Hydrogen cyanide (HCN)

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Hydrogen cyanide was one of the first fumigants to be used extensively under modern conditions. Its use for treating trees under tents against scale insects was developed in California in 1886 (Woglum, 1949). The use of HCN has been declining in recent years, but it is still important in certain fields of application.

HCN is one of the most toxic of insect fumigants. The fact that it is very soluble in water has considerable bearing on its use in practice. Thus, it may produce injury on moist materials, such as fruit and vegetables, because the solution of HCN in water is a dilute acid. Not only does this acid render these materials unpalatable and possibly hazardous for human consumption, but its action, by causing burning, wilting or discoloration, may make them unmarketable.

On the other hand, HCN has been widely used for fumigating dormant nursery stock that is sufficiently dry. It may be used for some living plants if they can be washed with water immediately after treatment to prevent burning by the acid.

HCN may be employed for fumigating many dry foodstuffs, grains and seeds. Although HCN is strongly sorbed by many materials, this action is usually reversible when they are dry, and, given time, all the fumigant vapours are desorbed. With many foodstuffs little, if any, chemical reaction occurs, and there is no detectable permanent residue.

Because of the high degree of sorption at atmospheric pressure, HCN does not penetrate well into some materials. It WAS largely because of this that vacuum fumigation was adopted.


HCN is a powerful, quick acting poison. In humans and other warm-blooded animals it induces asphyxiation by inhibiting the respiratory enzymes and renders tissues unable to absorb oxygen from the blood in the normal manner. The toxic action is reversible. In practice, this means that a person who is completely unconscious from the effects of cyanide, but whose heart is still beating, may still recover if suitable antidotes nod remedial measures are applied in time. HCN may be absorbed in toxic amounts through the unbroken skin (see below under "Precautions").

Toxicity of Residues

After ingestion by mammals, HCN is rapidly converted into thiocyanate (Lehmann, 1959), which is far less toxic. In a carefully controlled laboratory experiment conducted during a period of two years, rats were fed a regular diet containing 100 or 300 ppm by weight of HCN. The rats were reared on this diet from the weaning stage and during growth or maturity they did not exhibit any symptoms of poisoning. Growth and development were the same as in the controls (Howard and Hanzal, 1955). These results indicate that residues of HCN in fumigated foods, which would normally be far less than the amounts used in these tests, would be of no hazard to human consumers. The effect of HCN on specific types of foodstuffs is discussed under "effect on Plant Products".


Alternative name : hydrocyanic acid; abbreviation used : HCN

Odour Suggestive of almonds
Chemical formula HCN
Boiling point 26C
Freezing point -14C
Molecular weight 27.03
Specific gravity gas (air = 1) 0.9
liquid (water at 4C = 1) 0.688 at 20C
Latent heat of vaporization 210 cal/g
Flammability limits in air 6 to 41% by volume
Solubility in water Infinite at all temperatures
Pertinent chemical properties Weak acid. Relatively noncorrosive. When stored as a liquid without chemical stabilizer may decompose and explode in the container.
Method of evolution as fumigant Discharge from steel cylinder with aid of compressed air. By evaporation of liquid absorbed in porous material, such as card board discs or diatomaceous earth. By action of moisture in air on calcium cyanide, or of sulphuric acid on sodium cyanide
Commercial purity 96 to 99%

Natural vapour pressure at different temperatures

0C (32F) 264.3 mm Hg
10C (50F) 400.0 mm Hg
20C (68F) 610.0 mm Hg
25C (77F) 738.8 mm Hg
30C (86F) 910.0 mm Hg
40C (104F) 1 269.2 mm Hg

Weights and volumes of liquid

1 lb (avdp) at 20C has volume 659.7 ml
1 U.S. gal weighs 5.82 lb (2.643 kg)
1 Imp gal weighs 6.99 lb (3.171 kg)
1 kg has volume 1 454.3 ml
1 litre weighs 0.688 kg

Dosages and concentrations of gas in air (25C and 760 mm pressure)

By volume

Weight per volume

Parts per million Percent 1g/m lb/l 000 ft
210 0.001 0.011  
50 0.005 0.055  
100 0.01 0.11  
200 0.02 0.22  
500 0.05 0.55 0.03
905 0.09 1.00  
1 000 0.10 1.105 0.07
14 473 1.44 16.00 1.00
20 000 2.0 22.11 1.38

1 Ounces per 1000 cubic feet or milligrammes per litre
2 Threshold limit (ACGIH, 1981).

Toxicity to Insects

Among the commonly used famigants, HCN is one of the most toxic to insects. It aiso has a rapid paralyzing effect on most species. This action is an important consideration in dealing with insects, because sublethal concentrations may bring about apparent death. After exposure to the fumigant, the reversible action of the poison may permit the insect to recover. This reaction has already been referred to as protective stupefaction (Lindgren, 1938). It is important from the practical point of view because it means that the maximum recommended concentration should be atteined as quickly as possible during the application of the fumigant.


The flammability limits of HCN in air lie botween 6 and 41 percent by volume. These limits are well above the normally recommended fumigation doses of up to 1.5 percent (16 g/m or 16 oz/l 000 ft). However, it must be pointed out that, at the point of release of the gas at the beginning of a famigation, a concentration within these limits may exist for a short length of time. If there is any flame (such as a pilot light) or sparks near temporary high concentrations, a serious fire or explosion could occur. In working with this fumigant, great care must be exercised to extinguish all flames and turn of all electric switches before a treatment begins.



HCN applied in dosages effective ageinst insects does not affect the viability of seeds that are normally dry, with moisture contents suitable for storage. In a comprehensive study, the results of which were published between 1959 and 1961, Strong and Lindgren studied the effect of HCN in insecticidal concentrations on the germination of a wide range of seeds, including grain seeds, flax and small legumes. The variable factors considered were moisture content (8 to 14 percent), repeat fumigations and postfumigation storage. It wes concluded that germination of wheat, barley, oats, rice and flax seeds wes not impaired by one or two famigations with HCN. With the small legumes with a range of 5.8 to 12.2 percent moisture content (Ranger alfalfa, alsike clover, Ladino clover, Kenland red clever and Viking birdsfoot trefoil), all were tolerant to HCN in one or two fumigations, except aisike clover for which there was positive indication of impairment of germination.

Among 80 varieties of grain, vegetable and flower seeds tested by Lindgren et al (1955), six showed evidence of reduced germination. These were pole beans, burnet, California black mustard, smilo, marigold and snapdragon.

It may be concluded that HCN is a safe fumigant to use for seed treatment, especially for cereal grains under normal conditions, but with flower and vegetable seeds preliminary trials with local varieties are advisable.

Growing Plants and Trees

A considerable amount of injury, either temporary or permanent, may be sustained by actively growing plants fumigated with HCN. Because this gas is very soluble in water, special precautions have to be taken to reduce the amount of moisture on leaves and stems and in pots or soil balls of actively growing plants. Therefore, the plants should not be watered for one or more days before treatment. After treatment with HCN, it is necessary to wash the plants with water to remove any residual acid. HCN interferes with photosynthesis and other physiological processes (Moore and Willaman, 1917); for that reason, plants may be more susceptible to injury in daylight. Usually, treatments have to be carried out at night or in the dark. Also, following exposure, plants should be kept away from sunlight for several hours.

In the past HCN was widely used for fumigating ornamental and glasshouse plants, but it has been replaced by other fumigants that are less phytotoxic. The use of HCN generated from calcium cyanide to control glasshouse pests is discussed in Chapter 12. HCN was extensively used for may years to control scale insects on citrus trees in a tent fumigation procedure (Quayle, 1938). In this treatment, HCN gas (5 g/m ) from liquid HCN, or evolved from a salt such as calcium cyanide, was liberated into a relatively gas-tight tent installed over the tree and maintained for usually around 45 mintues. A number of factors, including temperature, humidity, physiological conditions of the tree (such as dormancy, disease, cultural conditions, stage of development and others), have considerable influence on the tolerance of the trees to the fumigant (Woglum, 1923). Since different species or varieties of citrus trees show wide variation in response and the insects in different areas may vary in tolerance, the actual conditions of treatment usually have to be developed to suit local circumstances. In scale insect eradication work, where complete kill of all insects is essential, the tent fumigation treatment with HCN has been found to be a valuable technique (Fosen et al, 1953). HCN was used by Del Rivero et al, (1974) to control larvae and eggs of the woolly white fly on orange trees.

While the concentrations of HCN required to kill insects may cause severe injury to growing plants, lower concentrations of HCN may stimulate growth and be useful in "forcing" growth in some species (Gassner, 1925).

Dormant Nursery Stock

HCN is still used in many parts of the world for the fumigation of the dormant nursery stock of deciduous trees, especially in quarantine treatments against the spread of scale insects (Ceder and Mathys, 1949; van de Pol and Rauws, 1957; Agarwala, 1956; Jen and Lai, 1959). As in growing plants, a marked stimulation has been noticed in some nursery stock fumigated with HCN (Cassner, 1925). Immediately following fumigation it is sometimes necessary to wash plants with water to avoid the burning of buds and new foliage.


For the most part, HCN does not react with the constituents of foodstuffs of normel moisture content. For example, materials such as honey and beeswax that have been exposed to HCN in treatments designed to destroy the bee colonies at the end of a season, were found to have less than 0.004 and 0.02 mg/kg CN (Ihnat and Nelson, 1979). Important exceptions or special considerations are noted in the paragraphs below.

Fresh Fruit and Veqetables

HCN is not generally recommended for the fumigation of fresh fruit and vegetables. It is used to some extent in the treatment of citrus and other fruit for the control of scale insect or thrips (Richardson and Balock, 1959). In some countries HCN is used in quarantine treatments of bananas, pineapple and other commodities for the control of aphids, mealy bugs and other exposed insects (Figure 19). The European Plant Protection Organization recommends HCN in a vacuum treatment for bulbs, rhizomes, tubers, asparagus roots and strawberry plants to control certain mites and nematodes (EPPO, 1976).

Cereals and Milled Foods

HCN has been used safely and successfully ageinst a wide range of dry plant products used as foods. An interesting exception to the general rule is that some HCN is retained by fumigated bran in combined form, hut this residue has no adverse effect on mixed feed containing average amounts of the treated bran (Page and Lubatti, 1948). At atmospheric pressure, it does not penetrate as quickly or as effectively as dues methyl bromide. This handicap can be overcome in bulk grain fumigations by the use of recirculating systems. Kunz et al (1964) studied the penetration of HCN throogh a mass of grain sorghum. The variables of temperature, moisture and dockage all reduced penetration in proportion to their increase.

Dried Fruit

Althoogh HCN has heen used for many years in the treatment of dried fruit, this practice is safe only under carefully controlled conditions. HCN may react with laevolose to form laevolose cyenohydrin, a poisonous compound that is feirly stable (Page and Lubatti, 1948). The amount of cyanohydrin formed in dried fruit varies with its moisture content and this should be kept to a minimum during famigation. With properly conducted treatments at normally recommended dosages and exposure periods, the small amounts of residues that may form would not present a danger. In one instance, careless application of the fumigant permitted liquid HCN to run into packages of dried fruit and an outbreak of food poisoning ensued from the excessive residues of hydrocyenic acid present at the time of consumption (American public Health Association, 1938).


Determination of Vopours

Various devices are available for determining whether a space fumigated with HCN is sufficiently aerated to permit human entry or whether a given material is adequately free of gas to allow handling. An important use of these devices is for testing bedding and clothing after fumigation in order to discover whether HCN is sufficiently desorbed so that warmth from the body may not release dangerous concentrations of the gas. All "handy" devices for indicating fumigant concentrations should be used with care. Instructions for their use should be followed meticulously. Judgement is required for the proper interpretation of the readings, and this improves with experience.

Test papers. Specially prepared test papers have been used successfully as indicators of the presence of HCN in dangerous concentrations. At present, two kinds are in general use, referred to respectively as methyl orange and benzidine copper acetate test papers.

Filter papers dipped in a solution containing methyl orange and mercuric chloride give reliable indications only down to 25 ppm of HCN in air at temperatures above 10C. However, if kept moist in a tightly stoppered bottle away from light, the papers will be in a fit condition for practical use for up to three months after preparation. The reaction of the papers is slow. In the presence of HCN they turn from an orange colour to various shades of pink, the usual period of exposure being two minutes (Sherrard, 1928). It is unnecessary for the observer to Stay in the space being tested; often the papers may be lowered into a space on a string or held through a door or window on the end of a stick.

The papers made from a solution containing benzidine acetate and copper acetate are sensitive to 10 ppm of HCN in air (Brown, 1952). The change in colour from white to various shades of blue, according to the concentration of HCN, takes place in ten seconds, and it is usually necessary for the operator to make the reading in the contaminated atmosphere. These papers have to be dipped in the mixture of solutions immediately before the test is made. Also, because the mixture is very unstable, the two solutions must be kept separate and only mixed shortly before use. The benzidine acetate solution itself is unstable and must be kept away from light in a brown bottle. Since benzidine compounds are known to have carcinogenic potential, appropriate precautions should be taken if this test method is used.

Manufacturers or suppliers of various types of cyanide fumigants give full instructions for making and using test papers. Some manufacturers also supply the methyl orange test papers ready for use and others have special test kits available for making the benzidine copper acetate papers on the job.

Automatic toxic qas detector. A portable, fully automatic detector based on the colour stain formed by reaction of the gas with reagents in a chemically impregnated paper is available for measuring hydrogen cyanide at TLV levels (J. and S. Sieger Ltd., Poole, Dorset. U.K.). This instrument provides in a rugged, lightweight unit, a sampling pump, sampling head and a simple photooptic system for direct reading of the gas concentrations. By changing the sampling period and/or the electronic gain, a whole range of concentrations can be analyzed. Tapes are available from the manufacturer for a number of different toxic gases.

Glass detector tubes. A number of glass detector tubes are available on the market for detection and measurement of HCN. Low range tubes that give concentration readings from 1 - 50 ppm can be used for health protection purposes and high range tubes designed to measure the levels used in fumigation treatments (0.05 to 2.0 percent) are available. (For calculations on conversion of concentration values from ppm or percent to g/m , see Chapter 2).

Chemical analysis. A simple method for determining HCN concentrations during fumigation is the silver nitrate method of Liebig, with modifications (supples, 1933). Samples of air/gas mixture are removed by aspiration and the HCN is absorbed in 2 percent sodium hydroxide solution. Titration is carried out with a standardized silver nitrate solution in the presence of potassium iodide.

White (1948) described a simple field method based upon the established phenolphthalein test for HCN. The air/gas sample drawn from the fumigation system is scrubbed through a trisodium phosphate solution containing phenolphthalein and copper sulphate and the fumigant concentration is measured calorimetrically.

The use of the thermal conductivity analyser with this fumigant is not recommended by Heseltine (1961) because of a number of technical difficulties involved.

Laboratory analysis. HCN can be measured over a range of concentrations by gas chromatography (Berck, 1965a; Donike, 1973, 1974; Darr et al, 19980).

Determination of Residues

A number of methods have been developed for the determination of residual HCN in foodstuffs. These are based on the original work of Lubatti (1935) on the analysis of cereals for this compound.

A satisfactory procedure for this analysis is given by the U.S. Food and Drug Administration (1964). Ihnat and Nelson (1979) described a method for determining cyanide residue levels in extracted honey, comb honey and wax cappings, and Toothill (1974) outlined a method for cyanide residues in grain.


HCN is generated and applied in several different ways; the choice of method depends on convenience, cost and the type of structure or material being treated.

Generation by Acid on Salt

HCN for fumigation was first generated by the action of an acid on a cyanide. Although largely replaced by other more convenient techniques, this reaction is still popular for certain purposes because it is economical and lends itself to emergency treatments. Both potassium cyanide, KCN, and sodium cyanide, NaCN, may be used. Sodium cyanide is cheaper and is the salt used principally today. Sodium cyanide for fumigation should contain not less than 96 percent NaCN and is best purchased in moulds (sometimes eggshaped) each weighing an amount convenient for measuring dosages. Sulphuric acid, H2S04, is the only commercial acid which gives a good reaction without undesirable side effects. It should be purchased in the commercially pure form at 66 Beaume' (specific gravity 1.84).

The procedure for generating HCN for fumigation from sodium cyanide and sulphuric acid in crocks, barrels or special generators is discussed in Chapter 8.

Generation from Calcium Cyanide

Calcium cyanide, Ca (CN)2, is a dark grey powder which yields HCN in reaction with the moisture in the air. The material is marketed, sometimes with a carrier, in different degrees of fineness of the granulations according to the intended use. For example, a fine grade is used for blowing small amounts into ants' nests with a hand duster. Larger granules are used for application to soil, for spreading on the floors of glasshouses or for mixing with grain. The amount of gaseous HCN yielded is between 25 and 50 percent of a given weight of granular material.

Volatilization of Liquid HCN

HCN may be purchased in cylinders as a liquid of 96 to 98 percent purity. The liquid contains a stabilizer which extends the storage life of the fumigant by inhibiting its tendency to generate heat during decomposition and to explode. Nevertheless, the manufacturers place a limit on the length of time a given lot may be kept, usually six months, after which the cylinder must be returned to the factory. This time limit should be strictly adhered to.

At normal temperatures, the natural vapour pressure of the fumigant is not sufficient to propel it into the open, especially when the liquid has to flow through piping. It is necessary to apply compressed air above the surface of the liquid to force it through the siphon tube out of the cylinder (Figure 20).

In vacuum fumigation, the reduced pressure in the chamber is sufficient to draw the liquid HCN from the cylinder. The discharge tube should pass through a heat exchanger en route to the vacuum chamber, as rapid volatilization of the fumigant brings about loss of heat with the possibility of freezing in the lines and tubes. The USE of the liquid in cylinders is a more expensive method of dispen ing the fumigant but, in large-scale operations, it is much safer and more convenient to handle HCN in this form and lengthy postfumigation cleaning is eliminated.

Absorption in Carriers

HCN may be purchased absorbed in inert materials. A convenient method of absorption is the use of highly porous cardboard discs. They are shipped in carefully sealed tin cans that withstand the pressure exerted by the absorbed fumigant at the highest temperatures normally encountered. Before the cans are opened, it is advisable to precool them in cold water, ice, or in a refrigerator at 0C. They are opened with special can openers (Figure 21) supplied by the manufacturers, and the discs are scattered in the space to be fumigated according to the required dosage. It is necessary that proper respirators be worn by those opening the cans and distributing the discs.

The discs are sometimes marketed with 2 to 5 percent chloropicrin used as a warning agent. Small amounts of ethyl bromoacetate have also been used for this purpose.

Disposal of Used Fumigant

In some countries the HCN remaining in an atmosphere after a fumigation must be absorbed and degraded or otherwise safely disposed of. A method and apparatus for decomposition of HCN to non-toxic compounds is given by Hatakoshi (1976).



Respirators fitted with the canister for acid gases will give protection against HCN and must be worn during all operations in which there is exposure to any concentration of this gas.

Absorption Through Human Skin

HCN may be absorbed in toxic amounts through the unbroken skin; the amount is increased if the skin is moist. With modern fumigation techniques it is unnecessary for an operator to remain in a full fumigation concentration of HCN. The fumigant is either discharged from cylinders outside the structure or the gas is generated indoors by one of the methods described above. In the second type of operation, workers who apply granular calcium cyanide or HCN discs, or who initiate generation by dropping sodium cyanide into acid, are able to move away before a heavy concentration of fumigant builds up. During the aeration process it is usually possible to open some doors and windows from outside the structure and to start exhaust fans and blowers so that the full concentration of HCN in the air may be reduced before any person goes inside.

Although the industrial-type respirator canister will remove concentrations of HCN in air up to 2 percent by volume for a limited length of time, it is recommended, in order to avoid absorption of dangerous amounts through the skin, that persons wearing respirators do not remain for more than 5 minutes in concentrations of 0.75 percent or for more than 20 minutes in concentrations of 0.5 percent.

Symptoms of Poisoning

Unless a person is immediately overcome by an overpowering concentration of HCN, a situation unlikely to be encountered in fumigation work when proper precautions are taken, there are preliminary symptoms which serve as a warning of poisoning. These symptoms are common to poisoning caused both by breathing HCN or by its absorption through the skin. More common warning symptoms are:

- irritation of the mucous membrane of the eyes, throat and upper respiratory tract;

- burning sensation on the tongue;

- metallic taste in the mouth

- feeling of pressure in the forehead;

- sharp pains in the head;

- giddiness and disturbed equilibrium;

- nausea and vomiting

If any of the foregoing, or related symptoms are experienced while a person is in the presence of HCN in any concentration, he should move immediately into fresh air, preferably where it is warm, and, if necessary, undergo the first aid treatment outlined below.

The concentrations of HCN that may bring about toxic effects if inhaled by humans are summarized in Table 12.

Although the poisonous action of HCN is rapid, a person may live for several hours after being completely overcome (Chen et al, 1935). Thus, even if there is some delay in the application of remedial treatments by the physician, it may still be possible to bring about the recovery of the patient.


HCN concentration in air

Possible effects on humans if gas is inhaled

Parts per million g/m (oz/1 000 ft )  
10 0.011 Threshold limit for continuous daily exposure of 8 hours.
25 0.027 Slight symptoms after several hours breathing. May lead to chronic poisoning.
50 0.055 Serious disturbances after breathing 0.5 to 1 hour.
100 0.11 Dangerous after breathing 0.5 to 1 hour with possible fatal results.
200 0.22 Quickly kills human beings and other mammals.


The following items, especially designed to combat HCN poisoning, should be included in the first aid kit:

- 12 pearls amyl nitrite;
- 2 ampoules of sodium nitrite (10 cmof 3 percent solution);
- 2 ampoules of sodium thiosulphate (50 cmof 25 percent solution);
- 1 sterile syringe, 10 cm;
- 1 sterile syringe, 50 cm;

This kit must be on hand every time HCN is used. The amyl nitrite is the only item which may be administered by the fumigators themselves; the others are for the use of a physician only.

The following procedure and instructions for administration of antidotes are based on the recommendations of a manufacturer of HCN fumigants (American Cyanamid Company, 1962).

Poisoning by the gas may not be fatal if prompt action is taken. Do not rush an unconscious person to the hospital. Prompt action on the spot is essential.

1. Do not breathe gas yourself even for a short time. If it does not overcome you, it will cut down your strength. Rescuers entering a contaminated area must be adequately protected with self-contained breathing apparatus and any necessary protective clothing. Canistertype gas masks are not dependable under such circumstances of possible high concentration.

2. Carry patient to fresh air, and lay him down. Fresh air does not mean out of doors in cold weather. Many persons have walked from a warm room containing gas only to collapse in the cold outside air. Take the patient to a comfortably warm room free of gas. Remove contaminated clothing but keep patient warm. Start following first aid treatment immediately and call a physician.

3. Break an amyl nitrite pearl in a cloth and hold lightly under the patient's nose for 15 seconds. Repeat five times at about 15-second intervals. Use srtifical respiration if breathing has stopped.

4. Never give anything by mouth to an unconscious person.

Suquested Care by a Physician

Antidotes should be administered by a physician only and the following method of administration is suggested.

1. The physician will decide on further administration of amyl nitrite. He will quickly load his syringes, one with 10 cmof a 3 percent solution of sodium nitrite and the other with 50 cm of a 25 percent solution of sodium thiosulphate. Only the specially prepared intravenous solutions in ampoules should be used.

2. Stop administration of amyl nitrite and inject intravenously 0.3 9 (10 cm of a 3 percent solution) of sodium nitrite at the rate of 2.5 to 5 cmper minute.

3. Inject by the same needle and vein, or by a larger needle and a new vein, 12.5 g (50 cm of a 25 percent solution) of sodium chiosulphate.

The patient should be watched for at least 24 to 48 hours. If signs of poisoning reappear, injection of both sodium nitrite and sodium thiosulphate should be repeated, but each in one half of the previous dose.

Even if the patient looks perfectly well, the medication may be given for prophylactic purposes two hours after the first injections.

If respiration has ceased but the pulse is palpable, artificial respiration should be applied at once. The purpose is not only to revive the respiration per se, but also to keep the heart beating. A handkerchief containing amyl nitrite should be laid over the patient's nose, for it may hasten the resumption of respiratory movements. When signs of breathing appear, injection of the above solutions should be made promptly.

Artificial Respiration

Artificial respiration by the traditional Schafer (prone pressure) method is no longer accepted as adequate. When breathing stops, artificial respiration should be given at once by an effective method. The HolgerNielson method is to be preferred and it is recommended that fumigation crews be trained in this. Although mouth to mouth resuscitation is the most effective technique for an emergency, it may be difficult to apply if the patient is being administered amyl nitrite. Furthermore, there may be danger to the rescuer himself if the patient is highly contaminated with HCN in or on the mouth or in the region of the face.

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