J.-X. Meng et al. / Spectrochimica Acta Part A 61 (2005) 823–827
825
log It − log It will be constant. We can get another expres-
2
1
sion 1/(t2 − t1) ∝ cCl or 1/(t2 − t1) = KcCl , where K is a
2
2
proportional constant, which means, if we could measure t1
and t2, chlorine concentration could be determined from a
working curve.
3. Experimental
3.1. Reagents
All the chemicals were of analytical reagent grade except
MO, which was of indicator grade. All solutions were pre-
pared with double distilled water.
3.2. Absorption solution
A 0.1000 g of MO was dissolved in 60 mL water at ∼50 ◦C
and the solution was cooled to room temperature. After
adding 60 mL ethanol, the solution was further diluted to 1 L
with water to get a stock solution. One gram of potassium
bromide was dissolved in 50 ml of the stock solution and di-
lute to 500 mL to get solution A. The absorption solution was
fabricated by mixing 8 mL solution A and 2 mL 3 M sulfuric
acid solution.
Fig. 3. Effect of the flow velocity of air samples.
4. Results and discussion
Air velocity could be adjusted with Pump 1 in the experi-
mental setup. It–t curve at different air velocity were shown
in Fig. 3. If the flow velocity of air sample is too large, ef-
fervesce in the absorption tube would be very furious and
lead to unstable of the transparent intensity. Large air veloc-
ity also leads to small values of (t2 − t1) and thus big relative
errors. Although a small velocity could give stable results, it
cost too long time to be adopted. A favorable velocity should
give medium values of (t2 − t1) and short analytical time.
A velocity no more than 0.5 L min−1 is appropriate for low
concentration samples (no more than 15.30 g L−1) and rise
with chlorine concentration.
3.3. Standard air sample
Pure chlorine is made from reaction of potassium per-
manganate and concentrated hydrochloric acid. Which was
desiccated with concentrated sulfuric acid and preserved in a
glass bottle. To make a standard air sample, definite volume
of pure chlorine was injected into a 10 L glass bottle. The
air was left to mix for more than 5 h before use. The above
produce may be repeated if necessary to get a more dilute
sample. Concentration of the standard air sample was con-
firmed before experiments with the recommended method
[1].
4.2. Effect of the absorption solution
3.4. General procedure for kinetic photometry
Similar to the effect in MO photometry method, acidity
of the absorption solution also affects fading velocity of the
solution and thus shapes of It–t curve as shown in Fig. 4. High
acidity caused fast fading and was favor to determination of
low chlorine concentration sample, while low acidity was
favor to that of chlorine rich sample. For most samples, an
acidity of 0.25 mol L−1 H2SO4 was reasonable.
A big drawback for MO photometry method is compli-
cated manipulation caused by unstable of the stock absorp-
tion solution. Since shift of A0 of the absorption will lead to
great error in ꢀA and determination of chlorine concentra-
tion. However, value of (t2 − t1) and thus the experimental
result are insensitive to shift of stock solution, as A0 is not
used in the present technique. Experiment has been done with
a stock solution preserved for 1 month at room temperature
and similar result with a fresh prepared solution was found.
Ten milliliters of absorption solution was put into the mod-
ified bubble absorption tube. Pump 2 was controlled to run
firstly for 2 s to ensure the flow cell give a stable signal. Then
Pump 1 began to run and drive air sample effervescem in the
absorption tube. In the mean time, the transparent light in-
tensity was measured and recorded with the computer. With
the present experimental apparatus, It is ∼0.03 before air ab-
sorption (which may vary with the concentration of MO in the
absorption solution) and 0.61 when all MO were consumed.
After measuring large number of samples with different chlo-
rineconcentration, goodlinearwasfoundbetween1/(t2 − t1)
and cCl when It = 0.3 and It = 0.5, which was adopted as
2
1
2
It and It in the following experiments. The computer pro-
1
2
gram was written to record It–t curve and output value of t1
and t2 after the experiment.