B. Phosphine (PH)
Phosphine or hydrogen phosphide (PH3) is a low molecular weight, low boiling point compound that diffuses rapidly and penetrates deeply into materials, such as large bulks of grain or tightly packed materials. The gas is produced from formulations of metallic phosphides (usually aluminium or magnesium phosphide) that contain additional materials like Ammonium salts for regulating release of the gas. It is formulated in tablets, pellets or small sachets of powder with additional materials such as ammonium carbamate, ammonium bicarbonate, wax, fillers, etc. to regulate release of fumigant and suppress flammability. Each pellet of 0.6 gm yields approximately 0.2 gm of phosphine and 3 gm of tablet releases 1 gm Phosphine (PH3). Magnesium phosphide (MP) is similarly manufactured in tablets and plates. Each plate weighing 117 gm. contains 32.3 percent of MP and liberates 33 gm. of phosphine. A high humidity (40 percent or more) is needed to generate the gas, and temperatures above 40° F are needed to produce satisfactory results. In the presence of moisture, phosphine (Hydrogen phosphide, PH3), a colourless gas, is emitted. PH3 boils at -87.7° C (-126° F) is slightly soluble in water, has excellent penetrative power, and has approximately the same density as air. The lower level of flammability is 1.79 percent in air. Phosphine ranks as one of the most toxic fumigants of stored product insects. It is a slow acting poison that is effective at very low concentrations if the exposure time is long enough. Usually, exposure times of four or more days are required to control insects, depending on temperature. The toxicity of phosphine to insects declines as the temperature falls to 5°C, so that longer exposure times are required for it to exert its effect. Phosphine is very toxic to all forms of animal life; hence exposure of human beings even to small amounts should be avoided.
Aluminium phosphide(ALP) is Solid; emits phosphine gas.
Pure Phosphine is odourless, however due to impurities it has Garlic-like, decomposing fish and stale urine odour.
|Boiling Point :
ALP : 2530°C
|Freezing Point :
i) At gaseous stage :
2.85 g/cm3 (20°C)
Explosive at 1.79% by volume in air. Auto-ignites at 38°C.
|Solubility in water: Very slightly soluble; reacts.
26 cc/100 cc at 17°C
Explosive when in contact with moist air. Corrosive to certain metals.
|Relation between weight and volume of Liquid:
As the fumigant remains in a state of gas, the weight/volume ratio is not relevant.
|Relation between dosages and concentration of Gas in air
The reading in Rekin Phosphine Monitor is shown in mg/l.
We should know that 1 mg/l is equal to 1 g/m3 and 1 g/m3 of phosphine gas is equal to 718 ppm.
|Natural Vapour pressure at various temperatures
Phosphine gas 40 mm Hg @ 129.4 C
|Mode of Action:
AlP + 3H20 = Al (OH)3+ PH3 (in air)
AlP + 3HCl = AlCl3 + PH3 (in air and stomach)
Phosphine gas is absorbed through the skin of the insects resulting in their kill.
|Threshold Limit value (TLV):
|Method of Evolution as Fumigant
Aluminium phosphide is available in pellets or tablets which contains gas with inert material and also ammonium carbamate. The addition of ammonium carbamate serves to reduce the danger of flammability of phosphine gas as it gives off carbon dioxide and ammonia.
|Effect on different commodities
Effect on Metals, Paints, Enamels, Plastics, etc.
Metals: Copper, Brass, Gold, silver: causes severe damage;
Other Metals: affects to some extent in high humidity.
Paints, enamels: No information.
Effect On Planting Material:
Germination of seeds: Germination not affected.
Phytotoxicity: No information is available.
Effect on Plant Products
No adverse effect on vitamins, particularly Vitamin A and B2. No adverse effect on baking quality of flour made from fumigated wheat.
It is toxic to Human beings and animals.
LD50 = 11.5 mg/kg
Conversion: Extensive work carried out so far has not indicated phosphine reacting with contents and leaving a permanent residue. The indications so far are that if such residues are left, they are extremely small and difficult to detect. However, phosphine has been found as residue in food grains in very low concentrations and further reduces after aeration.
The fact that phosphine aerates rapidly from foodstuff and that a residue of 0.1 ppm in a raw cereal would yield a residue below 0.01 ppm, the Codex Alimentarius Commission found no necessity to fix Acceptable Daily Intake (ADI).
|Usage as a Fumigant:
Phosphine is the toxic gas that is formed from hydrolytic reaction of aluminium phosphide. Immediately on contact with moisture, there is decomposition of ammonium carbamate component producing ammonia and carbon dioxide gas. Both these gases dilute the phosphine gas, which is evolved during subsequent hydrolysis, thus preventing explosion. As the breakdown progresses, the outer appearance of the tablet preparation will change from shiny grey-green to white. The tablet then disintegrates into a pile of grey white dust. The residue dust is aluminium hydroxide which may contain traces of aluminium phosphide.
Phosphine is an extremely useful fumigant in Phytosanitary treatment. It is highly toxic to pupal, larval and adult stages of insects. Its ovicidal properties are however a limiting factor. Because of this, the exposure period is longer than MB.
|Symptoms of Poisoning
Phosphine is highly toxic to human beings. The gas is not absorbed through the skin. There is no evidence for cumulative effect from intermittent, low level exposures of 10 ppm or less. At higher concentrations, inhalation of phosphine may produce symptoms including nausea, vomiting, diarrhea, headache, chest pain, and massive accumulation of fluids in the lungs before death.
Treatment: The treatment is only symptomatic and supportive.
(i) Remove patient from exposure, keep at rest. Rescuers should follow full safety procedures.
(ii) If the patient is unconscious, place in semi-prone recovery position or otherwise maintain the airway.
(iii) If patient is conscious but has difficulty in breathing, treat in a seated position and give oxygen if available.
(iv) Allow patient to recline with the legs slightly elevated.
(v) If breathing stops, immediately ventilate the patient artificially (mouth to mouth/nose or mechanically with oxygen available).
(vi) If the heart stops, begin cardiopulmonary resuscitation-CPR.
(i) Remove contaminated clothing. Wash contaminated skin with soap & water.
(i) Immediately flush with large amounts of water for at least 15 mins., occasionally lifting upper and lower lids.
|Best effective use:
· At relative humidity of over 75%, maximum release of PH3 occurs in 45 hours at 25°C;
· For bag stacks a minimum of 7 days exposure at a dosage of 6 to 9 tablets per tonne at 25°C gives best kill of insects;
· Perfect gas-tight enclosures give desired results; gas loss could be unmanageable at temperature exceeding 25°C ;
Only trained fumigation operator knowing toxicological and technological properties should use.
· Fumigation should not be attempted at Relative Humidity less than 50% (grain moisture contents around 10%);
· Some insect species are highly tolerant of PH3 especially at low temperatures and short exposures;
· Oil bearing commodities tend to air slowly and have to be checked for residual PH3;
· Odour threshold is 2 ppm; unfavourable on long exposures;
· Flammability hazard;
· Brass and copper are attacked.
EXAMPLE of Measurement of Concentration of Phosphine Gas
in an Air-tight Enclosure
Suppose we have to fumigate 3 Metric Tonnes (M.T.) of wheat by ALP (56% a.i.) in an airtight enclosure @ 9 gm./M.T. with the requisite humidity and temperature.
Nine ALP tablets of 3 gm. each would be required which would release 9 gm. of phosphine gas. We know that one gm./cu. metre of phosphine gas is equivalent to 718 ppm.
Now, to measure concentration in ppm, we must know the volume occupied by 3 M.T. of wheat. Normally, in an average, One M.T. of Indian wheat occupies 1.3 cubic meter of volume. Hence, 3.9 cubic metres of volume would be occupied by 3 M.T. of wheat.
From the above, 9 gm. of phosphine gas would be released in 3 M.T. of wheat or in 3.9 cubic metres of volume. Amount of phosphine gas released in one cubic metre would be= 9 divided by 3.9 = 2.3 gm. of phosphine gas in one cubic meter. Concentration in ppm would be = 2.3 gm X 718 = 1651 ppm approximately.
This gives us an inference that with the requisite temperature and humidity the maximum concentration of phosphine gas could reach theoretically upto 1651 ppm at one point of time in an air-tight enclosure.
Note: One should ensure that phosphine concentration is above lethal conc. of 200 ppm on the last 10th day of exposure period. If the exposure period is 7 days, the concentration should be 300 ppm on last 7th day.