Zborník príspevkov z vedeckej konferencie - Department of ...

Reagents
Analytical-reagent grade chemicals were used in all experiments.
Deionised and degassed water was used for the preparation of all solutions.
Carrier electrolyte and electrolyte for sample preparation: 0.1 mol L 1 Na 2HPO 4.
The bulk standard solution hydrazine was 1 g L 1 . The calibration solutions were prepared fresh before the measurement by
diluting the bulk standard solution in 0.1 mol L 1 Na 2HPO 4. The concentration of the standards was in the range of 1 –
700 g L 1 for macroporous electrode and 0.7 – 50 mg.L -1 for microporous electrode, respectively.
Sampling and sample preparation
The samples were taken into amber glass bottles and stored in a refrigerator. The control measurements were performed at
the same time as the chronopotentiometric measurements. Prior to analysis, the sample in the bottle was shaken and then let
to sediment for few minutes. For analysis the supernatant was pipetted. The signal for macroporous electrode E-104C at low
concentrations of hydrazine decreases because hydrazine is not stable and oxidizes the oxygen present. Therefore, the
stabilization of a boiler water samples with hydrochloric acid for macroporous electrode E-104C is necessary. After
stabilization, the signal is higher and more stable for up to several days.
Sample analysis
The sample was added to Na 2HPO 4.and on mixing the solution was immediately analysed.
The boiler water samples were analysed with the elaborated method independently in two laboratories by two different
operators.
The accuracy of the results for boiler water samples was checked independently in an accredited laboratory by the titration
method according to the Slovak Standard STN.
3. Results and discussion
Optimisation
The solubility of hydrazine facilitates the electrochemical determination of hydrazine through stripping analysis by making
use of an electrode material forming such a hydrazine. A porous glassy carbon electrodes offer the best performance.
Covering the surface with Triton X-100 (helps no to detain bubbles in the pores of electrodes) improved significantly its
performance. The same electrode could be used for several days virtually without loss of the sensitivity.
On deposition the cell is flushed with the electrolyte and the deposit is stripped, the latter is washed from the electrode during
the rinsing step. Porous electrodes possess a unique feature, absent in non-porous structures, namely there is a possibility to
strip and deposit the analyte many times in a stopped flow regime. This can be used for signal accumulation in order to
increase the signal to noise ratio [20], or to shift the signal to a potential range with lower background level.
The first step was to optimize the parameter for the current dependence 3 mg L -1 hydrazine on microporous electrode E-53C.
Changing the current between 20 A to 1000 A, while we observed in this measurement, in addition to current and stripping
time, peak symmetry and yield. We decided to stripping current 200 A for optimal stripping time, peak symmetry and yield.
Owing to the large electrode surface, stripping current larger than 200 A should be used. The measurement time at lower
currents is longer than 5 min. The current of 200 A ensured the best signal to noise ratio and fast measurement.
For a sample volume of 4 mL taken for preconcentration on the microporous electrode E53C, the response is linear up to
about 0.7–50 mg L 1 (regression data: slope 0.0980, intercept -0.0321, correlation coefficient 0.99938) with a limit of
detection and quantification [21] of 0.2 mg L 1 and 0.7 mg L 1 , respectively.
The boiler water of power, such a concentration of hydrazine normally occur only in rare cases such as start-or operation is
also the possibility of using industrial waste water discharged into surface waters of the energy industry (heat and power
plants), where regulation of the Government .296/2005 Coll is permissible maximum value of 4.0 mg L -1 hydrazine.
For a sample volume of 4 mL taken for preconcentration on the macroporous electrode E104C., the response is linear up to
about 1–700 g L 1 (regression data: slope 0.00133, intercept -0.02735, correlation coefficient 0.9989) with a limit of
detection and quantification [21] of 7.8 g L 1 and 23.4 g L 1 , respectively.
The boiler water from plants such concentrations of hydrazine already happening, what is important for the proposed method
and substantially. Their normal value is between 20 g L -1 to 100 g L -1 of hydrazine.
Testing of interferences of compounds typical for boiler water from power stations showed that determination was mainly
influenced cuprous Cu(II), ferrous Fe(II) and ferric Fe(III) cations because of precipitates creation with hydrogenphosphate
and ammonia, which increased the background. It was found that the ratio of N2H4 : interferent equal 1:10 significantly
affects the signal of hydrazine in all these interferent except ammonia, which affects the signal of hydrazine at a ratio of
1:1000.
Zborník príspevkov
z 18. medzinárodnej vedeckej konferencie
"Analytické metódy a zdravie loveka", ISBN 978-80-969435-7-9
- 86 -
hotel Falkensteiner, Bratislava
11. - 14. 10. 2010