flame tests.  Sodium sulfide solution, which can be added to a highly concentrated acidified or basified water sample (heavy metal ions react with sulfide ions to form sulfide  If a precipitate is formed when the sample is acidified, then one or precipitates): more of the following is present: lead, silver, mercury, copper, cadmium, arsenic.  If a precipitate is formed when the sample is basified, then one or more of the following is present: chromium, zinc, iron (Ill), nickel, cobalt, manganese, aluminium.

Eutrophication: The process of eutrophication involves the increase in nutrient content of a body of water resulting in excessive growth of plants, which when they die, causes oxygen depletion during the decay process (due to bacterial action). The oxygen depletion threatens the survival of fish. Tests to determine the extent of eutrophication of a waterway involve measurement of dissolved oxygen and nutrients, such as nitrates and phosphates.

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Hsc Chemistry Water Notes: Gravimetric Analysis. (2017, Oct 02). Retrieved from


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