Tuesday, November 27, 2012

Why Don't More Companies offer a Hydrogen Peroxide Monitor?

Hydrogen peroxide is a widely used chemical, finding application in healthcare for sterilization, metal fabrication mills for pickling and pulp & paper as a bleaching agent. According to Wikipedia, in 1994, world production of H2O2 was around 1.9 million tonnes and grew to 2.2 million in 2006. The hazards of exposure to hydrogen peroxide vapor are well known, the OSHA permissible exposure limit (PEL) for hydrogen peroxide is 1 ppm, calculated as an eight hour time weighted average (TWA) [29 CFR1910.1000 Tbl Z-1], which can be compared to the PELs for some other familiar hazardous gases and vapors: carbon monoxide (50 ppm), Hydrogen cyanide (10 ppm), chlorine (1 ppm ceiling). Hydrogen peroxide is electrochemically active and so it can be detected relatively easily by an appropriate electrochemical sensor.

Hydrogen peroxide is widely used, its vapors are known to be very hazardous, it is relativley easy to detect, but of the many gas detection companies operating the United States (estimated at over 80 at one point), why do so few offer monitors for hydrogen peroxide? We are aware of only three or four including ChemDAQ. The answer is calibration!

There are over twenty gases that monitors are readily available from the gas detection companies including Cl2, CO, H2, HCN. HCl, H2S, NH3, NO, N2O, NO2, SO2, O3 and combustible gases. The common feature that all these gases have in common is that the certified gas mixtures are available from specialty gas companies that can be used to calibrate the respective gas monitors. The availability of a test gas makes the design a monitor much easier. The new monitor will look like an old monitor with new sensor, tweaked circuitry and software and a new label. The sensor is calibrated with a new test and off it goes to a life of productive service. Of course, this is a gross simplification of the work that must be done to release a new product but it illustrates the point.

However, if the calibration gas does not exist, then the gas monitor manufacturer has two choices – find a surrogate gas or generate their own test gas. Many sensors respond to more than one gas and so if the ratio of the responses is constant (cross sensitivity), then calibrating with a gas that comes in cylinder from a specialty gas company, multiplied the cross sensitivity ratio should give a calibration to the new target gas.

The operative word there is ‘should’ because sometimes cross calibration is unreliable. Many hydrogen peroxide sensors give a strong response to sulfur dioxide, but the cross sensitivity can be affected by the environment. We once tested a hydrogen peroxide sensor with both hydrogen peroxide and sulfur dioxide and found that the sensor responded well to sulfur dioxide but gave almost no response to hydrogen peroxide. One of the components in the gas path was removing the hydrogen peroxide. Hydrogen peroxide is much more reactive than sulfur dioxide (stronger oxidizing agent, spontaneously decomposes over time); so the fact that there are contaminants that will react more with hydrogen peroxide than sulfur dioxide should not be very surprising.

Clearly, if this test had been an attempt to cross-calibrate the hydrogen peroxide sensor with sulfur dioxide, the sensor would have tested as normal, even though it barely registered hydrogen peroxide; creating a potentially dangerous situation. Even if the sensor module leaves the factor correctly calibrated, the same problem can potentially occur if the customer then attempts to field calibrate the sensor with a sulfur dioxide or other surrogate gas. After all, how many users have hydrogen peroxide test gas of known concentration readily at hand. Field calibration of reactive gas sensors with less reactive surrogates runs the risk of the sensor appearing to be functioning normally, but not actually detect the target gas.

The role of calibration in any gas monitor is two fold. The first function is a basic check that the monitor is working properly, that it responds to gas when the gas is applied. The second function is to ensure that the sensor reading is accurate. Obviously these two functions are closely related and overlap to some extent.

Since hydrogen peroxide test gas is not readily available, it must be produced by the sensor manufacturer. Hydrogen peroxide can be produced by evaporating hydrogen peroxide solution. There is no great mystery here, the chemical literature describes several way of achieving this goal, but the problem for the manufacturer is to produce a consistent concentration of hydrogen peroxide over time, and to be able to determine what that concentration is.

It may sound simple, and in principle it is, but the folks at ChemDAQ have spent a lot of time and effort engineering this process in order to produce a reliable and accurate calibration method. ChemDAQ generates hydrogen peroxide test vapor from hydrogen peroxide solution and we have our generators run continuously in order to ensure that the system is at steady state. We also regularly titrate the gas stream to measure the hydrogen peroxide concentration so that we know what hydrogen peroxide concentration used it. This hydrogen peroxide test gas is then used to calibrate the hydrogen peroxide sensors.

All of ChemDAQ’s hydrogen peroxide calibrations are performed with hydrogen peroxide test gas. Similarly, all of ChemDAQ’s peracetic acid sensor calibrations are performed with peracetic acid test gas. This approach means that users of our equipment can be assured that their hydrogen peroxide sensors will respond to hydrogen peroxide vapor and the that the readings will be accurate.

If you use hydrogen peroxide and your hydrogen peroxide vapor monitors are not calibrated with hydrogen peroxide test gas, but with another gas such as sulfur dioxide then you may want to question how reliable that calibration is.


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