ORIGINAL ARTICLES

 

Critical reviews on stability and Photosensitizer potential of metal
ferrocyanides: a possible prebiotic mineral; part (II)

 

Revisiones críticas sobre la estabilidad y el potencial Fotosensibilizante
de los ferrocianuros metálicos: un posible mineral prebiotico; parte (II)

 

 

Brij B. Tewari^1*, D. Usmanali¹, Rawl W. Webster¹,
I. Kadir¹, Ashish K. Tiwari², N. Ramchurjee^1,3
1Department of Chemistry, Faculty of Natural Sciences,
University of Guyana, P.O. Box 101110, Georgetown, Guyana
*Corresponding author: brijtew@yahoo.com
²lndian Institute of Technology ITI- Kanpur, Kalyanpur,
Kanpur, Uttar Pradesh 208016, India
³Department of Chemistry, University of Mysore, Mysore,
Karnataka 570006, India
Received 01 01 2018 Accepted 04 20 2018 Published 04 30 2018

 

 

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Abstract

Aluminum, barium, chromium, indium, platinum, stannous and vanadium ferrocyanides were synthesized and characterized by elemental analysis and spectral studies. Stability of synthesized metal ferrocyanides were recorded in heat, light (UV, VIS), various concentrations of acids (HCl, H₂SO₄), various concentrations of bases (NaOH, KOH, NH₄OH), sea water and from faucet at room and boil temperature. Stability of synthesized metal ferrocyanides was also recorded in organic solvents (ether, acetone, ethanol, formaldehyde) at room temperature. The oxidizing and photosensitizing potential of synthesized metal ferrocyanides were tested using potassium iodide and freshly prepared starch solution. The hexacyanoferrate (II) complexes of chromium, indium and platinum were found to be possible oxidizer and photosensitizer during the course of chemical evolution on primitive earth.

Keywords: Metal ferrocyanides, Stability, Oxidizer, Photosensitizer.

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Resumen

Los ferrocianuros de aluminio, bario, cromo, indio, platino, estannoso y vanadio se sintetizaron y caracterizaron mediante analisis elemental y estudios espectrales. La estabilidad de los ferrocianuros metalicos sintetizados se registro en calor, luz (UV, VIS), diversas concentraciones de acidos (HCl, H₂SO₄), diversas concentraciones de bases (NaOH, KOH, NH₄OH), agua de mar y del grifo a temperatura ambiente y de ebullición. La estabilidad de los ferrocianuros metalicos sintetizados tambien se registro en disolventes organicos (eter, acetona, etanol, formaldehído) a temperatura ambiente. El potencial oxidante y fotosensibilizador de ferrocianuros metalicos sintetizados se analizaron usando yoduro de potasio y solución de almidón recien preparada. Los complejos de hexacianoferrato (II) de cromo, indio y platino se encontraron como posibles oxidantes y fotosensibilizadores durante el curso de la evolución química en la tierra primitiva.

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INTRODUCTION

Since the classic experiments of Miller [1], it is believed that the origin of life processes began with the formation of biomonomers such as aminoacids, pentose sugars, nucleotides, purine and pyrimidine. It has also been proposed that main steps in the origin of life must have involved the formation of important biopolymers from biomonomers in primeval seas, but it is not well established as to how the bio-monomers have concentrated from dilute aqueous solution and polymerized to give large complex molecules. It is assumed that clay minerals and large metal oxidesavailable near sea shores might have played a key role in concentration of biomonomers through adsorption and desorption processes on their surfaces. The importance of clay minerals in chemical evolution was first proposed by Bernal in 1951 and proposed that clays near the hydro sphere-litho sphere interface might have adsorbed micro-biomonomers on and between their silicate layers facilitating thus the condensation considerably, leading to the formation of biopolymers and protecting them against hydrolysis [2].

It is assumed that divalent transition metal ions which were in abundance in the primeval sea would have formed complexes with the simple molecules readily available to them [3,4]. It is therefore, assumed that transition metal ions could easily have formed a number of soluble and insoluble complexes with the abundant CN^--, in the primeval sea. The insoluble cyanometal complexes thus formed could have settled at the bottom of the sea or at the seashore and might have catalyzed a number of reactions like condensation-oligomerization, oxidation and interaction reactions on their surfaces. The existence of ferriferrocyanide and metal ferrocyanides on the primitive earth has been reported by Arrhenius [5]. Activity of metal ferrocyanides as adsorbents [6, 7], ion-exchangers [8-10] and photosensitizes [11-13] is well established. The lower valent metal ions substituted in potassium ferrocyanide provide a good adsorption site that could have played an important role in chemical evolution and prebiotic chemistry. The insoluble double metal ferrocyanides, either settled at the bottom of the sea or on the seashore, on coming in contact with biomolecules could have acted as active surfaces for concentrating them.

A search for literature indicated some reports available on synthesis of metal hexacyanoferrate (II) complexes and very few report available on stability and photosensitizing activity of metal hexacyanoferrate (II). In view of this, attempt was made to study stability and photosensitizing activity of the aluminum, barium, chromium, indium, platinum, stannous and vanadium hexacyanoferrate (II) complexes. In addition, the present work describes a critical review on stability and photosensitizing potential of aluminum, barium, chromium, indium, platinum, stannous and chromium hexacyanoferrate (II) complexes.

 

RESULTS AND DISCUSSION

Elemental analysis of metal hexacyanoferrate (II) complexes

The percentage compositions of metals in metal hexacyanoferrate (II) complexes are given in Table 1. The percentage of metals (aluminum, barium, chromium, indium, platinum, stannous and vanadium) are found higher in comparison to iron in all complexes of hexacyanoferrate (II) studied. It is also clear from Table 1 that percentage of hydrogen is lowest in comparison to other elements in each metal ferrocyanides studied. The greater the percentage of hydrogen more the water molecules are expected to attached to metal hexacyanoferrate (II) complexes.

Spectral studies of metal hexacyanoferrate (II) complexes

The infrared special data of hexacyanoferrate (II) complexes are given in Table 2. It is observed from Table 2 that water molecules/OH groups and metal-nitrogen band shows highest and lowest absorption frequencies, respectively. The HOH bending C=N stretching and Fe-C stretching frequencies are observed around 1600 cm^-1, 2000 cm^-1 and 600cm^-1, respectively.

Effect of heat on the stability of metal hexacyanoferrate (II) complexes

It is clear from Table 3 that hexacyanoferrate (II) complexes of indium and platinum are stable at 100°C, while those of aluminum, barium, stannous and chromium platinum are unstable at 100°C.

Stability of metal hexacyanoferrate (II) complexes in various concentrations of acids at room temperature and at boiling temperature

It is observed from Table 4 that hexacyanoferrate (II) complexes of aluminum are slightly soluble in various concentrations of hydrochloric acid at room temperature with change in colour. Hexacyanoferrate (II) complexes of barium, stannous and vanadium are insoluble in various concentrations of hydrochloric acid with change in colour at room temperature. Hexacyanoferrate (II) complexes of chromium, indium and platinum are soluble at high concentration of hydrochloric acid (2.0 M), while slightly soluble or insoluble at low concentrations of hydrochloric acid with change in colour at room temperature.

Table 5 shows that hexacyanoferrate (II) complexes of aluminum, stannous, vanadium and chromium are partially soluble in various concentrations of boiling hydrochloric acid with change in colour, while those of indium are soluble at various concentrations of boiling hydrochloric acid and those of barium and platinum are insoluble in various concentrations of boiling hydrochloric acid.

It is observed in Table 6 that hexacyanoferrate (II) complexes of aluminum, stannous, chromium and indium are partially soluble in various concentrations of sulphuric acid at room temperature with change in colour,those of barium, platinum and vanadium are insoluble in various concentrations of sulphuric acid with change in colour at room temperature.

It is clear from Table 7 that hexacyanoferrate (II) complexes of aluminum, stannous, indium and chromium are partially soluble in various concentrations of sulphuric acid at boiled temperature with change in colour, those of barium, platinum and vanadium are insoluble in various concentrations of boiling sulphuric acid with change in colour. The colour change of metal hexacyanoferrate (II) complexes in various acids is mainly due to electronic transition within molecules of metal hexacyanoferrate (II) complex.

Stability of metal hexacyanoferrate (II) complexes in various concentrations of bases at room temperature and at boiled temperature

It is observed from Table 8 that hexacyanoferrate (II) complexes of aluminum, chromium and indium are partially soluble in various concentrations of sodium hydroxide with colour change at room temperature, those of barium, stannous, vanadium and platinum are insoluble in various concentrations of sodium hydroxide with colour change at room temperature.

It is clear from Table 9 that hexacyanoferrate (II) complexes of aluminum and barium are partially soluble in various concentrations of sodium hydroxide with colour change at boiling point, while those of chromium and indium are partially soluble at high concentration of sodium hydroxide and insoluble at low concentration of sodium hydroxide, respectively at boiling temperature, those of stannous, vanadium and platinum are in insoluble in various concentrations of sodium hydroxide at boiling temperature.

Table 10 shows that hexacyanoferrate (II) complexes of aluminum is partially soluble in various concentrations of potassium hydroxide with colour change at room temperature, while those of chromium and indium are partially soluble at high concentration of potassium hydroxide and insoluble at low concentration of potassium hydroxide, respectively at room temperature, those of barium, stannous, vanadium and platinum are insoluble in various concentrations of potassium hydroxide at room temperature.

It is evident from Table 11 that aluminum hexacyanoferrate (II) complexes are partially soluble in various concentrations of potassium hydroxide at boiling temperature, while those of chromium and indium are partially soluble at high concentration of potassium hydroxide and insoluble at low concentration of potassium hydroxide, respectively at boiling temperature, those of barium, stannous, vanadium and platinum are insoluble in various concentrations of potassium hydroxide at boiling temperature.

From Table 12 we observe that aluminum hexacyanoferrate (II) complexes are partially soluble in various concentrations of ammonium hydroxide with colour changes at room temperature, while those of chromium and indium are partially soluble at high concentration of ammonium hydroxide and insoluble at low concentration, respectively at room temperature, those of barium, stannous, vanadium and platinum are insoluble in various concentrations of ammonium hydroxide at room temperature.

Table 13 shows that aluminum hexacyanoferrate (II) complexes are partially soluble in various concentrations of ammonium hydroxide with colour changes at boiling temperature, while those of chromium and indium are partially soluble at high concentration of ammonium hydroxide and insoluble at low concentration, respectively at when boiling, those of barium, stannous, vanadium and platinum are insoluble in various concentrations of ammonium hydroxide at boiling temperature.

The colour changes of metal hexacyanoferrate (II) complexes in various bases are mainly due to electronic transition within metal hexacyanoferrate (II) complex molecules.

Stability of metal hexacyanoferrate (II) complexes in various concentrations in various solvents at room temperature

It is observed from Table 14 that complexes of aluminum hexacyanoferrate (II) and those of barium, chromium, indium and platinum are insoluble in various organic solvents; (ether, acetone, ethanol, and formaldehyde) at room temperature, those of stannous and vanadium are insoluble in ether, but they are soluble in acetone, ethanol and formaldehyde.

Stability of metal hexacyanoferrate (II) complexes in tap water and sea water at room and boiling temperature

It is clear from Table 15 that metal hexacyanoferrate (II) complexes of aluminum, stannous, chromium, indium, vanadium and platinum are found to be insoluble and stable in tap water and sea water at room and boiling temperature. Barium ferrocyanide is found to be partially soluble in tap water and sea water at room and boiling temperatures.

Effects of visible light on the stability of metal hexacyanoferrate (II) complexes

It is observed from Table 16 that hexacyanoferrate (II) complexes of platinum, stannous and vanadium are stable to visible light until 48 hours of irradiations. It is also observed from Table 16 that those of aluminum, stannous, chromium and indium are unstable in visible light at 12 hours of irradiations.

Effects of ultraviolet light on the stability of metal hexacyanoferrate (II) complexes

From Table 17 we see that complexes of hexacyanoferrate (II) of stannous, vanadium and platinum are stable to Ultraviolet light until 48 hours of irradiations. Also that those of aluminum, barium, chromium and indium are stable in ultraviolet light at 12 hours of irradiations.

Test on oxidizing and photosensitizing activity of metal hexacyanoferrate (II) complexes

Test on oxidizing and photosensitizing activity of metal hexacyanoferrate (II) complexes in potassium iodine and freshly prepared starch solution indicated platinum, chromium and indium hexacyanoferrate (II) complexes as possible oxidizer and photosensitizer during the course of chemical evolution on primitive earth.

 

CONCLUDING REMARKS

The following conclusions can be drawn from the present study:

1. The stability of hexacyanoferrate (II) complexes of aluminum, platinum, barium, chromium, stannous and vanadiumwere found to be affected by heat and light (visible, ultraviolet).

2. Complexes of hexacyanoferrate (II) of barium were found to be insoluble in various concentrations of hydrochloric acid at room temperature and temperature of boiling.

3. Barium, platinum and vanadium ferrocyanides are found to be insoluble in various concentration of sulphuric acid at room temperature and temperature of boiling.

4. Complexes of hexacyanoferrate (II) of barium, stannous, vanadium and platinum were found to be insoluble in various concentrations of potassium hydroxide and ammonium hydroxide at room temperature and temperature of boiling.

5. Chromium, indium, platinum, stannous, aluminum and vanadium ferrocyanides were found to be insoluble and stable intap water and sea water at room temperature and temperature of boiling.

6. Complexes of hexacyanoferrate (II) of platinum, stannous and vanadium were found to be stable to UV and visible light after 48 hours of irradiations.

7. Chromium, indium and platinum ferrocyanides were found to be possible oxidizer and photosensitizer during the course of chemical evolution on primitive earth.

8. Complexes of hexacyanoferrate (II) of aluminum, barium, chromium, indium and platinum were found to be insoluble in ether, acetone, ethanol and formaldehyde at room temperature.

9. It is also concluded from present study that hexacyanoferrate (II) complexes are insoluble and stable during the course of chemical evolution on primitive earth and played a significant role in condensation of precursors of early life in primeval seas.

 

EXPERIMENTAL

Chemicals

Potassium ferrocyanide, aluminum chloride, barium chloride, sodium vanadate, stannouschloride, chromic chloride, indium chloride, platinum chloride were obtained from BHD Poole, England.

All chemicals used without further purification. Solutions were prepared in doubly distilled water.

Synthesis of metal ferrocyanides

Aluminium, barium, indium and platinum ferrocyanides were prepared by adding metal chlorides (500 mL, 0.1 M) and potassium ferrocyanide (167 mL, 0.1 M) solutions with constant stirring [14].

The reaction mixture was heated on water bath for 3 hours and kept as such at room temperature for 24 hours. The precipitate was filtered under vacuum, washed several times with distilled water and dried in an air oven at 60°C. the driest product was grounded and sieved to 125 μm particles size.

Stannous ferrocyanide was prepared by mixing solutions of 0.25 M potassium ferrocyanide and stannous chloride in the ratio (2:1) with constant stirring [15]. The precipitate was cured at room temperature for 24 hours. The precipitate was filtered under vacuum, washed several times with distilled water and was dried in an air oven at 60°C. The driest product was grounded and sieved to 125 μm particles size.

Vanadium ferrocyanide complex was isolated by adding (10 mL, 1.0 M) HCl to mixture containing sodium vanadate (500 mL , 0.1 M) and potassium ferrocyanide (500 mL : 0.1 M) solution stirred constantly [16]. Reaction mixture to be heated on boiling water bath for 3.5 hours then allowed to cool at room temperature overnight. The precipitate formed was filtered and dried at 60°C in an air oven. The driest product was grounded and sieved to 125 μm particles size.

Chromium ferrocyanide was prepared under optimum experimental conditions according to the procedure of Malik et al. [17].

Characterization of metal ferrocyanides

Aluminium, barium, chromium, indium, platinum, stannous and vanadium ferrocyanides are light blue, pale greenish white, bright green, light blue, dark blue, blue and dark green colours, respectively. These metal hexacyanoferrate (II) complexes are amorphous insoluble solid and showed no X-ray pattern. The metal hexacyanoferrate (II) complexes were characterized on the basis of elemental analysis and spectral studies. The percentage compositions of metals were determined by IL-751 atomic absorption spectrophotometer [18]. Carbon hydrogen and nitrogen analysis were carried out by CEST-18, CHN analyzer (Table 1).

Infrared spectra of metal hexacyanoferrate (II) complexes were recorded in KBr disc on Beckman IR-20 spectrophotometer. All seven metal hexacyanoferrate (II) complexes show a broad peak at 3040-3700 cm^-1 is characteristic of water molecule and OH groups. A peak at around 1510-1660 cm^-1 is due to HOH bending [19], two sharp peaks, one at around 2000 cm^-1 and the other at around 600 cm^-1 in all seven spectra of complexes are characteristics frequencies of cyanide and Fe-C stretching, respectively [20].

Another sharp band at 489-500 cm^--1 in all seven metal hexacyanoferrate (II) complexes probably shows the presence of metal-nitrogen bond thus indicating a certain degree of polymerization in the products [21, 22] (Table 2).

Stability study on metal ferrocyanides

Effect of heat on the stability of metal hexacyanoferrate (II) complexes

A20 mg of each metal hexacyanoferrate (II) complexes was placed in a petri dishes. Petri dishes were dried in the air oven for 6 hours at 100°C. This process was repeated at 150°C, 200°C, 250°C to demonstrate the effect of heat on the various metal hexacyanoferrate (II) complexes. The colour of metal hexacyanoferrate (II) complexes at various temperatures was observed. The effect of heat on the stability of metal hexacyanoferrate (II) complexes is shown in Table 3.

Stability of metal hexacyanoferrate (II) complexes in various concentrations of acids at room temperature and at boiling temperature

The metal hexacyanoferrate (II) complexes (20 mg) were placed in the test tubes containing 10 mL of each 2.0 M, 1.0 M, 0.5 M, 0.2 M and 0.1 M acids (HCl, H₂SO₄).The mixture was agitated for 20 minutes at room temperature and observation for any change in colour of metal hexacyanoferrate (II) complexes were recorded (Tables 4, 6). The same reaction mixture boiled on a Bunsen flame for 20 minutes and any change in colour of metal hexacyanoferrate (II) complexes were recorded (Tables 5, 7). This process was repeated for each metal hexacyanoferrate (II) complexes. The colour change for metal hexacyanoferrate (II) complexes was recorded.

Stability of metal hexacyanoferrate (IT) complexes in various concentrations of bases at room temperature and at boiling temperature

Metal hexacyanoferrate (II) complexes (20 mg) were placed in the test tubes containing 10 mL of 2.0 M, 1.0 M, 0.5 M, 0.2 M and 0.1 M bases ( NaOH , KOH, NH₄OH).The mixture was agitated for 20 minutes at room temperature and observation for any change in colour of metal hexacyanoferrate (II) complexes were recorded (Tables 8, 10, 12). The same reaction mixture boiled on a Bunsen flame for 20 minutes and any change in colour of metal hexacyanoferrate (II) complexes were recorded (Tables 9, 11, 13). This process was repeated for each metal hexacyanoferrate (II) complexes. The colour change for metal hexacyanoferrate (II) complexes was recorded.

Stability of metal hexacyanoferrate (II) complexes in various organic solvents at room temperature

Metal hexacyanoferrate (II) complexes (20 mg) were placed in test tubes containing 10 mL each of ether, acetone, ethanol, formaldehyde. The mixture was agitated for 20 minutes and kept at room temperature for 1 hour then any change in colour of metal hexacyanoferrate (II) complexes were recorded (Table 14).

Stability of metal hexacyanoferrate (II) complexes in tap water and sea water at room temperature and at boiling temperature

The metal hexacyanoferrate (II) complexes (20 mg) were placed in the test tubes containing tapwater and sea water. The mixture was agitated for 20 minutes and observation for any change in colour of metal hexacyanoferrate (II) complexes were recorded (Table 15). The same reaction mixture boiled on Bunsen flame for 20 minutes and any change in colour of metal hexacyanoferrate (II) complexes were recorded (Table 15).

Effect of light (UV/Vis) on the stability of metal hexacyanoferrate (II) complexes

A 20 mg of each metal hexacyanoferrate (II) complexes were placed in a dry petri dish and the original colour was recorded. A 250W visible lamp was kept vertically above the sample at a distance of 28 cm. The observations for any change in colour of metal hexacyanoferrate (II) complexes were recorded at 12, 24, 36 and 48 hours (Table 16). The same experiment was repeated using a long wave (300 - 380 nm) UV lamp. The observations of any change in colour of metal hexacyanoferrate (II) complexes were recorded (Table 17).

Test on oxidizing and photosensitizing activity of metal ferrocyanides

The oxidizing and photosensitizing potential of metal ferrocyanides were compared by potassium iodide and freshly prepared starch solution. Oxidation of iodide to iodine in presence of starch gives blue colour. One drop of freshly prepared 2.0% starch solution was added into test tubes (length =10 cm; internal diameter = 1.30 cm) containing 10 ml of 0.1M potassium iodide solution. A 25 mg of hexacyanoferrate (II) complexes were added into each test tube and agitated, observation for any decolorization of blue color and potassium iodide and starch solution were recorded. The same experiment was repeated using a 250W visible lamp and a long wave UV lamp, which was kept vertically above the test tubes at a distance of 15.0 cm. Photosensitizer will decolorize the blue color of potassium iodide and starch solution in the presence of visible and ultraviolet light. The oxidizers will decolorize the blue color of potassium iodide and starch solution in the absence of light.

 

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