PHYSICO-CHEMICAL ASPECTS OF THE APPLICATION OF MERCURY TOWARDS PRESERVATION

OF STORED FOOD GRAINS

BY K. K. DOLE (Chemistry Laboratory, Fergusson College, Poona)

Received March 14, 1945

(Communicated by Prof. D. D. Karve, F.A SC.)

Tnz use of mercury for protecting stored food grains is an age-old custom in several parts of India. Dr. K. K. Kanan first brought the insecticidal property of mercury in connection with storage of grains, to the notice of the entomologists in the third Entomological Conference in India. 1 Since then various experiments have been made with mercury for preservation of wheat, rice and other grains from spoilage by insects. It is now well established that this property of mercury is due to the small amount of vapour which it emits at atmospheric temperatures. The mercury vapour sterilises the insect eggs and breaks the life-cycle of the insects feeding on the grain. It has been further observed 2 that not only the eggs but also small larvae of the insects are killed when mercury amalgams or colloidal mercury dispersions (dusts) are used for preservation of grain. There is an agreement among all previous workers that it is the mercury vapour that is responsible for checking insect growth. This means that mercury is acting as a fumigant and not as a contact or stomach poison. Naturally, therefore, the emission of mercury vapour from the different preparations of mercury is the chief factor of their efficiency as preservation of grain during storage.

At ordinary temperatures metallic mercury is a liquid with nonwetting properties. Evaporation of a liquid, in the space where diffusion of its vapour is rapid, depends upon its vapour pressure at the temperature at which the vaporisation is taking place, al~d the surface of the liquid exposed for evaporation. On account of its high surface tension mercury has a tendency to coagulate in droplets, which entails a great decrease in its eva- porating surface, as compared with that which it otherwise would have had, if mercury had been a liquid with wetting properties and low surface tension. Hence there is a ~,ery slow formation of vapour from mercury in its ordinary state. A larger surface of evaporation than the one in the liquid state of mercury, rapid diffusion of mercury vapour around the grain to be pre- served, minimum loss of mercury vapour away from the space of grain storage

* Proc. 3rdEnt. Meeting, Pusa, 1919, 761-62. a Dole, J. Uni. Born., March, 1943, 11, Part V, 117.

B4a 133

134 K . K . Dole

and the availability of all the amount of mercury in the preparation which is to be used for preserving grain, are some of the conditions essential for use of mercury for the preservation of stored food grains in an effective and efficient manner.

On account of the liquid state of mercury at ordinary temperatures it requires some vehicle to support it during the course of its application as a preservative in the storage of grain. Since the efficiency of mercury depends upon the evolution of its vapour and its diffusion, use of such a vehicle should facilitate vaporisation and diffusion of mercury. Various such bases or vehicles have been used in our laboratory, the selection of which was based on the following objects : -- (l) the mercury absorbed by the support should not be rendered inactive; (2) the vehicle used should increase the surface of evaporation of mercury ; (3) the vehicle itself should not add much to the cost of the preparation, and (4) since these prepara- tions are bound to be used on a large scale their manufacture on a commer- cial scale should be without any great technical difficulty.

Various preparations of mercury were made and used by the present author, amongst which were the tin-mercury, and copper-mercury amalgams. Mercury dispersed and enclosed in porous paper strips and the one dispersed in inert vehicle was also used. It was soon apparent that very few of the preparations fulfilled the above conditions. In amalgams a substantial part of the mercury was soon rendered inactive unless the concentration of mercury in it was high. Of the chief metals which amalgamate with mercury, copper is least soluble in it. 3 At the same time mercury adheres to its surface so that copper mercury surface amalgam would have served as an ideal material for the purpose of preserving grain and hence such copper plates coated with mercury were used by me in the first instance (Dole, loc. cit.). It was considered at the time that by the use of such surface copper amalgams, a small quantity of mercury would suffice for preser'vation of grain. But since then it was observed that these copper plates were rendered inactive after using them for few days. This inactivity is explained by the gradual dissolution of mercury by the copper and consequent lowering of vapour pressure of the mercury, which ultimately becomes zero if the quantity of mercury employed is less than its solubility in the copper. This is illustra- ted by an experiment which was carried out from 2-4-1943 to 1-5-1943.

Three copper plates measuring 10 • 5 cm. and weighing about 2.5 gin. each were coated on both sides with mercury. The amount of mercury adhering to the surface of copper plates was not uniform. Its weight varied

8 Ramsay, 1889, $5, 521.

,4 pplicaNon of Mercury towards Preservation of Stored Food Grains 135

f r o m pla te to plate. These plates were then p laced in a well ven t i l a ted r o o m ,

and they were loose ly cove red wi th an inver ted glass bel l - jar so as to p reven t

the depos i t i on o f a n y dus t par t ic les on the sur face o f the m e r c u r y bu t to

a l low the free d i f fus ion o f any v a p o u r tha t m i g h t be f o r m e d by the m e r c u r y

sur face on the c o p p e r plate. T h e th i rd c o p p e r p la te wi th m e r c u r y c o a t e d

on it was used as a c o n t r o l and p l aced in an e m p t y des i cca to r o f a b o u t one

l i tre capac i ty , cove r ed wi th a t igh t ly f i t t ing lid. This was d o n e wi th a view

to find ou t the loss o f m e r c u r y f r o m the p la te in an enc losed space. T h e

fo l lowing tab le gives the losses t ha t o c c u r r e d f r o m t ime to t ime in the weights

o f the di f ferent p la tes :--- TABLE I

Plate No. I Plate No. II (Kept under a lzell-jar

with free d,ftusion)

Plate No. III (Kept m an enclosed

space)

Grams Grams ~/eight of copper plate .. 2- 7616 2.7416 Weight of copper plate with"

mercury . . . . . . 3.6882 3.4272 Weight of mercury on plate 0.9266 0.6856 Weight of plate at the end of

32 days �9 3.5550 3-3466 ~Velght of mercury left on tlae

plate after the evaporatlon ceased . . . . . . 0.7934 0.6050

Loss cf mercury . . . . 0.1332 0.0815 Loss of rig 0.1332 0.0815 . . . . .

Mercury coated . . . . 0.9266 -- 0.1437 0.-q~- 6=U.14/j V, of rig lost . . . . . . 14.37% 14.73%

Grams 2.7244

3.5910 0.8666

3.5892

0,8648 0.0018

0.0018 . . . . 0.~6 =o .trOZO

0.26%

This shows that a substantial quantity of mercury is retained by copper under the most favourable conditions of evaporation of mercury.

It may be mentioned here that if an attempt is made to coat an increased quantity of mercury on the copper plates to compensate for the mercury held by them the copper plates lose their tensile strength and crumble to pieces.

TABLE II. Loss of mercury of the above plates with time

No. of days Total loss of Hg in grams after which

the less was noted Plate No. l Plate No. II Plate No. III

3 6 9

13 17 20 23 27 32

0.0 0.0459 0.0518 0.0656 0.0877 0.1221 0.1331 0.1331 0.1332

0.0327 0.0781 0.0811 0.0810 0-0809 0.0813 0.0815 0.0814 0.0813

0.0000 0 -0003 0.0009 0.0015 0.0015 0.0016 0.0017 0.0018 0.0018

136 K . K . Dole

The temperature of the room in which the plates were kept was sub- ject to varying atmospheric conditions, no attempt being made to keep it at constant temperature. No mathematical relationship can therefore be traced in the results obtained, but it can be seen that the losses in weight are obviously much greater in the earlier days than those after that period. After the first fortnight the losses in weight dwindle down materially. The plate No. III which was kept in an enclosed space does not show any appreciable loss. The slight difference in weights manifested is probably due to the loss of mercury vapour, required to saturate the one litre space of the desiccator; a very small part of it may also be due to exposure during the weighing operations.

One valuable inference that can be drawn from these observations of the negligible loss of mercury from this plate kept in an enclosed space is that a very small quantity of mercury will be sufficient for the preservation of grain provided the grain is stored in an enclosed space like a closed room or a metal storage tank. The quantity of mercury required for each place of storage can be calculated from the temperature of the rdom and the amount of enclosed space in which the grain is stored, irrespective of the weight of the grain stored.

A similar study of the loss by evaporation of mercury dispersed in finely precipitated calcium carbonate was also made. A powder con- taining 5070 of mercury dispersed in it was prepared and one gram of the powder was placed in each of two watch glasses, and the powder contain- ing the dispersed mercury was spread over an area of about 25 sq.cm, so that it formed a layer of about one millimetre in thickness. The watch glasses were then kept under cover in the manner described previously. The results of this experiment are given in the following table. The experiment was carried out from 2-4-1943 to 31-5-1943 and final weights were taken on 13-9-1943.

TABLF. III

Wt. of dispersion No, I--1-0003 grams Wt. of dlsperslcn No. II--1-0008 grams

No. of days No. I No, lI No. of days No. I No. II

3 6 9

13 17 20

0.0156 0.0326 0.0443 0.0557 0.0639 0.0700

0.0168 0.0331 0-0438 0.0527 0.0597 0-0649

23 27 32 62

164

0.0753 0.0817 0.0879 0-1070 0.3973

0.0690 0.0742 0.0773 0-0938 0-3916

Application of 3lercury towards Preservation of Stored Food Grains 137

The results indicate that the loss of mercury is gradual and about 80~o is lost from 25 sq. era. of evaporating surface in a period of five months. The comparison of these results with the results of the copper mercury surface amalgam indicates that the evaporation of mercury from the dis- persion is approximately four times as rapid as that from the surface of the amalgam. The evaporation of mercury in dispersions however would naturally depend upon the area of the evaporating surface which again is dependent on the fineness of particles of mercury. Ncw if the results of this experiment are studied mathematically one or two important points are brought out. The rate of evaporation of mercury from a constant surface at a constant temperature in an open atmosphere would be given

d~c by the equation ~- = Ks ( a - x ) where a is the original amount of mercury,

x is the amount lost and s, the surface area of evaporation, K is the evapo- ration constant depending upon the latent heat of evaporation, etc. On integration we get:

sK 1 a "= i • 1 7 6

This equation possesses the form of an equation of an unimolecular chemical reaction. On calculating the values of sK from the figures in Table III it is found the values are gradually falling. If this be taken into account it can be concluded that the evaporating surfaces are gradually diminishing. If the dispersions are thinly spread then they can be made to conform to the above equation. Similarly it may be expected that all the mercury contained in the dispersions will evaporate in due course. This expectation may very likely be fulfilled since the watch glasses with the powder, when weighed after a period of five and half months, showed a loss of 80~o of the mercury.

These experiments indicate that it is possible now to assess the value of any mercury preparation for using it as a preservative, especially if the dimensions of the godown and the prevalent temperatures therein are known. By this method it is possible to measure and compare the efficiency of mercury preparations for preserving grain in storage in actual practice.

The next important point which has to be decided is the least quantity of mercury required for controlling the attack of insect pests in stored food grains. Mercury has definite vapour pressures at definite tempera- tures and these vapour pressures are extremely small at ordinary atmo- spheric temperatures. Data concerning these vapour pressures was given in my earlier paper (loc. cit.). Mercury vapour is mono-atomic and one can

138 K . K . Dole

calculate from these vapour pressures the equilibrium concentrations of mercury at different temperatures using the ordinary gas laws. Thus :--

TemperatureC. . . [[ 15 ~ Eqm. con. per cu. meter in rngm

mercury . . . . 8-8

16 ~ 17 ~

9.4 10.2

18 ~

11.0

19 ~

12.0

20 ~

13.0

25 ~

20.0

30 ~

31.4

35 ~

47.0

One cubic metre of space would hold about the following quantities of different grains : m

/ Name of the gram . . . . . i/] Rice Weight of the gram in lolograms . 930 Intersntlal space m between the gram. 36%

I Wheat [ Jowar Bajarl

8!5 ] 850 850 38% 3~.5% 34%

Turdal [ Val 860 818

38.5% 97%

Varl 890 32%

Interstitial space in the grain has also been calculated in the above table to give a clear conception of the empty space available for the diffu- sion of mercury vapour around the grains, This data would give only a rough idea of the amount of grain stored in one cubic metre and the exact amount will depend upon on the mode of packing the grain. So it is ex- pected that about 10 to 12 mgm. of mercury per cu.m. of grain plus 20 to 32 mgm. per cu.m. for the empty space above, is the theoretical quantity required if the grain is enclosed in a closed space at about 25 ~ to 30 ~ C. But in practice allowance has to be made for various factors. If the grain is not stored in enclosed space, large losses of the vapour are expected by diffusion. Similarly, allowance will have to be made for the improper diffusion, especially with grains of small interstitial space. Tberefore, in the previous paper (lot. eit.) one gram of finely dispersed mercury was recommended for 240 lb. (110 Kilos.) of grain. Tbis expectation was actually tested in the present investigation by following experiments.

In the present investigation I have used a dispersion of mercury in calcium carbonate for the purpose of determining the minimum quantity of mercury required for protecting stored food grains from an attack of insect pests. The use of a dispersion instead of the mercury coated copper, or dispersed mercury deposited between two ribbons of porous paper was resorted to, because my study of the physical factors of the formation of mercury vapour has shown that such a dispersion or dust would prove more efficacious than the other two preparations used previously. The quantity of mercury to be used was first calculated from the known weight of the dispersion and this was afterwards checked by actual chemical examination agreed fairly well. The calculated quantities of dispersions added separately to each of the several samples respectively were first weighed on a chemical balance in watch glasses by the 'difference' method,

Application of Mercury lowards Preservation o/'Stored Food Grail, s 139

the weights of the dispersion taken for experiment being correct up to 0.5 of a milligram. The grain chosen was Val (Dolichus lablab), because this grain is easily attacked by a bean weevil (Bruchus phcesoli) and the difference between the state of the healthy grain and that of the insect-attacked grain could be quickly made out and also it could be secured in sufficient quantity as it was out of the range of rationed grains at Poona.

Beans already infected with eggs and insects were used in the experi- ment and the amount of further damage caused by the insect was estimated by counting the number of holes in the beans 200 Grams of the beans were weighed in several lots and the whole quantity in each lot was placed in glass bottles of one litre capacity with a loosely fitting cork so as to prevent the adult insects from coming out but allowing sufficient air for the insect life inside the bottle. The samples were examined from time to time. The following table gives the summary of the results.

TABLE IV. Beans infected with insects and eggs found on the beans Quantity of mercury added 17mgm. for 200 grams of beans. Number of holes for

25 beans selected at random from each bottle found to vary between 66 to 68.

Date Control Sample I Sample II no treatment 17 mgm. Hg 17 mgm. Hg

12-4-1943 12-5-1943 12-7-1943

12-8-1943

/ 66-68 holes per 25 beans[

130-140 do. ] All food material

exhausted [ Beans damaged, black [

with mould /

66--68 holes per 25 beans 68-72 do. 68-72 do.

68-72 do.

66-68 heles per 25 beans 68-72 do. 68-72 do.

68-72 do.

The effect of the addition of the mercury dispersion is obvious.

In the next experiment only sound beans were selected from the whole lot. Absence of holes bored by insects was the criterion used in selecting the beans. They were also examined for the presence of eggs on the beans becanse for our purpose the presence of eggs on the beans was desired and a few beans having eggs visible on them were deliberately included in all the different lots selected for trial. This experiment was carried out with a view to study the influence of mercury when used from the very beginning on grain coming directly from the fields for storage in the farm buildings of the cultivator. Also it is valuable from the point of preserva- tion of seeds on the farm by the cultivator.

140 K. K. Dole

TABLF, V. All beans without holes, but eggs present

Quantity of mercury added 17 mgm. per 200 grams of beans

Dates

Treatments

Control no treatment

Sample I preserved

with mercury 17 mgm.

Sample II preserved with Hg 17 mgm.

12--4-1943 12-5-1943

12-7-1943

12-8-1943

No holes Numerous holes and some adult

insects, beans hole Innumerable holes and beans

damaged Beans damaged. No food left

No holes do.

do.

do.

No holes do.

do.

do.

There can be no doubt about the effect of mercury on the eggs of the bean weevil.

The next experiment was carried out on mixed beans about 5% of which were affected by insect attack as judged by counting the number of beans with holes per 100 beans taken out at random from each bottle.

TABLE V]. Study of the minimum quantity of mercury sufficient to preserve 200 grams of beans in one litre of space

200 grams of beans in each bottle, affected beans (with holes) about 5~ m each botllr

Treatment

Control beans-- No treatment

17 mgm. Hg

6 mgm. Hg

4.5 mgm. Hg

3-5 mgm. I-Ig

1.5 mgm. Hg

Dates of observation

~

12-4-1943 ] 12-5-1943

5~ beans with About 90~ holes beans with

holes

do.

4~ beans with holes

6% beans wgh holes

5~ beans with holes

do.

5N beans with ho!es

do.

4~ beans with holes

5~ beans with holes

6% beans with holes

12-7-1943

All beans affected. All food matenal exhausted

6~ beans with holes

5% beans w~th holes

do.

6~ beans with holes

4~ beans with holes

12-8-1943

All beans blackened with moulds

5~ beans with holes

do.

6hg)lebse an s with

5~ beans with holes

do.

It can be concluded that the damage by the insects to the beans pre- served with mercury was completely checked and did not make any progress

,xlpplication o)C Mercury towards Preservation o[Stored Food Grains 141

whatsoever after the addition of the mercury dispersion. The small differ- ences observed are considered to be merely due to such errors as usually occur in sampling at random. Even the smallest quantity of mercury used in this experiment has proved effective. The ratio of 1.5 mgm. of mercury to 200 grams of beans amounts to 0.85 gram mercury for 240 lb. of beans or approximately 8 grams of mercury to a ton of grain (1016 Kilos) stored in a space of 1800 cu.ft. (about 50 cu.m.).

The above quantity of 1.5 mgm. per 200 grams in 1000 c.c. of space is the smallest I have so far used. In order to find out whether it could be reduced still further it is intended to use chemically precipitated mercury; but since in the course of such precipitation some mercurous chloride is likely to be formed to a smaller or greater extent in association with preci- pitated mercury, it is intended to see the effect of mercurous chloride (calo- mel) on the insect pests of the stored grain. Side by side it was intended to examine the effect of other chemicals which happened to be easily available in the chemical laboratory. Out of these, potassium metabisulphite was selected because it gradually liberates sulphur dioxide, which was consi- dered to have some effect on the stored grain pests. Sulphur was used as it is reported to be very successfully used in the field against some crop pests and fruit pests in agricultural and horticultural practice.

This experiment was tried on beans that were already affected by weevils to a small extent as in previous experiment. The results are given in the next table.

TABLE VII. 200 Grams beans partly attacked by insects and stored in one litre bottler

Dates of observation

Treatment

12--4-1943 12-5-1943 12-7-1943 12-8-1943

Control tno treatment)

1 Gram KHSOs ..

0.26 Gram HgCI ..

5 Grams sulphur ..

57. beans affected

do.

47. beans affected

670 beans .~ffected

75-80% loss most of the beans affected

24% beans affected

770 beans affected

About 6070 beans affected

Nearly 907* loss. Very few whole beans left

2/3 or 66% beans eaten up by insects

7% beans affected

Nearly 9070 heens attacked

Beans blackened with fungus

About 66~ affected ?

770 beans affected

All food material exhausted

142 K . K . Dole

The sulphur used in our experiment has proved to be ineffective. This does not however exclude the possibility that other brands of the sulphur used in agricultural and horticultural practice may prove more effective in overcoming the insect pests of stored grain. Potassium bisulphite has shown slight insecticidal activity as compared with the control but it is nothing as compared with the effect of mercurous chloride (calomel). This material is known to be non-poisonous to human beings and possesses appreciable vapour pressure at room temperatures. Its B.P. is 383 ~ C. as compared with the 357 ~ C. of mercury. Investigations with this pre- servating material will have to be further carried out in greater detail than has been possible hitherto.

In all these experiments one fact worthy of mention is that beans preserved with mercury or mercurous chloride remained intact without deterioration, but those preserved with other materials like sulphur and metabisulphite gave out a putrid odour. This was probably due to the fact that the dead bodies of the pupa~ or adult weevils killed by other materials used in the experiment got decomposed, and as these dead bodies were not removed they gave out the putrid odour on decomposition. Thence it may perhaps be a useful suggestion to bear in mind when selecting insecti- cides for preserving grain in godowns that it is better to choose such materials as kill the eggs and larvte rather than those that kill the pupae or the adults of insects.

How long would the effect of mercury last if no further insect attack takes place ? Stored grain which is preserved by mercury for a certain length of time should be expected to remain without any further damage even if the mercury is removed. Even if the grain preserved with mercury by killing the eggs of the insects is removed and stored in another place it should not show any damage by the insects. In order to test the correctness of this hypothesis, whole undamaged beans were selected from the lot preserved with the smallest quantity of mercury and were exposed to air for some days during which time any vapour of mercury that might have been surround- ing the grains would have been driven away from them. The beans thus freed from any mercury or mercury vapour were next stored in another fresh bottle of the same type and dimensions as before and examined at intervals. The beans were unaffected for a period of more than a month and were in the same condition as before quite sound and without any larv~ appearing in the grains or in the bottle, clearly indicating that all the eggs on the beans were actually rendered incapable of developing any insect larva~ therefrom.

Application ol c Mercury lowards Preservation olCStored Food Grains 143

SUMMARY

Different mercury preparations--amalgams, porous strips and dis- persions--have been studied with a view to measure their efficiency in pre- serving stored grains from insect attack. Out of the different preparations studied in this investigation mercury dispersions are the most efficient. In copper amalgam only about 14.5~o of mercury is available for vapour- isation.

Only very small quantity of mercury is required for preservation of grain. It is possible to preserve grain at the rate of 8 grams of mercury for one ton of grain stored in a space of 1,800 cu.ft, under suitable conditions.

Mercurous chloride, a non-poisonous compound of mercury, has been also found to be useful for preserving grain.

Since mercury is acting as a fumigant all conditions which increase the efficiency of fumigants in preserving grains would also be applicable to mercury.

Be