CVEN 3454/5404 Water Chemistry

Lab 2: Carbonate Alkalinity Determination

Purpose

The purposes of today's laboratory are to (1) understand the acid-base chemistry behind the carbonate alkalinity measurement and (2) to accurately measure the alkalinity of three Lefthand Creek watershed water samples.

Materials

• pH meter (Orion, model 250A or 290A)
• pH electrode (Orion Triode^® model 9107BN) and electrode stand
• pH buffers (4.01 and 7.00)
• stir plate, stir bars, beakers, 50 mL burette, stand
• sodium bicarbonate solution (NaHCO₃; about 0.2 mM; exact concentration will be provided)
• sulfuric acid solution of unknown concentration (H₂SO₄; about 0.001 M, or 0.002 N)
• water samples
  1. Lefthand Creek (about 40 m upstream of the Big Five Tunnel drainage)
  2. Lefthand Creek (about 40 m downstream of the Big Five Tunnel drainage)
  3. Big Five Tunnel Drainage (at entrance to Lefthand Creek)

Procedure

• Calibrate your pH meter with pH 4.0 and 7.0 standards. Remember to make sure that the auto-shutoff feature has been turned off (press 2nd+setup, press yes down to setting 1-4, press down arrow to change the setting to "no", press yes to go to setting 2-1, press measure to return to measurements).
  
  
• In the first titration, we'll determine the exact concentration of the sulfuric acid titrant solution (about 0.001 M) by titrating the sodium bicarbonate standard.
  a.  Fill the titration burette with the sulfuric acid titrant.
  b.  Add 100 mL (know the exact volume) of the sodium bicarbonate solution to a beaker and add a stir bar.
  c.  Arrange the beaker on the stir plate and insert the pH electrode into the beaker.
  d.  Titrate the bicarbonate solution with the titrant with additions ranging from 0.5 to 2.0 mL.
  e.  Monitor pH as a function of volume of titrant added until pH reaches about 3.
  f.  Use the Lab 2 spreadsheet to create a graph of pH versus volume of titrant added to show your progress.
  
  
• In the next three titrations, we will use the sulfuric acid titrant to determine the carbonate alkalinity of three water samples from Lefthand Creek and the Big Five Tunnel drainage.  Follow the titration directions listed above, but substitute the water samples for the sodium bicarbonate solution (with the exception that the sample volume should be 50 mL instead of the 100 mL for the sodium bicarbonate).  These titrations will be relatively quick (i.e., low titrant volumes).  Recall that water samples with pH values below the carbonate alkalinity endpoint (the pH at which {H⁺} = {HCO₃^-}) have negative alkalinity, and the alkalinity can be determined from using these titrations, too.

Determining the Endpoint using the Gran Function

The Gran function can be used to determine the end point of the carbonate alkalinity determination more precisely than other methods (e.g., a fixed pH value, an color indicator like methyl orange). Here's a quick review of the Gran function along with an application of it to a set of data gathered by the TAs.

The Gran function is a measure of the number of moles of H⁺ added to the solution after the end point has been reached:

excess moles H⁺ added = F₁ = (V_orig + V_titr) {H⁺} = (V_orig + V) (10^-pH)                       (1)

where F₁ is the Gran function, V_orig is the original volume of the sample solution (mL), V_titr is the volume of acid titrated into the solution (mL), and {H⁺} is the activity of protons.

Use the Lab 2 spreadsheet to create a graph of the   Gran function (F1) versus the titrant volume (Vtitr).  At a certain point along the x-axis, F1 will increase rapidly. These are the data points measured after the endpoint, where the acid added is no longer titrating carbonate species.  Instead, the added acid is decreasing the pH directly. Make a linear regression of these points with F1 > 0 and determine where the linear regression crosses the x-axis (where y = 0). The point at which the linear regression crosses the x-axis is called Vend, the end-point volume that you are seeking. The following figure contains an example using some data gathered by the TA for the titration of 2.00 mM NaHCO3. The top graph shows the titration curve -- you can see that it would difficult to pick out the end point just from the titration curve because the slope is not that steep where you would expect the end point (near pH 4 to 5). The bottom graph shows the plot of the Gran function vs. the volume of titrant added. The linear regression line passes through the x-axis at Vend = 9.40 mL, so that is the end point.

Lab Technique Note

As you may have noticed last week, titrations are not very exciting. However, a little perseverance and a some patience will produce some good data. If you have good data, answering the lab report questions will be much easier. Patience is a very important quality for anyone doing research in the lab. Once recent Nobel Prize winner in Biology who worked on the genetic make-up of flies said that 90% of his experiments were failures!

Questions to Address in Lab Report

• (results) Show the titration curve (pH versus volume) and Gran function curve (F₁ versus volume) for the bicarbonate standard.  Show the Gran endpoint on both graphs.
  
  
• (results) What was the molar concentration of the H₂SO₄ titrant solution?
  
  
• (results) Show the titration curves and Gran function curves for the three samples.  Show the Gran endpoints on all graphs.
  
   
• (results) What were the endpoint volumes and alkalinities (in molar concentration and as mg L^-1 CaCO₃) of the three Lefthand Creek water samples?  Show the equation used to make these calculations (use an Equation Editor).  Present these data in a table (with columns of endpoint volume, alkalinity (M), alkalinity (mg L^-1 CaCO₃)) for each sample.
  
  
• (discussion) Based on the pH and alkalinities you measured in the Lefthand Creek samples (not the Big Five Tunnel sample), estimate the concentrations of the various carbonate species (carbonic acid, bicarbonate, carbonate).  Assume that the Lefthand Creek water is closed to the atmosphere.  Present these calculated concentrations in a second table.
  
  
• (discussion) What other weak acids might contribute to alkalinity in the watershed samples?
  
   
• (discussion) Does the alkalinity measured for the Lefthand Creek sample taken downstream of Balarat Gulch make sense (i.e., as a mixture of upstream Lefthand Creek and the Big Five Tunnel drainage water)?  If so, calculate the relative flows of the Lefthand Creek (upstream) and the Big Five Tunnel Drainage (i.e., determine the ratio Q_Lefthand/Q_{Big Five}).

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