Stability of OrthoSilicic Acid in aqueous solutions

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Development of Eco-friendly (Green) Vapor Corrosion Inhibitor (VCI) for multi-metal packaging applications

Development of Green Vapor Corrosion Inhibitor (VCI) for metal packaging applications

ResiCor VCI – Corrosion Preventor

MICROFIGHT ARTICLE

Silicon (Si) is a chemical element having symbol Si atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster (1).

Silica (SiO₂) is a compound of Silicon Oxygen is represented as SiO₂ (Silicon Dioxide). It is amorphous in nature. Its crystalline varient is quartz (2).OrthoSilicic Acid (OSA) is the simplest soluble form of Silica (SiO₂). It is an inorganic compound of Silica (SiO₂) Silicates (SiO₄^4-) a precursor to other Silicates. It has Silicon (Si) tetrahedrally coordinated to 4 hydroxyl (OH) groups. It is represented by the chemical formula Si (OH)₄ or H₄O₄ Si. It is a weak acid. Its IUPAC name is Silicic Acid (4).In the earth’s crust, more than 90% Silicon (Si) occurs in the forms of Silica (SiO₂) or Silicates (Salts in which the anion contains both Silicon Oxygen – SiO (SiO₄^4-). It constitutes to 28% of the earth’s crust by mass.

It is also present in ocean water as Orthosilicic acid (Si(OH)₄) or its conjugate bases (Orthosilicate Ions). In ocean water, the algae known as diatoms regulate the biogeochemical
cycles of silica. These algae polymerize the Silicic Acid to Biogenic Silica (Amorphous Silica) to form their cell walls (3, 4).

Silicic Acid (OSA) has been reported to play important roles in biomineralization. OSA is of particular interest in agriculture as it is reported to be the most ‘assimilable’ or the
‘bioavailable’ form in which the plants uptake Silicon (Si) from the soil. The importance of Silicon (Si) in agriculture is very well documented for the last few years. It helps plants to
manage biotic and abiotic stress and enhance output. It is proposed to be working at a cellular level to impart benefits to plant cells at very low ppm (parts per million) concentration. We have found that every part of a living plant, the roots, trunk, branches , leaves, flowers, seed, fruit contain detectable concentration of silica.

Silicic Acid does not exist in nature. It has to be chemically synthesised using Silicates and Acid. It was first synthesised by Sir Thomas Graham (1861) by dialysing dilute silica sols
obtained by mixing an aqueous solutions of sodium silicate with hydrochloric acid. Nowadays, it is manufactured using Patrick’s process which consists essentially of gelation of an alkali metal silicate by means of acid. Monomeric and Polymeric Silicic Acids are formed by neutralising a solution of sodium metasilicate or water glass by acid. The condensation reaction of monomeric or polymeric silicic acid can be according to the following scheme (6). Fig 1 depicts the polymerisation of silicic acid.

(HO)₃SiOH + HOSi(OH)₃ (HO)₃Si-O-Si(OH)₃ + H₂O

Fig 1: Polymerisation of silicic acid molecules through formation of siloxane bond and water(5)

(Source: Dorota Napierska et al, The Nanosilica Hazard:Another Variable Entity; Particle & Fibre Toxicology, Dec 2010)

As a result of this condensation, a macromolecular silicic acid is formed, which gradually grows into a polymeric or ‘elementary’ particle of colloidal character. This particle consists of an irregular 3-dimensional network of SiO₄ tetrahedra, each silicon atom being linked to 4 oxygen atoms and each oxygen to 2 silicons as described previously. At a certain stage of the condensation, gelation of the colloidal solutions sets in and finally a gel is produced (5). It has been known for a long time that the pH drastically influences the condensation of monosilicic acid. This is evident from Fig 1 which depicts the variation of the gelling time of a silica sol with the pH of the sol (6)

Fig 1: Gelling time of a silica sol as a function of pH. SiO2 concentration:5% (pH 1-5, pH 8-10), 2.5% (pH 5-8) (6)

(Source: BJ Linsen et al, Physical Chemical Aspects of Adsorbants and Catalysts, Chapter 5 – Porous Silica, 1970, Pg 216)

Silica (SiO₂) is only sparingly soluble in water at neutral pH. Studies have proven that at low pH, the solubility of Silica increases slightly. However, the solubility dramatically increases at pH values above 9.5. Fig 2 depicts the Stability diagram of silicate chemistry. Here, the heavy line represents the solubility of amorphous silica. In the region below to the right of this, silicate ions are in true solution. In other words, it means, the components of the solution are molecular its composition is stable with time. In very dilute very alkaline solutions, the dominant anion will be the monomer. The degree of polymerisation, thus the number type of silicate anions present, increases as the Silicon (Si) concentration is increased, as pH is decreased or as temperature is decreased. Thus, the chemistry of Silica in aqueous solutions is critically dependent upon pH, concentration and temperature (7)

Fig 2: Stability Diagram for Soluble Silicates (7)

(Source: LE Datnoff et al, Silicon in Agriculture: Chapter 4 – A primer on the aqueous chemistry of silicon, 2001, Pg 63-64)

The mechanisms of Silica precipitation are further explained in Fig 3. In water, silicates silica gel are slowly hydrolysed into Orthosilicic Acid (OSA). OSA is poorly soluble in water. It polymerizes quickly to form small particles. These small particles eventually aggregate into longer chains. These longer chains form a colloid, opalescent turbid network. This process results in formation of a soft gel which is poorly bio-available (8).

Fig 3: Mechanisms of silica precipitation (8)

(Source:Sebastian Wilhelm and Matthias Kind, Influence of pH, Temperature Sample Size on Natural Enforced Syneresis of Precipitated Silica; Polymers 2015,7, Pg 2505-2506)

Thus, to use Silicon (Si) effectively, one has to prevent the gel formation of Silicic Acid. (As Silica or Silicate, it is abundantly available in the soil and no supplements would have been necessary.) This can be done by stabilizing the reaction with help of a strong base which will keep the Silicic Acid in solution. The base cannot be inorganic like, sodium, lithium, calcium or magnesium. Most of these silicates are insoluble. Sodium and lithium are not a part of bio activity. At NICHEM, we identified this core chemistry of silicon and hence developed a unique process to stabilise the synthesised Silicic Acid. NICHEM’s Silicic Acid consists of a weak acid which is processed through strong acid cation exchange resin. This results in a cation free acidic solution containing Silicic acid. This is combined with a strong free organic base a penetrating agent to obtain highly alkaline bioavailable form of Silicon (Si) known as Orthosilicic Acid (OSA/Si(OH)₄). The mole ratio of the weak acid to free base is 1:4. The resulting solution is highly alkaline having pH 13-14. It is called as CROPSIL. There were concerns raised on the impact of the pH of CropSil on plants. However, NICHEM’s numerous trials on multiple crops did not show any phytotoxicity of CropSil on the plants at the recommended dosage of 1 to 2 ml per litre water. NICHEM also conducted multiple compatibility studies of CropSil with popularly used nutrient mixes, pest & disease control products in India. CropSil was found to be compatible with most of the market formulations. In spite of being highly alkaline, it remained stable in the foliar spray solution & performed as expected. People frequently asked us the question, “If your product is Ortho Silicic Acid, how is the pH alkaline?” The answer to this is, it is proven in literature that OSA is only stable with a strong base. Inorganic bases do not allow OSA to be assimilated, hence an organic base needs to be used. The content of NICHEM’s OSA in the formulation is 2% of the total product. Most of the remaining product is basic due to the stabilizer. Silicic acid is a very weak acid. A salt of a weak acid and a strong base makes it essentially alkaline. Thus, the pH of the formulation becomes highly alkaline. When dissolved in water at a very low concentration of 2 ml per litre with 2% Silica, Silicic acid (OSA) will be fully dissociated being a salt of weak acid and strong base. This process makes the OSA plant available. Dilution of OSA in water during use decreases alkalinity of the formulation. Small amount of alkali is easily neutralised by atmospheric carbon dioxide. Thus, neither the acid nor the alkali causes harm to the plants at molecular levels. Due to the unique stabilization technology, NICHEM OSA does not poly-condensate to form a gel. It remains stable in recommended packaging and storage conditions for more than 2 years of its recommended shelf life. In fact, we had retained few samples in our lab for observation & found that they remained stable for more than 9 years. We believe our OSA can remain stable if atmospheric carbon dioxide is excluded. Thus, it is fully bioavailable when administered to the plants.

This process of stabilization of OSA in CropSil is patented in India, USA & Canada.
To know more, please visit our website www.nichem.solutions

About NICHEM

Long-standing Specialty Chemicals player with ISO 9001:2015 certification and a history of providing specialty solutions for over 25 years. The company is headed by senior chemical industry specialists with the combined expertise of more than 100 years. With an emphasis on eco-friendly, non-toxic products, the company’s primary strength is research, development, and customization. More information on NICHEM can be found at https://nichem.solutions.

References:
1. Silicon (https://en.wikipedia.org/wiki/Silicon#Applications)
2. Silicon Dioxide (https://en.wikipedia.org/wiki/Silicon_dioxide#Natural_occurrence )
3. Silicate (https://en.wikipedia.org/wiki/Silicate)
4. Ortho-silicic Acid (https://en.wikipedia.org/wiki/Orthosilicic_acid)
5. Dorota Napierska et al, The Nanosilica Hazard:Another Variable Entity; Particle &
Fibre Toxicology, Dec 2010
6. BJ Linsen et al, Physical Chemical Aspects of Adsorbants and Catalysts, Chapter 5 –
Porous Silica, 1970, Pg 216
7. LE Datnoff et al, Silicon in Agriculture: Chapter 4 – A primer on the aqueous chemistry
of silicon, 2001, Pg 63-64
8. Sebastian Wilhelm and Matthias Kind, Influence of pH, Temperature Sample Size on
Natural Enforced Syneresis of Precipitated Silica; Polymers 2015,7, Pg 2505-2506

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