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Lab #3 Instructions

Lab #3 Instructions - EXPERIMENT III Alkalinity PURPOSE To...

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1 EXPERIMENT III Alkalinity PURPOSE: To determine the alkalinity of a water sample using a double - endpoint titration for determination of [OH - ] and [CO 3 2- ], and compare the results to those of a potentiometric titration. INTRODUCTION Alkalinity is a parameter that is measured on almost all environmental samples -drinking water, natural waters, polluted waters, sewage, and industrial wastes. Alkalinity refers to the buffering capacity of water samples and to their ability to neutralize acidic pollution from rainfall or wastewater . For municipal sewage or industrial wastes , the amount of alkalinity is important in determining the type of treatment which should be employed. In the Northeast, alkalinity is primarily caused by the presence of carbonate (C0 3 2- ) and bicarbonate (HC0 3 - ) ions, although hydroxide (OH - ) ions may also contribute, especially when there is industrial pollution. Living organisms, such as aquatic life, function best in a pH range of 5.0 to 9.0 and levels of 20 to 200 mg/L are typical alkalinity values for fresh water. When the pH is above 8.3, carbonate (CO 3 2- ) is the primary contributor to alkalinity; when the pH is below 8.3, bicarbonate (HCO 3 - ) becomes the dominating factor. The values of alkalinity are reported in units of "mg CaCO 3 /L" because of its relationship to hardness, which is reported using the same unit. Although large environmental labs perform alkalinity tests using automated methods (1 sample/minute), we will employ the traditional analyses of using an acid-base titration with two endpoints. To gain insight into the double-endpoint titration technique you will perform a potentiometric titration, pH vs. titrant volume of your water sample. These techniques are described in more detail below. The alkalinity analysis consists of: [A] Preparation and Standardization of a Titrant. [B] Titration of a prepared unknown using the standardized titrant. [C] Titration of a natural water sample (well, river or lake water) using the standardized titrant. Part 1: DETERMINATION OF ALKALINITY A sample may contain any of the three anions (carbonate, bicarbonate, or hydroxide) or some combinations of them. Because these can all behave as bases, we will titrate with a strong acid. Hydrochloric acid (HCl) can be used , but we will be using sulfuric acid (H 2 S 0 4 ). Regardless of the source of the H + the following reactions will occur: (1 st endpoint) H + + CO 3 2- HCO 3 - (1 st endpoint) H + + OH - H 2 O (2 nd endpoint) H + + HCO 3 - H 2 CO 3 (really CO 2 + H 2 0) The two pKa values for the carbonate system are pK a1 = 6.35 and pK a2 = 10.33.
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2 The unknown and your aqueous sample could contain any one of the following combinations of the anions responsible for alkalinity: [1] carbonate only. ( CO 3 2- ) [2] bicarbonate only (HCO 3 - ) [3] hydroxide only, (OH - ) [4] carbonate and bicarbonate (CO 3 2- and HCO 3 - ) [5] carbonate and hydroxide (CO 3 2- and OH - ) We need to consider each of these five situations separately to see how the ions will behave at the two different endpoints of the titration (using two different indicators). One of the indicators we will use is phenolphthalein (we will use the abbreviation "phth" from now on). It has a pinkish
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Lab #3 Instructions - EXPERIMENT III Alkalinity PURPOSE To...

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