Report on alkalinity test

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Report on alkalinity test

  1. 1. PURPOSE:To determine the alkalinity of a water sample using a double – endpoint titration for -determination of [OH ] and [CO3 2-]. INTRODUCTION:Alkalinity is a parameter that is measured on almost all environmental samples -drinkingwater,natural waters, polluted waters, sewage, and industrial wastes. Alkalinity refers to thebuffering capacity of water samples and to their ability to neutralize acidic pollution from rainfallor wastewater. For municipal sewage or industrial wastes, the amount of alkalinity is importantin determining the type of treatment which should be employed.Alkalinity is primarily caused by the presence of carbonate (C03 2-) and bicarbonate (HC03-)ions, although hydroxide (OH-) ions may also contribute, especially when there is industrialpollution. Living organisms, such as aquatic life, function best in a pH range of 5.0 to 9.0 andlevels of 20 to 200 mg/L are typical alkalinity values for fresh water. When the pH is above 8.3,carbonate (CO3 2-) is the primary contributor to alkalinity; when the pH is below 8.3,bicarbonate (HCO3 -) becomes the dominating factor. The values of alkalinity are reported inunits of "mg CaCO3/L" because of its relationship to hardness, which is reported using the sameunit.Although large environmental labs perform alkalinity tests using automated methods (1sample/minute), we will employ the traditional analyses of using an acid-base titration with twoendpoints. To gain insight into the double-endpoint titration technique you will perform apotentiometric titration, pH vs. titrant volume of your water sample. These techniques aredescribed 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. DETERMINATION OF ALKALINITYA sample may contain any of the three anions (carbonate, bicarbonate, or hydroxide) or somecombinations 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 (H2S04). Regardless ofthe source of the H+ the following reactions will occur:(1st endpoint) H+ + CO3 2- → HCO3 –
  2. 2. (1st endpoint) H+ + OH- → H2O(2nd endpoint) H+ + HCO3 - → H2CO3 (really CO2 + H20)The two pKa values for the carbonate system are pKa1 = 6.35 and pKa2 = 10.33.The unknown and your aqueous sample could contain any one of the following combinations ofthe anions responsible for alkalinity: 1. carbonate only (CO3 2-) 2. bicarbonate only (HCO3 -) 3. hydroxide only (OH-) 4. carbonate and bicarbonate (CO3 2- and HCO3 -) 5. carbonate and hydroxide (CO3 2- and OH-)We need to consider each of these five situations separately to see how the ions will behave atthe two different endpoints of the titration (using two different indicators). One of the indicatorswe will use is phenolphthalein (we will use the abbreviation "phth" from now on). It has apinkish color at basic pH values and changes to colorless as we go below pH 8.3 (endpoint #1,EP1). The second indicator has traditionally been methyl orange, which changes from yellow toorange at a pH range of about 2-3; however, many people find this color change difficult toobserve. We will, therefore, use a mixture of indicators (bromcresol green/methyl red), whichgives a color change easier to see (from a blue-green to a very light pink, yellow, or colorless).This is endpoint #2 (EP2).Let us now consider the five different possibilities for analyzing and detecting the amount ofcarbonate, bicarbonate, and hydroxide that is present in a sample. 1. CARBONATE ONLY (CO3 2-):When carbonate ion is added to water the resulting pH falls in the range of 11-12. It can betitrated with acid to bicarbonate, the equivalence point occurring at a pH ≈ 8.3. Phenolphthalein(phth) will show a color change from pink to colorless in this pH range and it is ideal fordetecting the first endpoint (EP1).The bicarbonate ion can then accept another H+ to become "carbonic acid", which is reallycarbon dioxide (aq) and water. The mixed indicator will change color in this pH range and willallow us to detect the second endpoint (EP2).► Notice that if carbonate is the only original species, it should require equal amounts of H+ foreach endpoint. 2. BICARBONATE ONLY (HCO3 -)
  3. 3. If bicarbonate is the only substance in the sample, what would happen when phenolphthalein isadded to detect the first endpoint? The indicator would immediately turn colorless becauseHC03- would give a pH lower than the first endpoint. [Actually the indicator might appear as avery light pink color, but would require only a drop or two of acid to become colorless.]In other words, the bicarbonate is ready to be titrated only to the second endpoint and wouldrequire some amount of acid (Y mL [H+]).► Notice that if bicarbonate is the only original species, it should require approximately 0 mL of[H+] for the first endpoint. 3. HYDROXIDE ONLY (OH-)If hydroxide is the only substance in the sample, it is very basic and the phenolphthalein will turna deep pink. Thus acid needs to be added until the color disappears. At that time, we know thatall of the OH- has been converted to water, which can no longer react with additional acid.When we add the mixed indicator it should immediately change color (or it might require a dropor two of acid). This occurs because there isn’t any base left to react with the acid.► Notice that if hydroxide is the only original species, it should require approximately 0 mL of[H+] for the second endpoint. 4. CARBONATE (CO3 2-) and BICARBONATE (HCO3 -)During the first titration, only the carbonate reacts, producing bicarbonate at the first endpoint.Then bicarbonate reacts with acid to reach the second endpoint, but we now have two sources ofthe HC03-.
  4. 4. ► Bicarbonate that was originally in the sample► Bicarbonate that was produced from the carbonate during the first titration.Essentially theresults are identical to adding the two separate situations together.► Notice that this means the second endpoint volume (X + Y) will be larger than the firstendpoint and neither endpoint requires zero mL of acid. 5. CARBONATE ONLY (CO32-) and HYDROXIDE (OH-)Again we can add the two individual results together to predict the relative volumes of the twoendpoints. For the first titration, acid is needed to react with both the carbonate and thehydroxide ions, but when the first endpoint is reached, the hydroxide is finished, because it hasall been converted to water.Then the bicarbonate (produced from the carbonate) is left to react with the acid to reach thesecond endpoint.► Notice that this means the first endpoint volume (X + Z) will be larger than the secondendpoint volume (X) and neither endpoint is zero. Table 1 SAMPLE EP1 volume EP2 volume Comparison CO32-only X mL [H+] X mL [H+] EP1 = EP2 HCO3-only X mL [H+] Y mL [H+] EP1 ≈ 0 OH-only X mL [H+] 0 mL [H+] EP2 ≈ 0 CO32-and HCO3 X mL [H+] {X+Y} mL [H+] EP1< EP2 but EP1 ≠ 0 CO3 2-and OH- {X+Z} mL [H+] X mL [H+] EP1> EP2 but EP2 ≠ 0  To use this table we only have to know how much acid was required to reach each endpoint and match it up with one of the five possibilities. CALCULATIONS TO REPORT ALKALINITY VALUES
  5. 5. Results from the alkalinity analysis are reported in units of "mg CaC03/L sample". This is usedbecause traditionally "water hardness" is also reported with the same unit.We will have data from our experiment to represent the mL of acid required (A) and the molarity(M) of the sulfuric add. Using the reaction below, we can convert these measurements to thedesired unit of "mg CaC03/L sample". H2SO4 + CaC03 → CaSO4 + H20 + CO2Because we will use 20.0 mL of sample, the conversion looks like this:When all constants are collected and the units are cancelled, we get Equation (1):  Alkalinity (in mg CaC03/L sample) = A x M x (5.00 x103) Eq. 1The results from an alkalinity analysis are reported using five different parameters:  total alkalinity - the result based on the total amount of acid used  phenolphthalein alkalinity - the result based on the amount of acid required to reach the first endpoint  carbonate alkalinity - the result due to the presence of C03 2- in the sample  bicarbonate alkalinity - the result due to the presence of HCO3 2- in the sample  hydroxide alkalinity - the result due to the presence of OH- in the sampleTo report these five values, we use the same Equation (1) to do every calculation. Only the value ofA will be different {our molarity (M) is the same for the entire experiment}. It will be helpful to lookat another table to determine the value of A. EP1 and EP2 refer to titrant volumes (H2SO4) at the tworespective endpoints. TABLE 2 SAMPLE Total ALK phth ALK C032- ALK HCO3- ALK OH- ALK CO32-only A=EP1 + EP2 A=EP1 A=EP1 + EP2 A=0 A=0 HCO3-only A=EP2 A=0 A=0 A=EP2 A=0 OH-only A=EP1 A=EP1 A=0 A=0 A=EP1 CO32-and A = EP1 + A=EP1 A = 2 X EP1 A = EP2 – A=0 HCO3 EP2 EP1 CO3 2-and A=EP1 + EP2 A=EP1 A = 2 X EP2 A=0 A = EP1 – OH- EP2
  6. 6.  To use this table, we first have to identify what is in our sample by going through the analysis with Table III-l. Then, knowing what is in the sample , we come to Table III-2 and go to the line with that category. By following across the line, we know what value of A to substitute into Equation (1) for our parameters.EXAMPLE: Suppose CO3# - EC, required 8.11 mL of 0.00987 M H2SO4 to reach EP1 and anadditional 2.95 mL to reach EP2. This means that EP1 > EP2 and from Table III-1, that tells methat my unknown contains CO3 2- and OH-. I now need to go to Table III-2 to find the values ofA that should be substituted into Eq. (1): 3 Alkalinity (in mg CaC03/L sample) = A x M x (5.00 X 10 ) a) Total Alkalinity: A = EP1 + EP2 = 8.11 mL + 2.95 mL = 11.06 mL Total alkalinity = (11.06) x (0.00987) x (5.00 X 103) Total alkalinity = 546 mg CaCO3/L b) Phenolphthalein Alkalinity: A = EP1 = 8.11 mL Phenolphthalein alkalinity = (8.11) x (0.00987) x (5.00 X 103) Phenolphthalein alkalinity = 400. mg CaCO3/L c) Carbonate alkalinity: A = 2 x EP2 = 2 x 2.95 mL = 5.90 mL Carbonate alkalinity = (5.90) x (0.00987) x (5.00 X 103) Carbonate alkalinity = 291 mg CaCO3/L d) Bicarbonate alkalinity = 0 mg CaCO3/L (because there isnt any in the unknown. Bicarbonate alkalinity cannot be negative) e) Hydroxide alkalinity: A = EP1 – EP2 = 8.11 mL - 2.95 mL = 5.16 mL Hydroxide alkalinity = (5.16) x (0.00987) x (5.00 X 103) Hydroxide alkalinity = 255 mg CaCO3/L
  7. 7. Total Alkalinity Procedure 1. Clean the burette and fill almost to the top with N/50 sulfuric acid. Then run some acid to waste until the "zero" mark is reached. This should leave the stopcock and tip of the burette full of the solution. 2. Measure out 100 mL of the water to be tested and pour into a clean white porcelain evaporating dish. 3. With a dropping bottle, add 2 or 3 drops of methyl orange or methyl purple indicator to the sample and stir. When alkalinity is present, the solution becomes yellow when methyl orange is added or becomes green when methyl purple is added. (Note: When a high chlorine residual is present, the chlorine bleaches the color and makes the determination of endpoint difficult. The chlorine may be removed using sodium thiosulfate, or an additional indicator may be added.) 4. Slowly and carefully add N/50 sulfuric acid from the burette to the contents of the dish until the faintest pink coloration appears - that is, until the color of the solution is no longer yellow. While adding the acid, the solution should be gently stirred with the stirring rod. It is often advantageous to set up two 100 mL samples, adding methyl orange to each and acid to only one while the other is held alongside so that the colors may be compared and the color change to pink can be better recognized. (Note: When using methyl purple, the color is changed to purple. A gray tint precedes the end point and warns the operator of its approach.) 5. Record the volume of sulfuric acid used to reach the endpoint. 6. Calculate the total alkalinity, as follows: T. alk. = (mL of acid) × (10 ppm/mL)For example, if 1.5 mL of acid were used in the titration, then the total alkalinity would be: T. alk. = (1.5) × (10) = 15 ppm CaCO3(Note that this simple formula requires that you use the exact acid concentration and samplevolume listed in this procedure. If you use different values, you must calculate the alkalinity asfollows:  This more complicated equation can also be used to calculate phenolphthalein alkalinity.
  8. 8. Phenolphthalein Alkalinity Procedure 1. Clean the burette and fill almost to the top with N/50 sulfuric acid. Then run some acid to waste until the "zero" mark is reached. This should leave the stopcock and tip of the burette full of the solution. 2. Measure out 100 mL of the water to be tested and pour into a clean white porcelain evaporating dish. Stir the sample. 3. With a dropping bottle, add 5 drops of phenolphthalein indicator to the sample and stir. When phenolphthalein alkalinity is present, the solution becomes pink. No color indicates the phenolphthalein alkalinity is zero and that the test is complete. No color also indicates that free carbon dioxide is present, and the same sample may be used to test for carbon dioxide. 4. If pink color results after the indicator is added, slowly and carefully add N/50 sulfuric acid from the burette to the contents of the dish until the coloration disappears. While adding the acid, the solution should be gently stirred with the stirring rod. 5. Record the volume of sulfuric acid used to reach the endpoint. 6. Calculate the phenolphthalein alkalinity, in ppm, as follows: P. alk. = (mL of acid) × (10 ppm/mL)For example, if 2.4 mL of acid were used, the phenolphthalein alkalinity would be: P. alk. = (2.4) × (10) = 24 ppm CaCO3LAB PRECAUTIONSo You must wear safety goggles throughout this experiment.o We will be working with dilute acids and bases, but you may want to consider washing your hands well if in contact with the solutions.o You may wish to wear a lab coat or apron to protect your clothing.o If you have any cuts or sores on your hands, you may wish to wear gloves.o Clean any spills you may have with plentiful amounts of water.o Do not rub your eyes unless your hands have been thoroughly rinsed.o You must wash your hands before you leave this (and any lab)
  9. 9. Results & Discussions Test Type Water Source mL of acid Alkalinity (ppm as CaCO3) usedTotal alkalinity testPhenolphthalein alkalinity test1. What is the pH of the water? ______________________2. Does the water meet the first two criteria for being non-corrosive? ________________________3. Is the water caustic? ____________________4. How much of the alkalinity is hydroxide? ________________5. How much of the alkalinity is carbonate? __________________6. How much of the alkalinity is bicarbonate? _______________ CONCLUSIONS:o Has the goal of the experiment been met?.................................o Sources of possible or known errors and consequences of those errors (i.e., high results, low results, sporadic results, why? ………………………..o Suggestions for improvements……………………………

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