CVEN 3454/5404 Water Chemistry

Lab 3: Turbidimetric Measurement of Sulfate

 

Purpose

The purpose of this lab is to understand the role of precipitation and spectrophotometry by using the "turbidimetric method" to measure the concentration of sulfate in standard solutions and water samples from the Lefthand Creek watershed.  In this method, sulfate (SO42-) is precipitated as barium sulfate (BaSO4) following the addition of barium chloride (BaCl2).  The amount of light scattered by the barium sulfate suspension is measured on a spectrophotometer (actually, this is a measurement of turbidity) and compared to the amount of light scattered by a series of sulfate standard solutions.

Materials

The TA will supply the following:

Procedure

  1. Turn on the spectrophotometer, set the wavelength to 420 nm, and zero the absorbance with deionized water.

  2. Prepare a series of sulfate standards at concentrations of 0, 0.15, 0.30, 0.60, 1.0, and 2.0 mM Na2SO4 using the sodium sulfate stock solution (20 mM) and deionized water.  The standard with zero sulfate is referred to as the "blank."

  3. Pipette 10.0 mL of the blank and the standards into 50 mL beakers and add 1.0 mL of the acid "seed" solution to each.  The acid "seed" solution is a strong acid -- handle with extra care (gloves).  Swirl the flask gently to mix the solutions.

  4. Add 0.5 g of BaCl2 crystals to the flask containing the blank.  Let the flask stand for 1 minute.

     
  5. Put the flask on a stir plate, add a magnetic stir bar, and stir the solution for at least 3 minutes or until all of the BaCl2 crystals are dissolved.  Be consistent about the stirring time and stirring rate for all samples!  Try to estimate the stirring rate in revolutions per minute.

     
  6. With a pipette, withdraw a few milliliters of the suspension from the flask while the solution is still stirring.  Transfer the sample in the pipette to a spectrophotometric cell (a glass test tube).  Clean the outside of the cell, measure the absorbance at 420 nm in the spectrophotometer, and record the absorbance.

     
  7. Repeat steps 4, 5, and 6 for the five standards.

  8. Create a standard curve by plotting absorbance versus sulfate concentration for the blank and standard solutions.  Perform a regression (preferably first-order; i.e., linear) on the standard curve and solve the regression for the sulfate concentration.

  9. Perform steps 3, 4, 5, and 6 for the Big Five Tunnel drainage and Lefthand Creek samples.
    If the resulting absorbance for any sample exceeds the maximum absorbance measured for the standard solutions by more than 20%, dilute the sample added to the flask in step 3 (e.g., add 1.0 mL of sample and 9.0 mL of deionized water for a 10% dilution, or 0.5 mL of sample and 9.5 mL deionized water for a 5%).  For diluted samples, record the absorbance and note that the absorbance is for a sample of xx% dilution.  Later, the concentrations determined for these absorbances will be divided by the dilution factor to give the actual sulfate concentration for the sample.

  10. For one of the samples (presumably one of the samples for which a dilution is not necessary), perform two more measurements (a total of three measurements) of sulfate concentration to determine the reproducibility of the method.  Perform steps 3-6 for each replicate.

  11. Determine the concentrations of the samples using the standard curve regression.  For diluted samples, determine the concentration of the dilution, then correct the concentration for the dilution factor.  This step need not be completed during the lab.

Lab Technique Note:

       To obtain reproducible results using this method, careful attention must be paid to uniform temperature, time and rate of stirring, and time of standing of the suspensions before measurements.
       In the open air of the laboratory, we can only hope that Facilities Management has set up the HVAC system to maintain a uniform temperature, but we recognize that this is difficult because the fume hoods pull a large volume of air through the room in short times.  The best temperature control we expect in the lab air is ±1 °C.  Sometimes, we might use a water bath to maintain temperature (typical water baths can control temperature to ±0.1 °C, but water baths typically cost more than $1,000 and water baths containing built-in shakers cost nearly $3,000). 
       The rate of stirring is controlled by using a magnetic stirrer.  Be sure to maintain a constant stirring speed using the speed control.  Control the time of stirring carefully by paying attention to the time.  The original method specifies a stirring time of 3 to 8 minutes.  The actual time is less important than consistently stirring for the same amount of time.  Beware that prolonged stirring can heat up the suspension because the electric motor in the stirrer gives off heat.       

Questions to Address in Lab Report:

(Results) Recreate your sulfate standard curve (absorbance versus sulfate concentration) as a figure and provide the regression as an equation in the text (show it as [SO42-] = (Abs - Intercept)/Slope; don't leave y and x in the equation).

(Results) What was the reproducibility of the method for the water sample that you measured three times? Express the reproducibility of this method in terms of a coefficient of variance (the ratio of the standard deviation and the mean expressed as a percentage).

(Results) Estimate the detection limit for this method using the "effective" sulfate concentration of the blank solution and the reproducibility of the sample as discussed in lecture.  Compare your detection limit to the detection limit listed for this method in Standard Methods (discussed in lecture).

(Results) What were the sulfate concentrations (molar) in the Big Five Tunnel drainage and Lefthand Creek watershed samples?  Present these results in a table.  Include the absorbance and, if necessary, the dilution factor of each sample as table entries.

(Discussion) What factors affect the detection limit of the method?  What factors affect the reproducibility of the the method?

(Discussion) Explain the differences in sulfate concentration in the Big Five Tunnel drainage and Lefthand Creek watershed samples.

(Discussion) Based on the sulfate concentrations, estimate the ratio of flow rates for the Big Five Tunnel drainage and the Lefthand Creek upstream of the drainage (i.e., QBFd / QLh,up).  Compare this estimate to your flow rate ratio estimate determined with alkalinities (Lab 2) and comment on any differences.

Last updated on July 31, 2007 at 07:18 AM by Joe Ryan