PLS 671: Environmental Soil Chemistry

“Speciation of the Soil Solution”
Lab #4

I. Preparation of a Saturated Soil Paste Extract

1.) Weigh 20 g of soil into a 125 mL polyethylene bottle in duplicate. 

2.) Add 40 mL of deionized distilled water and stir with a spatula. 

3.) Shake for 1h.  

4.) Transfer to a Büchner funnel containing filter paper connected to a vacuum flask.

5.) Apply a vacuum and collect the filtrate.  You should be able to collect at least 20mL of filtrate to allow for
     characterization of anions and cations.  Note:  Be careful not to contaminate this “stock soil solution.”

II. Characterization of Soil Solution

1. Measure pH with Metrohm pH meters.  Between pH readings, rinse electrode with DI water.  Be careful not
    to contaminate your sample.

2). Measure electrical conductivity (EC). 

3.) Anion Concentration

 a.)  Common soil anions (chloride, nitrite, nitrate, orthophosphate, and sulfate) will be
     quantified using a Metrohm ion chromatograph, in which anions are separated on a
     column packed with an anion exchange resin and detected by a conductivity detector as
     eluent is pumped at a constant rate through the system.  You will inject approximately
     ~2 mL of your water sample and depending on the composition of your solution, the
     anions will separate.  Each will have a characteristic retention time that may be used to
     identify components in your sample by comparison with those in a standard solution.
     Each injection run time will be 18 min.  The chromatogram will be a plot of signal versus
     time.  The area under the peak is directly proportional to the amount of anion present.
     The software will allow you to integrate the peaks to determine anion concentration in
     your samples. 

4.) Cation Concentration

a.) You will measure dissolved Fe, Mn, and Zn concentrations using flame atomic absorption
      spectroscopy (FAAS) against a set of standards that you will prepare.  Atomic absorption
      spectroscopy is based on energy absorbed during transitions between electronic energy levels of an
      atom.  Atoms of a given element have a diagnostic set of allowed electronic energy levels.  Atomic
      absorption spectroscopy consists of passing light of a specific wavelength that can be absorbed by
      the element of interest through an atomic vapor of the element.  By measuring attenuation of the
      light intensity as a result of absorption and comparison with standards in the same background
      solution, elemental concentrations can be derived.  Standards calibration curves plotting
      absorbance versus concentration (mg L-1) are prepared.  Samples are run against these calibration
      curves to measure concentrations of specific elements of interest.

5.)  Prepare standard solutions for Fe, Mn, and Zn in deionized water from 1000 mg L-1 stock solutions.
      Standard concentrations are 0 (blank), 0.5, 1.0, 5.0, and 10.0 mg L-1 (at total of 15 standards per