Abstract :
"Rasa Shastra" deals with Indian Alchemy. Parada is the nucleus of 'Rasa Shastra' (Indian Iatrochemistry), and the whole science is under the influence of physical and chemical properties of Parada, hence it is very important for every students of Rasashastra to have a very clear idea about the physicochemical properties of Parada and different mercurial preparations, to have an optimum knowledge about this great subject. This study made a humble effort to eradicate the ignorance about the chemistry of mercurial preparation, and its structural approach.

Keywords :
Parada, Mercury, Iatrochemistry.

Conclusion :
Thus we see the building blocks of Ayurvedic mineral formulations are chemically and structurally differ very much from each other. They may have the same color, texture and therapeutic effect but they have different structural approach.
Implementation of modern analytical tools like Transmission electron microscopy, X-ray diffraction study etc. further increases the scope of scientific validation of the classical Ayurvedic processes and medicament of mineral origin.

Abstract :
"Rasa Shastra" deals with Indian Alchemy. Parada is the nucleus of 'Rasa Shastra' (Indian Iatrochemistry), and the whole science is under the influence of physical and chemical properties of Parada, hence it is very important for every students of Rasashastra to have a very clear idea about the physicochemical properties of Parada and different mercurial preparations, to have an optimum knowledge about this great subject. This study made a humble effort to eradicate the ignorance about the chemistry of mercurial preparation, and its structural approach.

Keywords :
Parada, Mercury, Iatrochemistry.

Objective :
Apart from classical knowledge of medicine, an ayurvedic student or researchers must have at least basic knowledge of chemistry, which deals with the structure and chemical properties of minerals that are used in Indian Alchemy.
The therapeutic efficacy of the minerals mostly inclined over the structural conformation; hence it is the guiding tool for the Drug development program in herbal sector. Rasashastra students must have clear knowledge of the basic pillars of herbo-mineral formulation like sindoora group, parpati group, kajjali group etc, to carry out the further drug development in the Ayurvedic field.

Pecularities of ‘Parada’ as a Metal ^[1] :
Parada (Mercury) belongs to Group IIB (Group 12) of the periodic table, along with zinc and cadmium. All the three elements occur in nature as sulfides. In this subgroup zinc and cadmium are rather close to each other but they differ widely from mercury. The anomalies are enumerated below:

1. Mercury is a liquid metal ^[2]: Most metals are solid because they share their valence electrons with surrounding metal atoms. Mercury-mercury bonding is very weak because its valence electrons are not shared readily. (In fact mercury is the only metal that doesn't form diatomic molecules in the gas phase).Heat easily overcomes the weak binding between mercury atoms, and mercury boils and melts at lower temperatures than any other metal. The thin valence electron sea makes mercury's ability to conduct electricity and heat much poorer than expected for a metal at that position in the periodic table. The s electrons are able to come very close to the nucleus. They swing around very massive nuclei at speeds comparable to that of light. When objects move at such high speeds, relativistic effects occur. The s electrons behave as though they were more massive than electrons moving at slower speeds. The increased mass causes them to spend more time close to the nucleus. This relativistic contraction of the 6s orbital lowers its energy and makes its electrons much less likely to participate in chemistry- they're buried deep in the atomic core.
2. High ionization potential: The first ionization potential (10.43 eV) is the highest first IP of all metals. The only other elements which have higher ionization potentials are hydrogen(13.53 eV), carbon, Nobel gases and the electronegative elements of nitrogen, oxygen, and fluorine families, thus mercury is considered as a Nobel metal, this is also supported by positive standard electrode potential(+ 0.789 volt).
3. Volatility: Mercury is the most volatile metal in periodic table. The melting points and boiling points of the three most volatile metals are given below:
   
   

   	Cs	Ga	Hg
   Melting Point (in celcius)	28.5	29.8	-38.9
   Boiling Point (in celcius)	705	2030	357

   Mercury vapor is mono atomic.
4. Amalgam formation: Mercury has a strong power of forming liquid metallic solution or amalgam, this property of forming metallic solution is also to be found with other metals above their melting points. In this case this is of special importance because of its very low melting point. Iron containers are used for mercurial preparation in Ayurveda because mercury does not form amalgam with iron.
5. Oxidation state of mercurial compound: Besides the usual group oxidation state of + 2(II), mercury exhibits a peculiar unparallel oxidation state in the mercurial (dimercury (I)) compounds. This state is correctly represented as Hg22+ and not as Hg+; the following experimental evidences support this formulation:
   (a) Variation of conductance with dilution of dimercury (I) nitrate Hg 2(NO3)2 resembles that of bi-univalent electrolyte Pb(NO3)2 rather than a univalent electrolyte AgNO3.
   (b) X-ray crystal structure determinations of several dimercury (I) compounds show that there is a strong Hg-Hg bond, the bond length being 2.5A(250Pico meter) (atomic radius of Hg=1.50 A) (150Pico meter)
   (c) Monoatomic formulation like HgCl is ruled out by diamagnetic behavior of all dimercury (I) salts. Mercury (I) with 79 electrons (d10 s1), must possess one spin giving paramagnetic behavior. Instead Hg22+ (Hg-Hg2+), will be diamagnetic due to spin pairing arising out of the covalent link between the two mercury atoms.
   A good degree of stability or inertness is associated with the S2 pair of electrons in the noble gas helium; certain non-transition elements often show chemical behavior which is explicable only on the assumptions of a model that their S2 pair of electron is inert.
   The inert pair effect is also shown in mercury but in a very unique way. Mercury has only two valence electrons (6S2), if both of this becomes inert valence become redundant. Two S electrons of two mercury atoms become unavailable for valence purposes through strong Hg-Hg link, thus leaving two other electrons for bonding purposes.

Structural Dissimilarities among Different Mercurial Preparations ^{[3, 4, 5]} :
Parada according to Ayurvedic Rasashastra is the most important ingredient; mercury and Gandhak (Sulfur) are the most important fundamental nucleus of Ayurvedic herbo-mineral preparation. Three most important fundamental formulation of mercury with sulfur, which forms the pillar of Rasa-shastra are as follows,

Kajjali :
etymology of kajjali is black corrylium, because due to its appearance, it is simply prepared by pounding sulfur with purified mercury (without any addition of fluid), until the color changes to black, and the formation of amalgam.
The process of constant trituration of parada (mercury) and shudha gandhaka (purified sulfur) will pack the sulfur molecules in between the layers of mercury molecules. As the trituration continues, the compact placement of sulfur molecules becomes more fixed. Therefore after oral administration of kajjali, the sustained release of active molecules takes place. Hence also in clinical practice it is observed that “timed release and sustained release” theories of the drugs triturated with kajjali are more than the drugs without kajjali. Trituration of herbal powders with kajjali brings out the structure of the compound as different layers of herbal medical principles with inert molecular layer of kajjali and this formation of chemically organized alternate layers of kajjali and herbal compounds continues proportionally with that of continued pounding.
Kajjali pharmacology can be discussed i.e. one is by considering the proposals that mercury and mercurial compounds like Kajjali are not absorbed through GI tract and other way of considering the mercurial absorption. When the theories say that Kajjali is not absorbed, it is postulated that the classically prepared kajjali eventually acts as GI stimulant, locally also acts as neuro chemical irritant for intestinal mucosa. Acts as a catalyst and hence through its catalytic activity, better absorption of remaining herbal pharmacological molecules is also augmented.

Parpati :
For the preparation of parpati, first kajjali is prepared and then it is melted at the temperature of 1190Celsius (melting point of sulfur), then melted kajjali is distributed over leaf (leaves of Musa acuminata, Butea monosperma, i.e. big leaves), covered square or rectangular shaped slabs made of cow dung cakes, after pouring the melted kajjali, it is pressed by another slab made up of similar materials, and after self cooling , the crunchy black layers of parpati is collected and powdered and stored in the bottle.
Both kajjali and parpati are black, metacinnabar (ß-HgS)

HgS is dimorphic with two crystal forms:

• Black, metacinnabar (ß-HgS) is less common in nature and adopts the zinc blend crystal structure.
• Red cinnabar (a-HgS, hexagonal) is the form in which mercury is most commonly found in nature.

(a)meta cinnabar, (b) Zinc blend structure
(Ball and stick model model, black-mercury, white-sulfur)

Need of the preparation of Parpati :
According to ayurvedic philosophy,

1. Parpati is one of the method among the 25 other methods known as ‘Parada bandha’, which means to control the movement and fragility of the Parad.
2. There is an application of heat for the preparation of Parpati, hence heat energy may enhance its therapeutic efficacy in treating dyspepsia, diarrheas, dysentery, etc.
3. There is the application of cow dung cakes, which comes from the alimentary canal of the cow, and may have some enzymatic activity.
4. Ghee is smeared on the leaves so that we can easily scrap out the Parpati, ghee is used to treat indigestion, and may have a synergistic effect along with Parpati.

Sindoor :
Sindoora group compounds are so named because their colour is of deep red just like red coloured sindoor, prepared when kajjali is made and then poured in a glass bottle covered with seven layers of mud smeared cloth, without the cork and heated in a muffled furnace or Valuka yantra (placed within the heap of sand and heated) at a series of temperature range.

1. Upto 250 0C: Melting of sulphur
2. 2500C-4500C: chemical transformation
3. 4500C-6500C: vaporization of the product and subsequent condensation.

Cork is placed in the bottle at appropriate time when,

1. The bottom of the bottle becomes red hot.
2. A coin placed over the bottle opening will turn white due to deposition of mercury particles.
3. No fumes appeared when a cool iron stick is put into the bottle.

After sealing, self cooling is performed and sindoora is scraped out from the neck portion of bottle.
Sindoora is nothing but Crystals of red a-HgS and they are optically active. This is caused by the Hg-S helices in the structure.

Ball-and-stick diagram of the alpha polymorph of Ras-Sindoor, a-HgS Mercury, Hg: grey, S: yellow.
Red a-HgS is more stable and found in nature in the name of ‘Hingula’.
Conversion of Black metacinnabar (ß-HgS) to Red cinnabar (a-HgS, hexagonal) is carried out when metacinnabar is heated, beta form converted to alpha form and become more stable.

Sindoora groups of drugs are red in colour but there are two exceptions. They are rasakarpura and rasa puspa. They are not red in colour and they are devoid of sulfur. Rasa karpura: Chemically they are mercuric chloride with the formula HgCl2. This is a white crystalline solid resembling to camphor (karpur) hence the name Rasa karpur. It was introduced by Bhavamishra , author of Bhav prakash in the 16th century and was then prepared by ferrous sulfate, copper sulfate etc.
But in Rasatarangini of 20th century, it is prepared by the action of sulfuric acid in the first phase and rocksalt in the second phase.

Heating a mixture of solid mercury (II) sulfate and sodium chloride affords volatile HgCl2, which sublimes and condenses in the form of small rhombic crystals.
Mercuric chloride is not a salt but a linear triatomic molecule; hence it has a tendency to sublime. In the crystal, each mercury atom is bonded to two close chloride ligands with Hg---Cl distance of 2.38 Å; four more chlorides are more distant at 3.38 Å.

Ball-and-stick model of part of the crystal structure of Rasa karpura, HgCl2.

Rasa puspa: It is prepared according to Rasatarangini by the action of ferrous sulfate and rock salt over mercury.
asa puspa is Mercury (I) chloride with the formula Hg2Cl2. Also known as calomel (a mineral form, rarely found in nature) or mercuric chloride, this dense white or yellowish-white, odorless solid is the principle example of a mercury (I) compound. Mercury is unique among the group 12 metals for its ability to form the M–M bond so readily. Hg2Cl2 is a linear molecule. The mineral calomel crystallizes in the tetragonal system. The unit cell of the crystal structure is shown below:
Ball-and-stick model of the unit cell of calomel, Rasa puspa, Hg2Cl2
Ball-and-stick model of the coordination environment of mercury in the crystal structure of calomel, mercury (I) chloride, Hg2Cl2
There is another red coloured controversial mineral known as Giri sundoor. It is not a Sindoor preparation as mentioned previously but it has bright red colour and found in the cracks and fissures of the rocks.
It is not mercury sulfide or cinnabar, but it is Red oxide of mercury and it has a formula of HgO. Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite (orthorhombic) and the second is analogous to the sulfide mineral cinnabar (hexagonal).
Both are characterized by Hg-O chains. At pressures above 10 GPa both of those structures convert to a tetragonal form.

Kajjali, Parpati, Sindoora are basic fundamental formulations of Rasa Shastra, they are pounded with other metals like gold or herbal extracts for their use in specific diseases.

Conclusion :
Thus we see the building blocks of Ayurvedic mineral formulations are chemically and structurally differ very much from each other. They may have the same color, texture and therapeutic effect but they have different structural approach.
Implementation of modern analytical tools like Transmission electron microscopy, X-ray diffraction study etc. further increases the scope of scientific validation of the classical Ayurvedic processes and medicament of mineral origin.

*Faculty of Ayurveda, Institute of Medical Sciences, Rajiv Gandhi South Campus, Banaras Hindu University, Barkachha, Mirzapur-231001 References :

1. R. N. Datta , ‘Inorganic Chemistry’, Part II (fourth ed.) : The new book stall publication.
2. L. J. Norrby, "Why is mercury liquid?”, Journal of Chemical Education, 68, 110-113 (1991)
3. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999),
   Advanced Inorganic Chemistry (6th ed.): Wiley - Interscience
4. Dr. K. R. C. Reddy, ‘Text book of Rasa shastra’(First ed.):Chaukhambha Sanskrit Bhavan.
5. Dr. D. N. S. Gautam, ‘Saral Ayurvediya Rasa shastra’ (First ed.) : Chaukhambha Survarati

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