Columbia ENVP U6220 - Environmental Chemistry Lab

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Question 5: Repeat the whole procedure with mangrove swamp water.Question 6: Explain the observed values in terms of the different environments they represent within each ecosystem and across ecosystems. Place a particular attention to these following environments:a) In the black mangrove waters, can you explain the observed pH considering that these waters should contain a relatively high alkalinity (brackish waters)b) Are the rain forest soil percolate waters different from those found in surface waters? How and why? What would that mean for example for transport of metals along a soil profile?c) In the algal mats, do you expect any parameter(s) to change over time? Which one(s)? In which direction and by what magnitude? Over what time scale?Environmental Chemistry U6220Lab #3Part 1: Investigation of the Carbonate-Bicarbonate System (Alkalinity of Aqueous Systems)Introduction:An aqueous solution of carbon dioxide produces a mixture of carbonate and bicarbonate ions. Determining the carbonate and bicarbonate ions in each other's presence is often important in environmental chemistry. a) CO2(g) + H2O(l)  H2CO3 (aq)b) H2CO3 (aq)  HCO3-(aq) + H+(aq)c) HCO3-(aq)  H+(aq) + CO32-(aq)The alkalinity of water is its capacity to act as a base by reacting with protons. There exists a fundamental difference between the expression of acid-base properties of pH and alkalinity. Whereas the pH can be considered to be an intensity factor which measures theconcentration of alkali or acids immediately available for reaction, the alkalinity is a capacity factor which is a measure of the ability of water sample to sustain reaction with added acids (in a sense, it is the ability of a water body to neutralize added acids). In practice, it may be determined by measuring the number of moles of H+ required to titrateone liter of a water sample to the end point. Acidification of a lake in its natural setting isitself analogous to a macro-scale titration and lakes are sometimes termed well-buffered, transitional, or acidic depending on their position on the titration curve.Alkalinity is therefore a useful measure of the capacity of water to resist acidification from acid addition (e.g. acid precipitation). The presence of carbonate, bicarbonate, and hydroxide ions imparts the alkalinity of natural or treated waters. Initially, your water samples may contain bases and will contain a positive alkalinity.Question 1. Based on the pH measured in you initial samples (seawater and mangrove swamp water):a) What contributes to the pH differences you observe in your two samples (i.e. in other words, what is the origin of the [H+] and alkalinity you observe).b) What do you think is the most important carbonate species present in you samples, and,c) Is [OH-] an important contributor to the total alkalinity? Why? How should you incorporate this base in any calculations associated with this experiment? The addition of acid to the water sample will convert the carbonate to bicarbonate (reverse of reaction c) until no carbonate remains. The addition of further acid will convert the bicarbonate to carbonic acid until no more bicarbonate remains (reverse of reaction b). The carbonate and carbonic acid equivalence points may be determined either by titration using indicators or by pH titration. The first end point determined (in the pH range 8.3-10) represents the completion (equivalence point or stoichiometric end point) of the following reaction:H+(aq) + CO32-(aq)  HCO3-(aq)i.e. the carbonate has been neutralized by the acid-forming bicarbonate ions. In the pH range 3.2-4.5, all of the bicarbonate ions initially present in the water sample, together with all of those produced from the reaction of the carbonate ions, will be neutralized. The resulting alkalinity is known as the total alkalinity.HCO3-(aq) + H+(aq)  CO2(g) + H2O(l)Question 2 . Please write the equation for the estimated total alkalinity (what are the assumptions you need to take? Are these valid? Say why)The importance of the carbonate/bicarbonate system in natural waters stems from their ability to act as buffers in natural waters. The oceans are described as being buffered since relatively large quantities of acid or base can be added to seawater without causing much change to its pH. However, many freshwater lakes do not have a large buffer capacity and consequently a small addition of acid (e.g. from acid precipitation or industrial effluent) can cause large changes in pH without warning. The carbonate alkalinity and the total alkalinity are useful for the calculations of chemical dosages required in the treatment of natural water supplies.Summary of the Method:Alkalinity is measured by titrating a water sample with sulfuric acid. The Vernier sensor is used to monitor pH during the titration. The equivalence point will be at a pH of approximately 4.5, but will vary slightly, depending on the chemical composition of the water. The volume of the sulfuric acid added at the equivalence point of the titration is then used to calculate the alkalinity of the water.Material checklist:1) Computer 7) 100 ml graduated cylinder2) Vernier computer interface 8) 250 ml beakers (2)3) Logger Pro 9) Wash bottle with DI water4) Vernier pH sensors (2) 10) Utility clamps & Ring stand5) Sampling bottles 11) 0.01 M H2SO4 solution6) 25-50 ml burets 12) Magnetic stirrer and barProcedure:pH TitrationNote (Caution!): Please wear gloves and safety goggles to perform this experiment and beware that H2SO4 is corrosive. Avoid spilling it on your skin or clothing1) Obtain at least 300 ml of seawater into a beaker (seawater and mangrove swamp water).2) Using a 100 ml graduated cylinder, carefully add 100 ml of each water sample in each beaker3) Place each beaker on the base of a magnetic stirrer and drop a stir bar carefully into the beaker. Set the stirrer to a speed that mixes the sample well, but does not splash.4) Keep the water away from the computer at all times.5) Prepare the computer for data collection by opening “Test 11 Alkalinity” from the Water Quality with Computers experiments files of Logger Pro. On the Graph window, the vertical axis has pH scaled from 2 to 10 units of pH. The horizontal axis has volume scaled from 0 to 20 ml. There is also a Meter window which displays real time pH readings.6) Insert the electrodes of the pH meter into the beaker.7) Ensure complete coverage of the electrodes. It is essential that adequate clearance is


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Columbia ENVP U6220 - Environmental Chemistry Lab

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