Environmental Engineering Reference
In-Depth Information
Table 7.5 Calculation of Eh data with respect to the Technical Standard Line of the NTCHS
Day
pH
Eh of TS line
Measured Eh
Eh relative to the TS line
1
4.1
349
655
306
2
4.3
337
342
5
3
4.5
325
371
46
4
4.7
313
342
29
5
4.9
301
209
92
6
5.1
289
30
319
These calculated values can then be subtracted from the measured Eh values
(made using Pt electrodes). Positive values indicate that they are above the TS
line (oxidizing) and if the values are negative, this means the data would plot
below the TS line (reducing). If the data shown in Fig.
7.8
are plotted relative to
the TS line, we obtain the graph shown in Fig.
7.9
. On the graph in Fig.
7.9
, the
“zero” of the Y axis (Eh relative to the TS) represents the TS line itself. So any
values that are greater than zero imply that the data plot above the TS line (in an
Eh pH diagram) and are oxidizing with respect to Fe, and any negative values
imply that the data plot below the TS line and are reducing with respect to
Fe. This plot allows a quick visual evaluation of how these conditions change
with time and at what point in time they become reducing.
Submission of Lab Report
Each student will write and submit a laboratory report based upon the data collected
by the group (in the table below, data collected by students are shown by the shaded
boxes). Each student should then calculate the Eh (considering the correction for
the reference electrode). A graph similar to Fig.
7.8
should be constructed. The Eh
of the TS line should be calculated for the pH values measured (and interpolated)
for the soil. Then you should calculate, for each electrode, the Eh relative to the TS.
Using data measured and calculated in the student's version of Table
7.6
, each of
the following figures should be prepared:
• A figure showing Eh over time (similar to Fig.
7.8
)
• A figure showing Eh and pH in relation to the TS line and to the goethite stability
line (similar to Fig.
7.9
)
• A figure showing Eh relative to the TS as a function of time (similar to Fig.
7.10
)
Each student should provide a discussion of the data and figures that should
include some reference to the following:
• Reproducibility and replication of the data measurements;
• Comparison of redox data collected at the two soil depths;
• The meaning of the Eh/pH data collected, implications of the data based on
Eh/pH stability diagrams and where data plot relative to the TS line; and
• Comparisons between methods used - in particular the Eh/pH data and the
reaction of the soil with alpha-alpha dipyridyl dye.
