edited by
JAMES 0. SCHRECK Univemity of Northern Colorado
Filtrates e residues
Greeley. CO 80639
Microscale Experiments Dissolved Oxygen and Chloride Determination in Water Mary Crosson Wellesley High School, Wellesley, MA 02181 Reen Gibb Brookline High School. Brookline. MA 02146 As part of a project sponsored by the National Science Foundation, "Application of Basic Science in Industry and Scciety to Enhance Secondary School Science", we adapted experiments used in environmental studies to microscale. Decreasing budgets and increasing awareness of chemical hazards make microscaling experiments very timely because of greater safety, less waste disposal, and decreased costs by saving time, money, and storage space. Two experiments that we adapted to microscale are the determination of dissolved oxygen (DO) in water, based on the Winkler method (I), and the determination of chloride concentration in seawater, based on the Mohr method (2). I n microscale titrations the concentrations are expressed in terms of the mass of the solution (mol solutelg solution) rather than as the molarity (mol solute1L solution). Burets are replaced by Beral pipets, and mass measurements replace volume measurements. One milligram digital balance with tare is sufficient for a class of 12 laboratory groups. Students weigh Beral pipets (supported in a small beaker or wire holder) before and after the titration. The reagents can be saved in taped, labelled Beral pipets for future use. Determining Dissolved Oxygen DO in a water sample is determined by a sequence of reactions (3).In alkaline solution, manganese(I1) hydroxide is precipitated (eq 1) and oxidized (eq 2) by the dissolved 0 2 to Mn(OHI3.This is converted by the addition of sulfuric acid (eq 3) to manganese(II1) (as sulfate), which immediately oxidizes (eq 4) iodide ion (previously added a s KI) to iodine. The liberated Iz in the form (eq 5) of soluble I3 is determined by titration with sodium thiosulfate solution of known concentration, using starch indicator (eq 6). ~ n ' + +2 0 K + Mn(OH),
(1)
4Mn(OHIz+ 0, + 2Hz0 + 4Mn(OHI3
(2)
for each mole of oxygen in the sample, 4 mol of Na2S2O3is required. 1mol 0, x
4 rnol Mn(OH), 1mo102
1 ma^ M ~ ~ +1mol I2 X 1mol Mn(OH), zm01&*
The 1976 EPA Quality Criteria for Water recommends a minimum concentration forDO of 5.0 mg/L (which is 5 ppm when the density of sample is 1 g/mL) to maintain good fish population (4). DO values decrease with rising temperature. They also depend on the atmospheric pressure and the chemical content and depth of the water. At constant temperature, the DO decreases a s the chloride ion concentration increases. Reference I gives the DO (in mg/L) a s a function of temperature and chloride concentration for water exposed to water-saturated air (see Table 421:l of that publication). Determining Chloride Concentration I n this microlaboratory, the chloride concentration in seawater is determined by microscale titration of the saltwater sample with silver nitrate solution of known concentration in the presence of chromate (CrOz-1, which serves a s the indicator. Chloride ions precipitate as silver chloride. Silver chloride is less soluble than silver chromate. Therefore, when chloride has been removed from solution, the orange-red silver chromate begins to precipitate, signalling the endpoint.
The chloride concentration in seawater is around 1.93%. Location and depth cause slight variations in this value (5). Dissolved Oxygen Experimental Materials Starch reacts with the iodine to produce a n intensely blue complex. Upon reduction of iodine to iodide, the blue disappears, indicating the endpoint. The amount of DO in the water sample is reported in parts per million (ppm). As the balanced equations show,
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Each group should have the following solutions in labelled, small Beral pipets: manganese sulfate, potassium hydroxidelpotassium iodide, concentrated sulfuric acid (Caution), and freshly prepared starch solution. Sodium thiosulfate solution should be put in a large Beral pipet.
Preparing Solutions (based on 40 titrations)
Sodium Thiosulfate (Na~Sz03)Solution This solution should be an approximately 5 x lo4 molfg solution. Dissolve 0.080 g of NazSz03in approximately 100 g of distilled water i n a preweighed beaker. Weigh the beaker again to get the mass of the total solution. Sample calculations for the concentration of the solution are given in Figure 1.
1 mol mol Na,S,O, = 0.081 g x ----- 5.1 x lo4 158.11 g - 4.9 x 10" mol mol NaB,O, - 5.1 x lo4 mol g solution 103.714g solution g solution
Figure I. Sample calculations for NazSz03 wncentration tor 0.081 g Na2S203 dissolved in 103.714 g solution. Manganese Sulfate (MnSOd Solution Add 100 mL water to a beaker containing 1.0 g MnSO4 and mix. Potassium Hydroxide /Potassium Iodide Solution C a u t i o n : Patasium hydroxide is caustic. Avoid contact with skin. Add 100 mL water to a beaker containing 2.1 g KI and 0.7 g KOH and mix. Starch Solution (3%) Add 1 g soluble starch to 33 g water. Heat (do not boil) and stir until the solution becomes clear.
blue just disappears. Then reweigh the NazSz03 Beral pipet. Record the mass of the pipet before and after addition. A sample student data table is given in Table 1, and calculations based on this data are given in Figure 2. Chloride Experimental Materials
Procedure Caution: The solution of MnSO&I/KOH is very corrosive. Avoid contact with the skin and eyes.
Premark a large test tube for 25 mL by adding 25 mL HzO from a graduated cylinder. The DO content is apt to change if the sample remains in contact with air or is agitated. Collect a 25-mL sample, being careful to avoid air bubbles. Record temperature and immediately 'fix" the DO by adding 20 drops (1 mL) of MnS04 solution and 20 drops of KVKOH solution. Mix well and then add 2 drops of concentrated H2S04(Caution)to the sample in a large test tube. Stopper the test tube and swirl gently to mix contents. Measure approximately 10.000 g of this prepared water sample into another test tube (25-mL size). Save the remaining sample for a sewnd trial. Add 1 drop of freshly prepared starch solution to produce the blue of the starchiodine. After weighing t h e NazSz03 B e r a l pipet, a d d t h e NazS203solution to the water sample dropwise until the Table 1. Sample Data for the Titration of Water Sample with Sodium Thiosulfate
Mass of water sample used Initial mass of NazSz03 solution and Beral pipet Final mass of NazSz03solution and Beral pipet Concentration of NazSz03 solution (provided by instructor)
Figure 2. Sample calculations to determine the oxygen concentration in water.
10.303 g 6.728 g 5.310 g 4.9 x lod mollg of solution
For each gmup have the following solutions in labelled, small Beral pipets: potassium chromate, silver nitrate, and seawater or a n unknown NaCl solution. Directions for Preparing Solutions
Silver Nitrate (AgN03)Solution Caution: Silver nitrate will stain the skin. The concentration should be approximately 5 x 104 moWg solution. Dissolve approximately 2.000 g of AgN03 in approximately 25 g of deionized water in a preweighed beaker. Weigh the beaker again to get the total mass of the solution. Saltwater Solution For the water sample, use ocean water or make up a salt solution by dissolving 1.45 g of NaCl in 50 g of deionized water. Put the solution in a Beral pipet. Table 2 contains sample data for the preparation of the salt and silver nitrate solutions. Sample calculations for the solution data are provided in Figure 3. Table 2. Sample Data Table for the Preparation of the Salt and Silver Nitrate Solutions
mass of sodium chloride taken mass of sodium chloride and water mass of silver nitrate taken mass of silver nitrate and water
1.489 g 50.623 g 0.852 g 11.060 g
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1. Concentration of the standard silver nitrate solution. 0.852 g AgN03 11.060 g solution
I mol AgN03 mol AgNO, = 4.53 lod Solution 169.879 AgN03
3.121 g - 2.243 g = 0.878 g saltwater used 2.820 g - 1.818 g = 1.002 g silver nitrate solution used Molecular weights:
1 mot of AgN03 = 1 mol Ag' = 1 mol CL
1 rnol NaCl = 58.443 g
because Ag++ Cf + AgCl
1 mol AgN03 = 169.872 g
2. Theoretical value of %CI- (value students should obtain).
35.45 g Ct 1.489g NaCI 50.823 g solution 58.45 g NaCl
1.002 g AgNO, solution x 4.53 x 10"
mol AgNO, solution
= 4.54 x 1od mol AgNO,
= 1,78%CI
4.54 x 10" mol AgNO, Figure 3. Calculations lor the solution data
4.54 x
= mot
of ~ g +mol = CI-
35.5 g cr = 1.61 x 10-'g Cr mol CI-x ----1 mot CI-
Potassium Chromate Solution Caution: Potasium chromate is carcinogenic. Avoid contact with the skin. Dissolve 1.94 g of K2Cr04i n 10 g of deionized water. Figure 4. Sample student calculations.
Procedure Weigh the saltwater pipet. Then put 20 drops of the saltwater into a test tube (10-mL size), and reweigh the saltwater pipet to determine the mass of saltwater to be titrated. Add 1drop of the potassium chromate solution. Record the initial mass of the &NO3 pipet. Add the &NO3 solution dropwise, swirling between additions to mix the contents. Continue "titratin8 until a red ~ r e c i ~ i tete pwsists Roughly 20-30 drops "i'AgS03 w ~ l "suaily l be r r ~ r u ~ r eRewe~eh d the ArNO, Rrral v~r,etto determine of the &i\To8use; in t h e titragon. Sample stuthe dent data is given in Table 3. Calculations to determine the percentage of chloride are provided i n Figure 4.
Discussion Although the chloride determination does not require special precautions, the collection and analysis of the DO i n a sample is subject to considerable error. Our students collected their samples in the same test tube in which the chemicals, brought to the site i n cassette camers, were " DO. Obtaining s a m ~ l e from s deer, locaadded to " f ~the tions requires special d l e c t i o n bottles. 1)iscussion.i of the collection of DO sarnoles are found in ref's 1 and 3. I n the DO analysis, if t h e water sample is colorless, starch can be omitted. The disappearance of the yellow of
-
Table 3. Sample Data Table for the Titration of the Saltwater with Silver Nitrate Solution initial mass of saltwater and pipet final mass of saltwater and pipet initial mass of silver nitrate and pipet final mass of silver nitrate and pipet
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Journal of Chemical Education
3.121 2.243 2.820 1.818
g g g q
the dilute iodine will then signal the completion of the "titration". Our students did a profile of DO in neighboring aqueducts. During their vacations, students could collect saltwater samples from various locations for chloride analysis.
Exercise for Students What about the possible consequence of interfering substances? Chlorinated tapwater contains both Cl2 (chlorine) and OCT (hypochlorite). How does the presence of these species affect results? What affect would NO; (nitrite), a common constituent of polluted water, have on DO results? All three species (C12(eq I ) , OC1-(eq 81,and NO5 (eq 9)) are capable of oxidizing I- to 12, thus increasing the apparent oxygen content of the sample. c12+ 2 r + 2 ~ 1 -+ I~
(7)
HI+OCT+2T+C1~+12+OK
(8)
Acknowledgment The authors are indebted to Walter Weibrecht for his support and helpful suggestions. Literature Cited I. standard ~ e t h o d for s the ~zominationofwater and was& wohr. 15th ed: h e n c a n Public Health Association: NewYork. 1981: pp 388,390. 2. Skmg. D. A: West. D. M. Fundamentals ofAnalyt~co1Ckemisfq, 3rd ed: Holt: USA, 1976:p 179. 3. Stagg,W. R. J Chom Educ. 1972.49.428. 4. Dlsolud &wen woter Qualit" Standards Criteria Diaesi:Government Pnntine Office: w&.pton, ~.i,19is:I. 5. Ross, D. A. Inimducfion lo Oceenogmphy, 2nd ed: Plentie-Hall: USA, 1917; p 98.
