DETERMINATION IN WATER ANALYSIS USING THE PHENANTROLINE METHOD: HOW IT WORKS?
The
phenanthroline method is the preferred standard procedure for the measurement
of iron in water at the present time, except when phosphate or heavy metal
interferences are present. The method depends upon the fact that 1,10 phenanthroline
combines with Fe²+ to form a complex ion that is orange-red in color. The color
produced conforms to Beer's law and is readily measured by visual or
photometric comparison.
Water
samples subjected to analysis have usually been exposed to the atmosphere;
consequently, some oxidation of Fe(II) to Fe(III) and precipitation of ferric
hydroxide may have occurred. It is necessary to make sure that all the iron is
in a soluble condition. This is done by treating a portion of the sample with
hydrochloric acid to dissolve the ferric hydroxide:
Since
the reagent 1,10-phenanthroline is specific for measuring Fe(II), all iron in
the form of Fe(III) must be reduced to the ferrous condition. This is most
readily accomplished by using hydroxylamine as the reducing agent. The reaction
involved may be represented as follows:
By
proper modifications of the test procedure, measurements of total, dissolved,
and suspended iron can be made. These considerations are not normal, however,
and, whenever they are, special precautions must be taken in sampling and
transportation of samples to ensure that no changes occur before analyses are
performed. Because of the possible errors that may result, it is best that the
analyst assumes full responsibility for sampling as well as for analysis. When
interfering materials such as phosphates and heavy metals are present,
satisfactory results can be obtained by use of a procedure that involves
acidification of the sample with HCl and extraction of the iron into
diisopropyl-ether prior to the addition of the phenanthroline solution.[5]
Departing
from that point, we consider the first experimental conditions for measurement
of iron immediately (wait for 0 min) must have the precise detection value of
iron as prescribed in The U.S. EPA secondary drinking water standard is 0.3
mg/L. However, because of the instability of ferrous iron, which is changed
easily to the ferric form in solutions in contact with air, determination of
ferrous iron requires special precautions and may need to be done immediately
after at the time of sample collection. Then, if we consider result of the
first condition, the value of dissolved iron
(for 2 times test) is may vary from the range of 0.065 mg/L – 0.108 mg/L
with the average value is 0.087 mg/L. In other side, for the total iron it is
obtained that the range value of 0.15 mg/L – 0.394 mg/L with the average value
is 0.272 mg/L. Since 0.087 mg/L and 0.272 mg/L < 0.3 mg/L, therefore, we can
conclude that from the first experimental conditions the value of iron is
acceptable for drinking water standard in this context.
Next
, we consider the second experimental conditions which is let waiting the water
samples in 20 minutes. These conditions may cause the precipitation settlement
take place into water sample that influence the increasing of iron on the water
and it could be water sample that contain the ferrous iron, which is already changed
easily to the ferric form in solutions due contact with air exposure. In this
experiment we use the Phenanthroline method which is the procedure for
determining ferrous iron using 1,10- phenanthroline has a somewhat limited
applicability; avoid long storage time or exposure of samples to light. A
rigorous quantitative distinction between ferrous and ferric iron can be
obtained with a special procedure using bathophenanthroline. This type of
method limitation may influence all experimental conditions especially for the
second conditions when we have waiting for 20 minutes and after that doing the
measurement and may have the iron detection value higher than other
experimental conditions or from the drinking water standards(0.3 mg/L). Then,
if we are looking for the result of iron value, the dissolved iron (for 2 times
test) shows the range of 0.039 mg/L – 0.033 mg/L with the average value is
0.036 mg/L. In other side, for the total iron shows the range of 0.335 mg/L – 0.398
mg/L with the average value is 0.367 mg/L. Since 0.036 mg/L and 0.367 > 0.3
mg/L, therefore, we can conclude that from the second experimental conditions
the value of iron is not acceptable for drinking water standard in this context.
Lastly,
we consider the third experimental condition when the water sample should
aerate for 20 minutes and then wait until 5 minutes, after that do the
measurement process. Generally, we know that the aeration process mostly used
as the one of method of iron removal in water/wastewater treatment. Aeration
systems are ways of adding air to water in order replenish its oxygen levels.
The more air that is pumped into the water the better the circulation, thus
reducing chances of bacteria build up and stratification. This process is most
commonly used with non-free flowing bodies of water such as lakes, ponds, farm
dugouts, and reservoirs. One of the requirements for the successful
precipitation of iron would be to provide it with sufficient contact time to
oxygen so that the minerals can react(the effect from second experimental
conditions). Iron removal is technique used to remove excessive iron from
water. One of these techniques is aeration of drinking water. After we know
this concept, that makes these conditions special for sure because it is clear
that we can get the detection value of iron is very lower than other
experimental conditions. Let’s consider this graph about the effect of the
aeration time and the iron concentration value in water.[6]
From
this graph we can see that the Fe concentration measured on
well water after aeration was decreased when compared with before aeration,
which was statistically significant. But the iron concentration remained above
WHO guideline value of 0.3 mg/L . Increasing the aeration time produced a
reduction in the Fe level. Aeration caused lowering of Fe concentrations in the
well water. As per WHO Guideline, the iron concentration in drinking water
should be less than or equal to 0.3 mg/L . But the iron concentration was more
than above value [6]. This study prove that the aeration can decrease the iron
concentration value when compared without aeration, it means the value of this
condition must lower than the other experimental conditions or standard values.
Then, we consider the iron value from these conditions, the dissolved iron (for
2 times test) shows the range of 0.033 mg/L – 0.0310 mg/L with the average is 0.032
mg/L. In other side, the total iron shows the range of 0.4103 mg/L – 0.4111
mg/L with the average is 0.411 mg/L. Since 0.032 mg/L and 0.411 > 0.3 mg/L,
therefore, we can conclude that from the second experimental conditions the
value of iron is not acceptable for drinking water standard in this context.
Based
on this graph, its shows the different theoretical relationship as stated
before when the Fe concentration measured on well water after aeration was
decreased when compared with before aeration or other experimental conditions.
We know that the third experimental conditions which is aeration for 20 minutes
and wait 5 minutes should be lower than the other experimental conditions. This
error condition may influence from the 5 minutes waiting process due the
exposure of the air and light that easily interfere the water sample and from
the process of boiling.
Based
on this dissolved iron graph, its shows the different theoretical relationship
as stated before in first experimental conditions. Because we already know that
the instability of ferrous iron, which is changed easily to the ferric form in
solutions in contact with air, determination of ferrous iron requires special
precautions and may need to be done immediately after at the time of sample
collection. Also, we consider about the opportunity of precipitation settlement
like in second conditions that may not happen in the first condition, it is
because the measurement doing immediately without waiting like other
conditions. This point support why the determination of iron in first condition
should be lower than the other experimental conditions. But in this graph shows
that it higher than other conditions, that means it will be some interfere that
influence the result, it could be from the same thing as total iron when the
water sample get the exposure from the air and light which might be already converted
to other conditions and from the process of boiling.
References
[5] Sawyer,
C. N., Parkin, G. F., & McCarty, P. L. (2003). Chemistry for
environmental engineering and science (5th ed.). McGraw-Hill Education.
[6]
Marjani, A., Nazari, A., & Seyyed, M. (2009). Alteration of iron level in
drinking water by aeration in Gonbad Kavoos (North East of Iran). American
Journal of Biochemistry and Biotechnology, 5(2), 94–97.
https://doi.org/10.3844/ajbbsp.2009.94.97
Komentar
Posting Komentar