AL-2021 Hydrogen Peroxide (H2O2) in Air and Water Monitor

The Model AL-2021 is a state-of-the-art digital hydrogen peroxide (H₂O₂) analyzer that is available in several different application-specific configurations. Both single and dual channels models are available for air or water H₂O₂ measurement applications.

The detection of peroxides is based on the liquid phase reaction of peroxides with P-Hydroxyphenylacetic Acid catalyzed by Peroxidase. This reaction produces a fluorescent dimmer that can be excited at 326 nm (Cd-lamp) and detected between 400 and 420 nm. The technique is sensitive to all peroxides in the solution.

To distinguish between H₂O₂ and organic peroxides two parallel channels are used. In one channel (channel B) of the instrument, H₂O₂ is destroyed selectively by catalase prior to the fluorescent detection in the instrument. The amount of H₂O₂ is then given by the difference between the signals from the two channels - signal for total peroxide (channel A) minus the signal for total per-oxide without H₂O₂ (channel B) - corrected for the destruction efficiency of the catalase solution.

catalase

2 H₂O₂ 2 H₂O + O₂

The above reactions are carried out in aqueous solution of peroxides and other reagents. Therefore, for the measurement of gaseous peroxides, these have to be trapped in aqueous solution first. This is achieved in a stripping coil by pumping air and a stripping solution (pH-Buffered water free of H₂O₂) continuously at known flow rates. The air and liquid streams are afterwards separated in a glass separator and the solution is then analyzed for peroxides.

In the instrument, capable for measuring H₂O₂ in air and water samples, the H₂O₂ mixing ratio in air is then calculated from the concentration in solution and the ratio of air and stripping solution flow rates.

The coil size and the flow rates of air and stripping solution are optimized for quantitative stripping of H₂O₂. Due to lower solubilities however, the stripping efficiency for other peroxides is lower than that for H₂O₂ and probably varies between the 60% found for Methylhydro peroxide and 100% for H₂O₂. As the composition of organic peroxides in air is unknown, the signal from channel A (after destroying H₂O₂ by catalase) gives only an approximate estimate of the concentration of organic peroxides. Therefore, the instrument cannot determine exactly the amount of organic peroxides in air but a relative measure of the concentration is indicated.

Provided the sensitivity is known for each channel, this is the case after performing an calibration cycle, the calculation of concentrations is done in two steps. In the first step the equivalent mixing ratios are calculated from signals from both channels. In the second step, the H₂O₂ concentration is calculated from the difference of two equivalent mixing ratios. This difference is then corrected for the transmission efficiency of the inlet tubes, for the catalase destruction efficiency, and for the efficiency of the zero trap.

Peroxides in water solutions can be measured directly. In this mode, stripping is not necessary and, consequently, either zero air has to be applied to the samples inlet or the internal zero trap has to be switched on. Model AL2021W has been designed for measuring H₂O₂ only in water samples. Therefore, no stripping coil is required.

Due to time response, in applications requiring two channels (such as input vs. output process checks) or simultaneous air and water measurements two analyzers are used ganged together. The fully automated nature of the system allows the system to be tailored to many configurations and on line process applications.