Hydrazine and carbohydrazide are both used as oxygen scavengers to protect boiler systems and closed water loops from corrosion, but they differ significantly in form, toxicity and handling. Hydrazine is a powerful, highly toxic liquid that has been the traditional choice for decades, while carbohydrazide is a solid derivative that offers a safer, easier-to-handle alternative with comparable oxygen-scavenging chemistry. This article compares the two chemicals across composition, physical form, reactivity, applications and safety, and explains when each one is preferred.
Two related but different chemicals
Hydrazine (N2H4, CAS 302-01-2) is the simplest diamine: a colorless, fuming liquid with an ammonia-like odor, a strong reducing agent that reacts rapidly with oxygen. Carbohydrazide (CH6N4O, CAS 497-18-7) is the condensation product of hydrazine with carbonic acid, a white crystalline solid that is stable at room temperature and dissolves readily in water. Both molecules share the reducing hydrazine functionality, which is what makes them effective oxygen scavengers, but the solid form and lower volatility of carbohydrazide change how it is handled, dosed and stored.
Key differences at a glance
| Property | Hydrazine | Carbohydrazide |
|---|---|---|
| CAS number | 302-01-2 | 497-18-7 |
| Chemical formula | N2H4 | CH6N4O |
| Physical form | Colorless fuming liquid | White crystalline solid |
| Odor | Ammonia-like | Odorless |
| Reactivity | Very high; strong reducing agent | Strong reducing agent, more stable |
| Toxicity | Toxic and carcinogenic | Much lower toxicity |
| Typical use | Oxygen scavenger, propellant, intermediate | Oxygen scavenger, intermediate, cross-linking agent |
How oxygen scavenging works
In boiler water treatment, dissolved oxygen is the main cause of pitting corrosion on metal surfaces. Hydrazine removes oxygen by reacting with it to form nitrogen and water, and it also passivates metal surfaces by promoting a protective oxide film. Carbohydrazide behaves similarly: it reacts with oxygen at low temperatures and pressures, and the reaction products are volatile, so they do not add dissolved solids to the boiler water. Like hydrazine, carbohydrazide also passivates metal surfaces, which is why it is accepted as a direct replacement in many treatment programs.
Application comparison in boiler water treatment
Hydrazine has been the standard oxygen scavenger in high-pressure boilers because of its fast reaction and zero solid residues, but its toxicity and carcinogenicity have pushed many operators to switch. Carbohydrazide provides the same corrosion protection with much lower occupational risk, and its solid form simplifies transport, storage and dosing: it is dissolved into solution on site instead of being pumped as a hazardous liquid. In practice, the choice depends on system pressure, feedwater quality, the available dosing equipment and the operator's safety policy. Some programs use blends of scavengers or supplement carbohydrazide with catalysts to improve reaction speed at low temperatures.
Safety, handling and storage comparison
Hydrazine is a very hazardous material: it is toxic, carcinogenic, corrosive and can ignite under certain conditions, so it requires dedicated handling systems, ventilation, personal protective equipment and strict regulatory compliance. Carbohydrazide, while still requiring care, has much lower toxicity and is not carcinogenic; it can be handled with standard chemical hygiene, though skin contact should still be avoided. Both materials must be stored away from oxidizers and strong acids, in clearly labeled containers, and both react with oxygen in air, so containers should be kept sealed.
How to choose between hydrazine and carbohydrazide
For new installations and for operators who want to reduce occupational risk, carbohydrate is usually the preferred choice because of its solid form, lower toxicity and comparable performance. For existing systems designed around liquid dosing, the switch is still straightforward because both chemicals are dosed as aqueous solutions, but the concentrations and feed rates differ, so the treatment program should be revalidated. Whichever chemical is selected, monitoring of residual scavenger, dissolved oxygen and metal passivation should continue as part of the routine water-treatment control program.
Frequently Asked Questions
What is the main difference between hydrazine and carbohydrazide?
Hydrazine is a toxic, carcinogenic liquid, while carbohydrazide is a solid derivative that is much safer to handle and provides similar oxygen-scavenging performance.
Why is carbohydrazide used in boiler water treatment?
Carbohydrazide removes dissolved oxygen, passivates metal surfaces and produces only volatile products, protecting boilers without adding dissolved solids.
Is carbohydrazide as effective as hydrazine?
Yes. Carbohydrazide delivers comparable corrosion protection and is widely accepted as a safer alternative, sometimes with catalysts to speed the reaction.
What is the CAS number of hydrazine?
The CAS number of hydrazine is 302-01-2; the CAS number of carbohydrazide is 497-18-7.
Can hydrazine be replaced by carbohydrazide in existing plants?
Usually yes, because both are dosed as aqueous solutions; the feed rate and monitoring program should be revalidated when switching.
Is hydrazine used for anything besides boilers?
Yes. Hydrazine is also used as a rocket propellant and as an intermediate in pharmaceuticals, agrochemicals and polymer blowing agents.
