Flameproof (EXd) VS Intrinsic Safety (EXi): A Hardcore Comparison Guide to Industrial Explosion-Proof Technology

Flameproof (EXd) VS Intrinsic Safety (EXi): A Hardcore Comparison Guide to Industrial Explosion-Proof Technology

Introduction: When Industrial Sites Face Explosion Risks

In the grand landscape of industrial production, industries such as petrochemicals, coal mining, and oil and gas storage and transportation are like "minefields" hiding countless risks. In the working environments of these industries, explosive gases, dust, and air mix intimately, creating dangerous environments everywhere. In such environments, electrical equipment is like a "powder keg" that could be ignited at any time. The electric sparks and high temperatures generated during operation are like the fatal "fuse." A moment of carelessness may trigger a devastating explosion.

To build a solid safety defense line for industrial production, two major technologies—flameproof (EXd) and intrinsic safety (EXi)—emerged, becoming the "guardian deities" protecting industrial safety. They appear similar in function, both born to prevent explosions, but they are worlds apart in principles, applications, and other aspects, each with its own "special skills." Next, let us deeply explore the mysteries of these two explosion-proof technologies and see how they demonstrate their prowess on industrial sites to ensure production safety.

I. Core Principles: Two Paths to Block Explosions from the Source

(1) Flameproof (EXd): Resisting Explosions with a "Hard Shell," Isolating Danger Inside and Out

Flameproof explosion-proof technology is like putting an indestructible "armor" on electrical equipment. Its core lies in using a high-strength enclosure to "lock" the dangers that may be generated inside the electrical equipment—sparks, arcs, and high temperatures—inside, thereby completely isolating them from the external explosive environment.

Flameproof enclosures are usually made of metal materials such as cast iron and cast aluminum. These materials are high in strength and impact-resistant. The thickness of the enclosure is generally above 3 mm, or even thicker, and can withstand the enormous pressure generated by an explosion of an explosive mixture inside, usually ≥1.5 MPa. It is like a sturdy safe: no matter how much "turmoil" occurs inside, the outside remains safe.

Moreover, the design of the joint surfaces and threaded gaps of the flameproof enclosure is also very particular. These gaps are very small; for example, common threaded gaps are required to be below 0.2 mm. The purpose of this design is that when an internal explosion occurs, the flame generated by the explosion will be rapidly cooled as it passes through these tiny gaps, reducing its temperature to a level insufficient to ignite the external dangerous gas. Imagine the flame is like a "crazy dancer." When it tries to pass through these narrow "channels," it is rapidly cooled by the channel walls and finally loses the ability to ignite the external environment. Therefore, even if the inside "explodes," the flameproof enclosure, by virtue of its structural design, can firmly "trap" the flame and high temperature inside, ensuring the safety of the external environment.

(2) Intrinsic Safety (EXi): Essentially Safe with "Weak Current," Extinguishing Sparks from the Root

The concept of intrinsic safety technology is completely different. It starts from the root and eliminates the possibility of explosion by limiting energy, just like cutting off the "source of fire" at the source.

When designing intrinsically safe electrical equipment, various parameters in the circuit are strictly controlled, limiting factors that may generate energy, such as voltage, current, inductance, and capacitance, to extremely low levels, ensuring that under normal operation and possible fault conditions, the energy generated cannot ignite the surrounding explosive gas. For example, the working voltage of an intrinsically safe circuit is usually limited to ≤30 V and the current to ≤100 mA. With such weak energy, even in unexpected situations such as short circuits or component failures, the energy of the electric spark generated will be lower than 0.28 mJ, which is far below the minimum ignition energy of most combustible gases. For example, intrinsically safe equipment is like a "gentle little angel." The energy it generates is like small water droplets and cannot trigger the "raging fire" of an explosion.

At the same time, the safety barrier, which is indispensable in an intrinsically safe system, is like a precise "energy valve." It is installed between the non-hazardous area and the hazardous area, isolating and limiting the high energy from the non-hazardous area, allowing only low energy that meets safety standards to pass through and be delivered to the intrinsically safe equipment in the hazardous area, thereby ensuring that the equipment in the hazardous area always operates within the safe energy threshold range. The safety barrier is like a loyal "guard," always protecting the safety of the hazardous area, preventing the "invasion" of high energy, and allowing intrinsically safe equipment to work with peace of mind in hazardous environments.

II. Structural Design: The Safety Philosophy of Rigidity and Flexibility

(1) Flameproof: Precision Craftsmanship Under a Bulky Enclosure

The structural design of flameproof electrical equipment can be called a perfect combination of "strength and precision." Its enclosure is undoubtedly the core defensive "fortress" of the entire equipment. It is usually carefully crafted from metal materials such as cast iron and cast aluminum. These materials are as solid as steel and have high strength and excellent impact resistance. Take common flameproof motors as an example. Their enclosure thickness is often above 3 mm. For equipment used in some special occasions, the enclosure thickness can even reach 5 mm. This is like putting a thick "armor" on the equipment, enabling it to easily withstand the enormous pressure generated by an internal explosion, usually ≥1.5 MPa.

To ensure the reliability of this "armor," the enclosure of flameproof equipment must undergo a series of rigorous tests. During the hydrostatic test, the enclosure must withstand water pressure far exceeding normal working pressure, simulating the high-pressure environment during an internal explosion, to ensure that the enclosure will not develop any cracks or deformation. In the impact test, the equipment is subjected to violent impacts from heavy objects to verify the sturdiness of the enclosure under accidental impact. Only enclosures that pass these strict tests are qualified for use.

In addition to the strength of the enclosure, the design of the joint surfaces and threaded gaps of the flameproof enclosure is the key of keys. These seemingly minor parts actually contain great safety mysteries. Parameters such as the roughness of the joint surface, the length of thread engagement, and the size of the gap are strictly constrained by national standards (GB3836.2). For example, the thread engagement length is required to be more than 6 threads, so as to ensure that when an explosion occurs, the threads can effectively transmit pressure and prevent the enclosure from rupturing. The gap size is strictly controlled; common threaded gaps are required to be below 0.2 mm. Such a tiny gap allows the flame generated by the explosion to be rapidly cooled as it passes through, reducing its temperature to a level that cannot ignite the external dangerous gas.

However, although the structural design of flameproof equipment is powerful, it also has some obvious drawbacks. Due to the particularity of its enclosure design, the equipment must not be opened while energized, which to some extent limits the convenience of equipment maintenance. When performing equipment maintenance, maintenance personnel need to use professional tools to carefully disassemble the enclosure, and the operation process is cumbersome and time-consuming. Moreover, because the enclosure is heavy, flameproof equipment is often large in volume and relatively heavy, which makes it seem somewhat "powerless" in scenarios with narrow spaces and strict requirements for equipment volume and weight. For example, in some small equipment installation areas for underground operations, or in work scenarios where equipment needs to be moved frequently, the bulkiness of flameproof equipment becomes an obvious disadvantage.

(2) Intrinsic Safety: A Lightweight and Exquisite Circuit Revolution

Completely different from flameproof equipment, the structural design of intrinsically safe electrical equipment follows a "lightweight and exquisite" route. It abandons the heavy enclosure and instead relies on the ingenious combination of "low-power circuits + safety barriers" to achieve explosion-proof functionality, like a graceful dancer moving flexibly in a hazardous environment.

One significant advantage of intrinsically safe equipment is its small and light volume. Take an intrinsically safe pressure transmitter as an example. Its volume is only 1/3 that of a flameproof one, and its weight is 50% lighter. This lightweight design allows it to be easily installed in various equipment with limited space, such as sensors, instruments, and communication modules. In industrial automation production lines, a large number of sensors need to collect data in real time. With their small size, intrinsically safe sensors can be conveniently installed in every corner of the equipment without occupying too much space, while efficiently completing data collection tasks.

The circuit design of intrinsically safe equipment can be called a "precise revolution." It uses a series of dedicated intrinsically safe components, such as low-capacitance capacitors and current-limiting resistors. These components are like "little guards" in the circuit, constantly monitoring and limiting the energy in the circuit. During the design process, engineers strictly calculate the distributed inductance and capacitance in the circuit to ensure they meet safety standards. For example, the distributed capacitance of the wires is usually limited to ≤0.1 μF, which can effectively avoid dangers caused by capacitor energy storage in the circuit.

In the structural design of intrinsically safe equipment, the safety barrier plays a crucial role. As a "bridge" connecting the non-hazardous area and the hazardous area, it can accurately isolate and limit the high energy from the non-hazardous area, allowing only low energy that meets safety standards to pass through and be delivered to the intrinsically safe equipment in the hazardous area. The existence of the safety barrier is like adding a solid "protective lock" to the equipment in the hazardous area, ensuring that the equipment always operates within the safe energy threshold range.

Intrinsically safe equipment has a unique advantage in maintenance. Due to the low-energy characteristics of its internal circuits, it supports "live maintenance." This means that maintenance personnel do not need to cut off the power supply. They only need to perform specific operations in the safe area to calibrate, replace, and maintain equipment in the hazardous area. At petrochemical production sites, some instruments need regular calibration. Intrinsically safe instruments can be calibrated live without stopping production, greatly improving production efficiency and reducing production interruption time caused by equipment maintenance.

III. Application Scenarios: Precise Adaptation to Hazardous Areas

(1) Classification by Hazard Level: How to Choose for Zone 0, Zone 1, and Zone 2?

(1) Zone 0 (continuous explosion risk): This is an extremely dangerous area where explosive gas mixtures exist continuously or for long periods. A moment of carelessness may trigger a massive explosion. In this "powder keg" environment, only intrinsically safe (ia level) equipment is competent. For example, in coal mine gas outburst areas, the gas concentration is extremely high and it may be ignited at any time. Also, inside chemical reactor vessels, various chemical substances react under high temperature and high pressure, and the environment is extremely complex. With its extremely low energy output, intrinsically safe equipment is like a "safety lamp" lit in a hazardous area. Even in the harshest environments, it can ensure safety and eliminate the possibility of explosion from the source.

(2) Zone 1 (explosive gas may appear during normal operation): In this area, explosive gas mixtures may appear during normal operation, and the degree of danger should not be underestimated. Here, both flameproof and intrinsically safe equipment have their own uses. For high-power equipment such as motors and switchgear, which generate large amounts of energy during operation, the sturdy enclosure of flameproof equipment is like an indestructible "firewall," capable of withstanding the impact of possible internal explosions, confining the danger inside, and ensuring the safety of the external environment. For low-power instruments such as temperature transmitters and sensors, intrinsically safe equipment is more advantageous. Intrinsically safe equipment is small in size and low in energy consumption. It can not only meet the data collection and transmission needs of instruments but also ensure safe operation in hazardous environments, achieving both safety and convenience.

(3) Zone 2 (explosive gas appears occasionally): In Zone 2, explosive gas mixtures are unlikely to appear during normal operation, and even if they do, they exist only briefly. The explosion-proof choice in this area is relatively flexible. Both flameproof and intrinsically safe equipment can be used, and other explosion-proof types such as increased safety are also applicable here. When choosing, comprehensive consideration can be given based on cost and maintenance needs. If the budget is limited and the equipment is unlikely to generate sparks or high temperatures during normal operation, increased safety equipment may be a good choice. If the requirements for equipment safety and stability are extremely high, then flameproof or intrinsically safe equipment will be more suitable.

(2) Segmentation by Industry Scenario: Optimal Solutions for Different Fields

(1) Petrochemicals: This is an industry full of explosion risks. Distillation towers, tank farms, and other areas often belong to Zone 1 scenarios. In these places, large pumps, compressors, and other equipment have high power and generate much energy during operation. Once an explosion occurs, the consequences are unimaginable. Therefore, flameproof equipment has become their "standard configuration." Its sturdy enclosure can effectively resist the impact of internal explosions and ensure production safety. For field instruments, wireless transmitters, etc., which need to collect and transmit data in real time and have certain requirements for volume and power consumption, intrinsically safe equipment can better leverage its advantages. By pairing with safety barriers, intrinsically safe equipment builds an intrinsically safe system, like weaving a safety net in a hazardous environment, ensuring that instruments operate stably under safe conditions.

(2) Underground coal mines: Gas explosions are the "number one killer" in underground coal mines. In Zone 0 and Zone 1 environments, gas concentrations are high and explosion risks are extremely high. In such high-risk environments, intrinsically safe equipment has become the mainstream choice. Intrinsically safe explosion-proof mobile phones facilitate communication by workers underground, ensuring timely information transmission. Intrinsically safe sensors monitor parameters such as underground gas concentration and temperature in real time. Once abnormalities are found, they immediately issue alarms, safeguarding the lives of underground workers. These intrinsically safe devices are like "safety guards" in coal mines, always protecting the safety of workers.

(3) Dust explosion environments: In places with dust explosion risks such as flour mills and aluminum powder processing plants, explosion-proof requirements are more special. In addition to considering gas explosion protection, it is also necessary to combine dust explosion-proof markings (such as Ex tD). Flameproof equipment can be used in Zone 21 and Zone 22. Through its sturdy enclosure, it prevents dust from entering the interior of the equipment and avoids dust explosions caused by sparks or high temperatures generated inside the equipment. When intrinsically safe equipment is used in dust environments, it must be paired with dust-specific safety barriers to further limit energy and ensure safe operation in dust environments.

IV. Comparison of Advantages and Disadvantages: There Is No Absolute "King of Safety"

(1) Flameproof (EXd)

Advantages

(1) Suitable for high-power equipment, no reliance on associated equipment: Flameproof equipment is like a "strongman," easily driving high-power equipment such as large motors and heaters. In large distillation towers in the petrochemical industry, motors need strong power to drive them. Flameproof motors, with their strong power support, can operate stably and ensure the normal operation of the distillation tower. Moreover, flameproof equipment does not rely on associated equipment such as safety barriers, making installation and use relatively simple and reducing system complexity.

(2) Impact-resistant and vibration-resistant, adapting to harsh mechanical environments: The sturdy enclosure of flameproof equipment gives it excellent impact and vibration resistance. In underground coal mines, equipment is often subjected to impacts from falling rocks and mechanical collisions, as well as strong vibrations during mining. Flameproof equipment is like a strong warrior, able to stand firm in such harsh mechanical environments, ensuring the normal operation of the equipment and providing reliable support for coal mining.

(3) Wide temperature class coverage, handling high-temperature scenarios: The temperature classes of flameproof equipment range from T1 (maximum surface temperature 450°C) to T6 (maximum surface temperature 85°C), covering a very wide range. This enables it to be used safely in various high-temperature environments. Whether beside high-temperature industrial furnaces or at oil and gas extraction sites in hot desert regions, flameproof equipment can operate stably with its broad temperature adaptability and play its explosion-proof role.

Disadvantages

(1) Large volume and high weight, limited installation space: Due to its sturdy enclosure design, flameproof equipment is often bulky and heavy. In some places with limited space, such as small chemical laboratories, flameproof equipment may not be installable, or after installation it may occupy a large amount of space, affecting the layout and operation of other equipment. Moreover, the transportation and installation of the equipment also become difficult due to its weight, requiring professional lifting equipment, which increases installation cost and difficulty.

(2) Complex maintenance, power-off and cover opening affect production efficiency: Flameproof equipment must be powered off and opened during maintenance. This process is not only cumbersome but also causes production interruption. In petrochemical production, every equipment maintenance may affect the continuous operation of the production line and cause economic losses. Moreover, frequent power-off and cover-opening operations also increase the risk of equipment damage and reduce the service life of the equipment. Maintenance personnel also need to be extra careful during operation to avoid safety accidents caused by improper operation.

(3) If the enclosure is damaged, explosion-proof performance directly fails: The explosion-proof performance of flameproof equipment depends entirely on the integrity of the enclosure. Once the enclosure is severely damaged, such as scratched joint surfaces or damaged threads, the explosion-proof performance will be greatly reduced or even directly fail. In actual use, the equipment may suffer enclosure damage due to collisions, corrosion, and other reasons. This requires regular inspection and maintenance of the equipment to timely discover and repair enclosure damage and ensure the explosion-proof performance of the equipment. But even so, safety hazards caused by enclosure damage cannot be completely avoided.

(2) Intrinsic Safety (EXi)

Advantages

(1) The only explosion-proof type suitable for Zone 0, with the highest safety level: In the field of explosion protection, Zone 0 is the most dangerous area, and intrinsically safe equipment is the only explosion-proof type that can be safely used in Zone 0. Take coal mine gas outburst areas as an example. The gas concentration is extremely high, and explosion risks exist at all times. With its extremely low energy output and strict circuit design, intrinsically safe equipment is like building a solid safety defense line in a hazardous area, eliminating the possibility of explosion from the source and providing the most reliable guarantee for the lives of workers and the smooth progress of production.

(2) Lightweight and compact, easy to integrate into smart devices, supporting digital transformation: The lightweight and compact characteristics of intrinsically safe equipment make it an ideal choice for smart devices. Under the wave of Industry 4.0, devices such as smart sensors and wireless communication modules are increasingly widely used in industrial production. Intrinsically safe equipment can be easily integrated into these smart devices without occupying too much space, while ensuring safe operation of the devices in hazardous environments. In smart factories, intrinsically safe sensors can collect equipment operation data in real time and transmit the data to the control system through wireless communication modules, realizing remote monitoring and intelligent control of equipment and providing strong support for industrial digital transformation.

(3) Live maintenance, reducing downtime costs, suitable for continuous production scenarios: Intrinsically safe equipment supports live maintenance, which is one of its outstanding advantages. In continuous production industries such as petrochemicals and electric power, equipment downtime for maintenance brings huge economic losses. The live maintenance function of intrinsically safe equipment allows maintenance personnel to maintain and repair equipment without stopping production, greatly reducing downtime costs and improving production efficiency. Maintenance personnel can calibrate and replace parts of intrinsically safe equipment in the hazardous area through remote operation or special tools in the safe area, ensuring that the equipment is always in optimal operating condition.

Disadvantages

(1) Depends on safety barriers, system design requires professional matching: Intrinsically safe equipment must rely on safety barriers to work properly, and the matching between safety barriers and intrinsically safe equipment requires professional design and calculation. When designing an intrinsically safe system, engineers need to consider parameters such as distributed capacitance and inductance in the circuit to ensure they meet safety standards. A slight negligence may lead to reduced explosion-proof performance of the system or even safety accidents. Moreover, the selection and installation of safety barriers must also be carried out strictly in accordance with specifications, increasing the difficulty of system design and implementation.

(2) Limited transmission distance, long distances require dedicated intrinsically safe cables: The transmission distance of intrinsically safe equipment is usually limited, generally not exceeding 500 m. If signals need to be transmitted over long distances, dedicated intrinsically safe cables must be used. In some large factories or mines, the distance between equipment is relatively long. When using intrinsically safe equipment, a large number of dedicated cables need to be laid, which not only increases costs but also brings difficulties to construction and maintenance. Moreover, cable laying also needs to consider environmental factors such as electromagnetic interference and temperature changes to ensure the stability and reliability of signal transmission.

(3) Strict power limitations, unable to drive high-power equipment: The design concept of intrinsically safe equipment is to limit energy, so its power limitation is very strict and it cannot drive high-power equipment such as motors and heaters. In some industrial scenarios requiring high-power equipment, intrinsically safe equipment is powerless, and other explosion-proof types must be chosen. This limits the application scope of intrinsically safe equipment to a certain extent, making it unable to meet all industrial explosion-proof needs.

V. Selection Guide: Three Moves to Choose the Right Explosion-Proof Solution

In industrial production, correctly choosing an explosion-proof solution is crucial. It directly relates to the safety and stability of production. Flameproof (EXd) and intrinsic safety (EXi) each have their own strengths. How do you choose between them? Don't worry. Here are three selection guide moves to help you choose the most suitable explosion-proof solution.

(1) Look at the Environment: First Determine the Hazardous Area and Gas Group

(1) Zone 0 / high-precision scenarios: Zone 0 is a high-risk area with continuous explosion risk, such as coal mine gas outburst areas and the interior of chemical reactor vessels. Here, intrinsically safe (ia level) equipment is the first choice. It is like a "reassurance pill," providing the highest level of safety guarantee. At the same time, pair it with an isolated safety barrier, like adding a "safety lock" to the equipment, to further ensure system safety. In high-precision electronic instrument production workshops, intrinsically safe equipment can also show its talents because the electromagnetic interference it generates is extremely small and will not affect the precision measurement and control of instruments.

(2) Zone 1 / high-power equipment: Zone 1 may have explosive gases during normal operation. For high-power equipment such as motors and switchgear, flameproof equipment, with its sturdy enclosure, can withstand the impact of internal explosions, like a loyal "guard," protecting the safety of the equipment. When choosing flameproof equipment, pay attention to the enclosure protection rating. An IP65 or higher protection rating can effectively prevent the ingress of dust and water, ensuring normal operation of the equipment in harsh environments.

(3) Dust environments: In dust environments such as flour mills and aluminum powder processing plants, explosion-proof requirements are more special. Not only must gas explosion protection be considered, but dust explosion-proof markings (such as Ex tD) must also be combined. Flameproof equipment can be used in Zone 21 and Zone 22. Through its sturdy enclosure, it prevents dust from entering the interior of the equipment and avoids dust explosions caused by sparks or high temperatures generated inside the equipment. When intrinsically safe equipment is used in dust environments, it must be paired with dust-specific safety barriers to further limit energy and ensure safe operation in dust environments. Be careful to avoid mixing gas explosion-proof equipment, so as not to leave safety hazards.

(2) Calculate Requirements: Comprehensive Consideration of Power, Maintenance, and Cost

(1) Low-power instruments: For low-power instruments such as 4-20 mA transmitters, intrinsically safe equipment has obvious advantages. It has low energy consumption and small size, like a "small elf," ensuring safety while meeting the functional requirements of the instrument. Moreover, intrinsically safe equipment is convenient for later maintenance and supports live maintenance, greatly reducing maintenance costs and downtime. At petrochemical production sites, a large number of temperature and pressure transmitters use intrinsically safe equipment, which can stably collect data while being easy to maintain, ensuring production continuity.

(2) High-power motors: High-power motors (≥1 kW) generate large amounts of energy during operation. Intrinsically safe equipment cannot meet their power requirements, so only flameproof equipment can be chosen. When choosing flameproof motors, pay attention to the enclosure material. Stainless steel enclosures are more corrosion-resistant and can be used for a long time in harsh chemical environments, extending the service life of the equipment. In chemical enterprises, many motors of large pumps and compressors use flameproof stainless steel enclosures to ensure safe operation of the equipment in highly corrosive chemical media.

(3) Frequent maintenance scenarios: If equipment requires frequent maintenance, the advantages of intrinsically safe equipment become prominent. It supports online maintenance. Maintenance personnel can inspect and repair the equipment without shutting it down, like a "considerate doctor," treating the equipment without affecting production. This greatly reduces downtime losses and improves production efficiency. In the power industry, monitoring instruments in some substations use intrinsically safe equipment, and maintenance personnel can perform maintenance without power outages, ensuring the stability of power supply.

(3) Check Certification: Explosion-Proof Markings Hide Key Information

(1) Flameproof markings: The marking of flameproof equipment is usually Ex d [IIB/IIC] T4. Take "Ex d IIC T6" as an example. "Ex" indicates explosion-proof, "d" represents flameproof, "IIC" indicates suitability for highly dangerous gases such as hydrogen and acetylene, and "T6" indicates a maximum surface temperature of 85°C. Through this marking, we can clearly understand the explosion-proof performance and applicable scope of the equipment. When choosing flameproof equipment, be sure to carefully check the explosion-proof marking to ensure that the equipment meets the requirements of the usage environment.

(2) Intrinsic safety markings: The marking of intrinsically safe equipment is Ex ia [I/II] T5. For example, "Ex ia IIB T4": "Ex" also indicates explosion-proof, "ia" indicates ia-level intrinsic safety, suitable for Zone 1 and Zone 0, "IIB" indicates suitability for IIB gases, and "T4" indicates temperature class T4 (≤135°C). The explosion-proof marking of intrinsically safe equipment makes its safety level and applicable environment clear at a glance. When purchasing intrinsically safe equipment, choose equipment with the appropriate explosion-proof marking according to actual needs.

VI. Conclusion: Explosion-Proof Selection—Suitability Is the "Safety Answer"

Flameproof and intrinsic safety are not a zero-sum game of "which is safer," but precise safety solutions for different industrial scenarios. Flameproof is like a "heavy warrior," using a sturdy enclosure to resist explosion impacts. Intrinsic safety is like a "precision assassin," eliminating danger from the energy source. When choosing, factors such as hazardous area level, equipment power, and maintenance needs must be combined so that professional explosion-proof technology truly becomes the "guardian" of industrial safety. After all, on the battlefield of flammables and explosives, there is no best explosion-proof type, only the most suitable safety choice. (Note: The data and technical standards in this article refer to the GB3836 series explosion-proof specifications, IEC60079 international standards, and industry practice cases. For specific selection, please rely on certification by professional institutions.)