Chemical Pollutants in Air, Water, Soil, and Solid Wastes Phần 3 pptx

SPECIF IC CLASSES OF SUBSTANCES AND AGGREGATE PROPERTIES 2 © 1997 by CRC Press LLC © 1997 by CRC Press LLC ALKALINITY Alkalinity of w ater is a measure of its acid-neutralizing ability. The titrable bases that contribute to the total alkalinity of a sample are generally the hydrox- ides, carbonates, and bicarbonates. However, other bases such as phosphates, borates, and silicates can also contribute to the total alkalinity. The alkalinity value depends on the pH end point designated in the titration. The two end points commonly fixed in the determination of alkalinity are the pH 8.3 and pH 4.5 (or between 4.3 and 4.9, depending on the test conditions). When the alkalinity is determined to pH 8.3, it is termed phenolphthalein alkalinity . In such alkalinity titration, phenolphthalein or metacresol purple may be used as an indicator. On the other hand, the total alkalinity is measured by titrating the sample to pH 4.5 using bromocresol green as the indicator. Alkalinity may also be determined by potentiometric titration to the preselected pH. An acid standard solution, usually 0.02 N H 2 SO 4 or HCl, is used in all titrations. The procedure for potentiometric titration is presented in Chapter 1.6. In this titration, a standard acid titrant is added to a measured volume of sample aliquot in small increments of 0.5 mL or less, that would cause a change in pH of 0.2 unit or less per increment. The solution is stirred after each addition and the pH is recorded when a constant reading is obtained. A titration curve is constructed, plotting pH vs. cumulative volume titrant added. The volume of titrant required to produce the specific pH is read from the titration curve. CALCULATION where V is mL standard acid titrant used and N is normality of the standard acid. Since the equivalent weight of CaCO 3 is 50, the milligram equi valent is 50,000. The result is, therefore, multiplied by the factor of 50,000 to express the alkalinity as mg CaCO 3 /L. Alkalinity, mgCaCO L = mL sample 3 / ,VN××50 000 2.2 © 1997 by CRC Press LLC © 1997 by CRC Press LLC BROMIDE Bromide (Br – ) is the anion of the halogen bromine, containing an e xtra electron. It is produced from the dissociation of bromide salts in water. It may occur in ground and surface waters as a result of industrial discharges or seawater intrusion. Bromide in water may be analyzed by one of the following three methods: 1. Phenol red colorimetric method 2. Titrimetric method 3. Ion chromatography While the fi rst method is used for low level detection of bromide in the range 0.1 to 1 mg/L, the concentration range for the titrimetric method is between 2 and 20 mg/L. The samples may be diluted appropriately to determine bromide con- centrations at higher range. Ion chromatography is used to analyze many anions including bromide and is discussed in Section 1.8. PHENOL RED COLORIMETRIC METHOD Bromide ion reacts with a dilute solution of sodium p -toluenesulfonchlora- mide (chloramine-T) and is oxidized to bromine which readily reacts with phenol red at pH 4.5 to 4.7. The bromination reaction with phenol red produces a color that ranges from red to violet, depending on the concentration of bromide ion. An acetate buffer solution is used to maintain the pH between 4.5 and 4.7. The presence of high concentration of chloride ions in the sample may seriously interfere in the test. In such cases, the addition of chloride to the pH buffer solution or the dilution of the sample may reduce such interference effect. Remove free chlorine in the sample by adding Na 2 S 2 O 3 solution. In addition, the presence of oxidizing and reducing agents in the sample may interfere in the test. Pr ocedure A 50-mL sample aliquot is treated with 2 mL b uffer solution followed by 2 mL phenol red indicator and 0.5 mL chloramine-T solution. Shake well after 2.3 © 1997 by CRC Press LLC © 1997 by CRC Press LLC CHLORIDE Chloride (Cl – ) is one of the most commonly occurring anions in the en vi- ronment. It can be analyzed using several different methods, some of which are listed below: 1. Mercuric nitrate titrimetric method 2. Argentometric titrimetric method 3. Automated ferricyanide colorimetric method 4. Gravimetric determination 5. Ion-selective electrode method 6. Ion chromatography Methods 1 and 3 are EP A approved (Methods 325.3 and 325.1-2, respec- tively) for chloride determination in wastewater. For multiple ion determination, ion chromatography technique should be followed (see Section 1.8). MER CURIC NITRATE TITRIMETRIC METHOD Chloride reacts with mercuric nitrate to form soluble mercuric chloride. The reaction is shown below for calcium chloride (CaCl 2 ) as a typical e xample. The analysis may be performed by titrimetry using a suitable indicator. Diphenyl carbazone is a choice indicator that forms a purple complex with excess mercuric ions in the pH range of 2.3 to 2.8. Therefore, the pH control is essential in this analysis. Xylene cyanol FF is added to diphenyl carbazone to enhance the sharpness of the end point in the titration. Nitric acid is used to acidify the indicator to the required low pH range. Other halide ions, especially bromide and iodide, are interference in this analysis. Acidify alkaline samples before analysis. Fe 3+ , CrO 4 2– , and SO 3 2– at concentrations above 10 mg/L are often used to interfere with the analysis. CaCl Hg NO HgCl Ca NO 2 + () ⇒+ () 3 2 23 2 2.4 © 1997 by CRC Press LLC © 1997 by CRC Press LLC CYANATE The formula of c yanate is CNO – . It is a uni valent anion formed by partial oxidation of cyanide (CN – ). Under neutral or acidic conditions, it may further oxidize to CO 2 and N 2 . The analysis of c yanate is based on its total conversion to an ammonium salt. This is achieved by heating the acidified sample. The reaction is shown below: The concentration of ammonia (or ammonium) minus nitrogen before and after the acid hydrolysis is measured and the cyanate amount is calculated as equivalent to this difference. Calculation Thus, the amount of NH 3 –N produced from mg CNO – /L = A – B where A = mg NH 3 –N/L in the sample portion that w as acidified and heated, and B = mg NH 3 –N/L in the original sample portion. Therefore, the concentration of c yanate as mg CNO – /L = 3.0 × ( A – B ). [In the above calculation for cyanate, the concentration of ammonia–nitrogen was multiplied by 3 because the formula weight of CNO – is (12 + 14 + 16) or 42, which is three times 14 (the atomic weight of N, as ammonia–N).] Pr ocedure Add 0.5 to 1 mL of 1:1 H 2 SO 4 to a 100-mL portion of sample acidifying to pH 2 to 2.5. Heat it to boiling for 30 min. Cool to room temperature and bring up to the original volume by adding NH 3 -free distilled w ater. CN O CNO – → – 24 2 42 44 3 KCNO + H SO H O NH ) SO KHCO 22 +⇒ +( 2.5 © 1997 by CRC Press LLC © 1997 by CRC Press LLC CYANIDE, TOTAL Cyanides are metal salts or complexes that contain the cyanide ion (CN – ). These cyanides could be subdivided into two categories: (1) simple cyanides such as NaCN, NH 4 CN, or Ca(CN) 2 containing one metal ion (usually an alkai or alkaline-earth metal or ammonium ion) in its formula unit, and (2) complex cyanides such as K 4 Ce(CN) 6 or NaAg(CN) 2 containing two different metals in their formula unit, usually one is an alkali metal and the other a heavy metal. The complex cyanide dissociates to metal and polycyanide ions. The latter may further dissociate to CN – which forms HCN. The degree and rate of dissociation of complex cyanides depend on several factors, including the nature of the metal, pH of the solution, and dilution. Cyanide ion and HCN are highly toxic to human beings, animals, and aquatic life. Cyanide in water may be determined by the following methods: 1. Silver nitrate titrimetric method 2. Colorimetric method 3. Ion-selective electrode method 4. Ion chromatography SILVER NITRATE TITRIMETRIC METHOD Cyanide reacts with silver nitrate as shown below forming the soluble cyanide complex, Ag(CN) 2– . When all the CN – ions in the sample are complexed by Ag + ions, any further addition of a few drops of titrant, AgNO 3 , can produce a distinct color with an indicator that can determine the end point of the titration. Thus, in the presence of a silver-sensitive indicator, p-dimethylaminobenzalrhodamine, Ag + ions at first com- bine preferentially with CN – . When no more free CN – is left, little excess of Ag + 2 32 3 CN AgNO Ag(CN NO – – – )+⇒ + 2.6 © 1997 by CRC Press LLC . end points commonly fixed in the determination of alkalinity are the pH 8 .3 and pH 4.5 (or between 4 .3 and 4.9, depending on the test conditions). When the alkalinity is determined to pH 8 .3, . such interference effect. Remove free chlorine in the sample by adding Na 2 S 2 O 3 solution. In addition, the presence of oxidizing and reducing agents in the sample may interfere. phenolphthalein alkalinity . In such alkalinity titration, phenolphthalein or metacresol purple may be used as an indicator. On the other hand, the total alkalinity is measured by titrating