The Crystal Growth Has Given New Dimension Biology Essay

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Crystal growth has given new dimension to fulfill the need of modern technology. Natural crystals are not available in adequate quantities and also having imperfection and low purity. Synthetic crystals, prepared in the laboratory can replace these natural crystals. These synthetic crystals are pure having less imperfection and can be grown of desirable size.

With the help of gel method a lot of work has been done [35]. Afterwards several investigators [73-85] have used gel method to grow various types of crystals and characterized it, also due to its simplicity of the process it can be successfully used to grow the crystals at room temperature with good control over nucleation [14-19, 86-89].

A lot of works have been devoted in recent years to the preparation of oxalates of metallic ion or mixture of metallic ions. Crystals of calcium oxalates were grown by gel method [47-48]. Gadolinium samarium oxalates were grown using gel method [50]. Barium oxalate crystals were grown by gel technique [6, 7, 49, 90], sodium oxalate crystals [8], cerium oxalate crystals [20], lanthanum neodymium oxalate single crystals [21], cadmium oxalate single crystals [51] were grown using gel method. For getting full information about growing crystals, they are characterized by various techniques.

In this work, the grown strontium oxalate crystals were characterized by

X-ray diffraction (XRD)

Fourier Transform Infrared Spectroscopy (FT-IR)

Thermal analysis

Thermal Gravimetric Analysis (TGA)

Differential Thermal Analysis (DTA)

Differential Scanning Calorimetry (DSC)

Chemical Analysis

i) Gravimetric

ii) Volumetric

5) Energy Dispersive Analysis by X-ray (EDAX)

6) Scanning Electron Microscope (SEM)

The present chapter describes growth of crystals of undoped strontium oxalate by establishing optimum conditions and their characterization.

3.2 Gel technique

The growth of crystal in the gel method is based on the diffusion of reactant in gel. Following two techniques for the process of diffusion in gel media were employed.

Single diffusion technique.

Double diffusion technique.

These techniques are discussed in chapter I and shown in fig. 1.1 and fig. 1.2.

Both techniques were used in the present work but single diffusion technique was found most suitable for the growth of good quality crystals in present work. Requirements and procedure of both techniques are discussed below.

3.3 Apparatus used

Corning glass test tube

Corning glass U shape tube

Magnetic stirrer

Digital pH meter

Burette and pipette

Beaker, measuring cylinder

Specific gravity bottle, Electronic Balance

Gas for heating, funnel, stand, glass rod, filter paper, wire gauge, cotton plug etc.

3.4 Chemical used

All are A.R. Grade as follows

Sodium Meta silicate (Na2SiO3, 5H2O)

Strontium Chloride (SrCl2, 6H2O)

Strontium Nitrate [Sr(NO3)2 ]

Oxalic acid (H2C2O4)

Agar-agar powder

Double Distilled Water

3.5 Experimental procedure

The chemical reaction method is used to grow undoped strontium oxalate crystals. The technique involves growing of crystals by allowing the reaction of two solutions of soluble salts by diffusion through a gel with subsequent nucleation and the crystal growth, which continuous due to the gradual precipitation of insoluble product.

3.5.1 Chemical reaction

Strontium chloride reacts with oxalic acid and it forms strontium oxalate,

SrCl2 + H2C2O4 ï‚® SrC2O4 + 2HCl

Strontium Nitrate reacts with oxalic acid and it forms strontium oxalate,

Sr(NO3)2 + H2C2O4 ï‚® SrC2O4 + 2HNO3

3.5.2 Preparation of Silica gel

I) Preparation of sodium metasilicate solution

Sodium metasilicate solutions of different specific gravity were prepared by adding proper amount of sodium metasilicate in double distilled water. This solution was continuously stirred for half an hour by using magnetic stirrer and filtered directly into the airtight coloured bottle.

II) Preparation of silica gel from sodium metasilicate

Silica gel was prepared by mixing solution of sodium metasilicate and oxalic acid which is one reactant of known concentration and then continuously stirred for ten minutes using magnetic stirrer. To get good proper of gel various trials were made.

3.5.3 Single diffusion

In this procedure 12 ml of 0.5 M oxalic acid solution was taken in a small beaker, in which sodium meta silicate solution of 1.04 gm/cc was added drop by drop from burette and continuously stirring it by using magnetic stirrer for maintaining the pH in the range 3 to 4.5 of given solution and noted burette reading. Continuous Stirring is done to avoid the excessive local ion concentration, which may otherwise cause premature gelling and make the final medium inhomogeneous and turbid. The solution was continuously stirred till the pH of the resulting solution has reached a specific value. The crystallizing vessel were used essentially consist of standard glass tube of 2.5 cm inner diameter and 20 cm in length. Gelling mixture, after noting pH was allowed to set in glass test tubes. These tubes were hermitically sealed to prevent evaporation and contamination of the exposed surface by dust particles of atmosphere or atmospheric impurities and were kept undisturbed.

The mixture appeared to be quite transparent initially. However, with the lapse of time, its colour changed and became milky white when the gel was completely set. The gel was usually found to set 25 to 30 days, depending on the environmental temperature. After ensuring firm gel setting, it was kept for aging for 3 to 4 days. The aging of the gel reduces the diameter of the capillaries in gel so that the speed of the reaction is automatically controlled. After ensuring proper, firm gelation, supernatant strontium chloride solution was introduced carefully over the set gel with the help of pipette, the solution being allowed to fall along the wall of the test tube to prevent the gel surface from breaking or cracking. Again these tubes were hermitically sealed to prevent evaporation and contamination of the exposed surface by dust particles of atmosphere or atmospheric impurities and were kept undisturbed. Tubes were again kept idle for further processes to occur.

The supernatant solution started slowly diffused into the gel medium. Nucleation was observed after 5 to 6 days and crystals started to grow. In silica gel very small crystals (micro crystals) were observed inside the gel medium. Though different concentration of reactant and supernatant were tried, but very small sized crystals were observed as shown in fig. 3.1.

3.6 Results and discussion

A series of experiments were conducted by varying the concentrations of inner and other reactants as well as the pH of the medium, gel density, gel setting time and gel aging time. In single diffusion, after a few days better quality, well shining, transparent micro crystals were observed as shown in fig. 3.1.

3.6.1 Optimum conditions

i) Density of sodium meta silicate solution -- 1.04 gm/cc

ii) Volume of sodium meta silicate solution -- 20.2 ml

iii) Volume of 0.5M oxalic acid solution -- 12 ml

iv) pH of gel -- 4.0

v) Concentration of supernatant, strontium chloride -- 1 M

vi) Room Temperature -- 30oC

vii) Gel setting period -- 25 days

viii) Gel aging period -- 4 days

ix) Growth period -- 70 days

x) Quality of crystals -- Micro crystals

3.6.2 Effect of various parameters on crystal growth

I) Different densities of silica gel

Different densities of Silica gel from 1.02 to 1.06 gm/cc were tried. It was observed that, higher densities gel set more rapidly than the lower densities. Also the transparency of gel decreased as the density increased. The good results were obtained with the gel density 1.04 gm/cc. Low density gel could not set in required period and found to be unsuitable for diffusion of reactants. Pore size of gel was reduced with denser gel, thereby decreased nucleation density; this is well agreed with the conclusion [35].

II) Concentration of reactant

Different concentrations of reactant oxalic acid 0.3 M to 1 M were used. The good result was obtained with 0.5 M concentration of reactant, oxalic acid which is used for setting of gel as well as one of the reacting component. Different concentrations of strontium chloride or strontium nitrate were also tried and the optimum value of the same are mentioned.

III) pH of gel

The effect of pH of silica gel was studied using different ratios of sodium meta silicate and oxalic acid in the mixture of silica gel. As the relative quantity of sodium meta silicate was increased in the mixture, pH values were increased and leaded to poor formation of crystals. However at low pH values the crystals were not formed in the gel. pH values 3 to 4.2 were tried, it was observed that below 3.5 pH the gel was not set completely while above 4 pH the gel was set but the transparency of the gel was decreased. Therefore pH was kept 3.5 to 4. It was observed that good results were obtained at 4 pH. It had been found that when pH was low in the range of 3 to 4 it took long time about four weeks to set the gel completely and it took more than one month to add the supernatant. With this long time of setting and aging the gel was got shrinked and the outer surface of the gel in the test tube which works as an interface for the diffusion of supernatant becomes completely uneven and the height of the gel column is much reduced because of gel shrinking. To avoid this difficulty and to get sufficient height of gel column and to get better quality crystals below 3cm depth of the gel column, experiment repeated with initial more height of the gel column that means slightly more than half of the height of the test tube. But this process again leads to growth of very small crystals.

Therefore to get better quality crystals another media of gel that means agar- agar gel was employed to grow the crystals. Barium oxalate crystals were grown in agar-agar gel to get better quality crystals [6, 7].

3.7 Preparation of Agar-agar Gel

Agar-agar gel was prepared by dissolving 1 to 3 gm of agar-agar powder into 100 ml hot double distilled water to get homogenous solution. This solution was then poured into the test tubes with one reactant and kept for setting gel by covering the mouth of the test tube with cotton plug. After setting gel, it was kept for aging.

3.7.1 Experimental Procedure

1 to 5 gm of agar-agar powder was dissolved in to hot double distilled water mixed with 0.5 M to 1 M oxalic acid solution. The crystallizing vessel were used essentially consist of standard glass tube of 2.5 cm inner diameter and 20 cm in length. Gelling mixture poured in glass test tubes. These tubes were hermitically sealed to prevent evaporation and contamination of the exposed surface by dust particles of atmosphere or atmospheric impurities and were kept undisturbed. The gel was usually found to set 3 to 4 days, depending on the environmental temperature. It was observed that the mixture in a glass tube was initially transparent and slowly turned light yellowish. The water slowly evaporated and gel was completely set. After ensuring firm gel setting, it was kept for aging for 3 to 4 days. After that 0.5 M to 1 M solution of strontium chloride or strontium nitrate was added as a supernatant over the set get. Nucleation was observed after 5 to 6 days and crystals started to grow. It was observed that for 1 gm, 2 gm agar-agar powder and with 2 ml, 3 ml solution of 0.5 M to 1M oxalic acid, the gel did not set completely within specific time and then by adding supernatant proper reaction could not take place and no formation of crystals.

Solutions prepared with 3 gm to 5 gm of agar-agar powder and with 2.5 ml to 3 ml solution of 0.5 M to 1 M oxalic acid, the gel could set completely but after adding supernatant the rate of diffusion was slow. Due to slow rate of diffusion nucleation formation was found to be slow and therefore very few nuclei were nucleated. Therefore to avoid this problem the reactant and supernatant were interchanged.

By interchanging the reactant and supernatant in the above single diffusion method good expected results were obtained. Fibrous growth, crowded and opaque, tiny but lustrous and isolated crystals were obtained in the gel, as shown in fig. 3.2.

3.8 Results and discussion

A series of experiments were conducted by varying the concentrations of inner and other reactants as well as gel setting time and gel aging time. In single diffusion, after a few days' better quality, fibrous growth, crowded and opaque, tiny but lustrous and isolated crystals were obtained in the gel, as shown in fig. 3.2.

3.8.1 Optimum conditions

i) Concentration of agar Gel -- 3 %

ii) Concentration of reactant, strontium chloride -- 1 M

iii) Concentration of supernatant, oxalic acid -- 1 M

iv) Room temperature -- 30oC

v) Gel setting period -- 4 days

vi) Gel aging period -- 2 days

vii) Growth period -- 30-45 days

ix) Quality of crystals -- 1mm x 1mm

(nearly) isolated

Crystal

To get better quality crystals another technique that means double diffusion

with agar- agar gel was employed to grow the crystals.

3.9 Double diffusion

3.9.1 Experimental procedure

3% of agar-agar powder was added in hot double distilled water with continuous stirring to get homogeneous mixture and then this solution was poured in corning glass U - tube of diameter 2.5 cm up to appropriate height. These tubes were hermitically sealed to prevent evaporation and contamination of the exposed surface by dust particles of atmosphere or atmospheric impurities and were kept undisturbed for gel setting and aging. The gel was usually found to set 3 to 4 days, depending on the environmental temperature. It was observed that the mixture in a glass tube was initially transparent and slowly turned light yellowish. The water slowly evaporated and gel was completely set. After ensuring firm gel setting, it was kept for aging for 2 to 3 days. After aging, strontium chloride and oxalic acid were added into two limbs of the U-tube simultaneously of the same height. After 4 to 6 days very few shining very few micro crystals were nucleated near to the oxalic acid side and then the nucleation of the crystals started spreading from oxalic acid side to strontium chloride side in the U-tube. It had been also observed that as the crystal nucleated towards the strontium chloride side, the size of the crystals had been slightly increased.

As the crystal initially nucleated at the oxalic acid side rather than the centre of the U-tube, this indicates that the rate of diffusion of oxalic acid in the agar-agar gel was less than the strontium chloride. Therefore very few micro crystals were obtained in the gel as shown in fig. 3.3.

3.10 Results and discussion

A series of experiments were conducted by varying the concentrations of inner and other reactants as well as gel setting time and gel aging time. In double diffusion, after a few days' very few micro crystals were obtained in the gel as shown in fig. 3.3.

3.10.1 Optimum Conditions

i) Concentration of agar-agar gel -- 3 %

ii) Concentration of reactant I, strontium chloride -- 1 M

iii) Concentration of reactant II, oxalic acid -- 1 M

iv) Room temperature -- 30oC

v) Gel setting period -- 4 days

vi) Gel aging period -- 2 days

vii) Growth period -- 30 days

ix) Quality of crystals -- Very few micro crystals

3.11 Effect of various parameters on crystal growth

Different parameters such as nucleation, concentration of reactant, aging period of gel etc. have considerable effect on the growth rate of crystals. These parameters are discussed as follows.

I) Nucleation

In single diffusion it was observed that the rate of nucleation depends upon concentration of supernatant solution. It was found that at dilute solution, the rate of nucleation was slow and size of it was also small. Under these conditions very few nuclei were nucleated. However, on increasing the concentration of supernatant, the rate of nucleation and growth of nucleation was also large. This indicated variation of number of nuclei formed with respect to change in concentration of supernatant.

In single diffusion, at 1M concentration of supernatant, tiny, lustrous and shining isolated crystals were grown in the agar-agar gel.

In double diffusion it was observed that by varying the concentration of both the reactants, the rate of diffusion through the agar gel was changed. As a result rate of nucleation and size of crystals was also affected.

II) Effect of concentration of agar gel

It was observed that the transparency of the gel was decreased as the concentration of gel was increased and colour was changed to yellowish. It was also observed that

i) very small size and few crystals were obtained using strontium chloride as a supernatant while

ii) good quality, large size and more crystals were obtained in the same concentration and same conditions using oxalic acid as a supernatant, which suggested oxalic acid to be used as supernatant.

III) Effect of aging period in agar gel

More aging period reduces the number of growing nuclei. This might be due to reduction of cell size and consequently the rate of diffusion of supernatant into gel. Good quality and large size crystals were obtained at moderate aging period of gel about a week.

3.12 Characterization

3.12.1 X-ray Diffraction (XRD)

X-ray diffractogram is useful in the analysis of crystal structure. Cell parameters,'d' values, unit cell, volume and lattice system etc. can be evaluated by using x-ray diffractogram.

X-ray diffractogram of gel grown undoped strontium oxalate crystals were recorded using powder rotated diffraction on 'Miniflex Rigaku' X-ray diffractometer at Department of Physical Sciences, North Maharashtra University, Jalgaon, (M. S.).

CuK radiation ( = 1.54051 Ao) was used as a target material. The sample rotated in the range 5o to 80o (2). The recorded X-ray diffractogram of undoped strontium oxalate crystals by hydro silica gel is as shown in fig. 3.4 and by agar-agar gel is as shown in fig. 3.5. From these diffractograms, intensity ratio I/Io,'d' spacing, corresponding miller indices (h k l) were computed as shown in table- 3.1 and table-3.2.

These calculated 'd' values are matched with the reported ones. (h k l), (a b c) and system calculated by the computer program POWD (Integrative Powder Diffraction and Indexing program). These unit cell parameters and system are shown in table- 3.1.1 and table- 3.2.1. These parameters satisfy the conditions for triclinic system, i.e. the grown crystals have triclinic structure with a ≠ b ≠ c and  ≠  ≠.

Table - 3.1 XRD data of undoped strontium oxalate crystals by hydro-silica gel

Peak

No.

2

Deg.

FWHM

d- value

Intensity

I/Io

Indices

h k l

1

28.050

0.118

3.1783

485

54

0 1 0

2

29.600

0.176

3.0153

128

15

1 1 0

3

37.750

0.059

2.3810

190

21

0 0 1

4

37.800

0.059

2.3779

265

30

-1 0 1

5

37.900

0.059

2.3719

207

23

0 -1 1

6

38.750

0.059

2.3218

108

12

-1 -1 1

8

38.950

0.059

2.3103

101

12

4 1 0

10

42.650

0.059

2.1181

183

21

-4 0 1

12

43.150

0.059

2.0947

115

13

5 1 0

17

45.250

0.059

2.0022

557

62

5 -1 1

19

49.050

0.059

1.8556

108

12

-5 -1 1

Table - 3.1.1 Unit cell parameters and system of undoped strontium oxalate crystals by hydro-silica gel

Parameters

Strontium Oxalate

System

Triclinic

a

16.7505 Ao

b

3.4772 Ao

c

2.5623 Ao



111.393o



90.441o



100.384o

V

136.41 Ao3

These parameters satisfy the conditions for Triclinic System since a  b  c and     .

Table - 3.2 XRD data of undoped strontium oxalate crystals by agar-agar gel

Peak

No.

2

Deg.

FWHM

d- value

Intensity

I/Io

Indices

h k l

1

31.60

0.176

2.8289

107

24

0 1 0

2

36.35

0.294

2.4694

233

51

1 0 0

3

37.85

0.176

2.3749

352

77

0 1 1

4

41.10

0.176

2.1943

136

30

-1 1 0

5

41.70

0.294

2.1641

103

23

1 0 1

6

42.65

0.176

2.1181

112

25

-1 1 1

7

47.65

0.235

1.9068

460

100

-1 -1 2

8

49.50

0.176

1.8398

147

32

-1 -1 5

9

51.30

0.176

1.7794

135

30

1 -1 3

10

57.60

0.176

1.5989

121

27

0 -2 2

11

61.00

0.176

1.5176

102

23

0 -2 1

12

63.00

0.176

1.4742

132

29

1 1 1

13

69.40

0.235

1.3530

112

25

-2 0 3

14

76.20

0.176

1.2483

161

36

-1 -2 2

15

77.70

0.176

1.2279

105

23

2 -1 1

16

79.25

0.235

1.2078

117

26

-2 -1 4

Table - 3.2.1 Unit cell parameters and system of undoped strontium oxalate crystals by agar-agar gel

Parameters

Strontium Oxalate

System

Triclinic

a

2.7219 Ao

b

3.2973 Ao

c

11.0085 Ao



116.524o



108.889o



95.492o

V

80.20 Ao3

3.12.2 Fourier Transform Infrared Spectroscopy (FT-IR)

Nearly all-academic and industrial laboratories make use of infrared spectroscopy as a bench tool for structural analysis. Although the IR spectra is the characteristics of the entire molecule, it turns out that certain groups of atoms give rise to bands at or near the same frequency regardless of the structure of the rest of the molecule. Mostly IR spectra are used in conjunction with other spectral data to determine molecular structure of the sample under study.

In the present work IR spectra of Strontium Oxalate sample was recorded on JASCO instrument, Model 460 Plus at AISSMS College of Pharmacy, Pune, (M. S.) and University Department of Chemical Technology, North Maharashtra University, Jalgaon, (M. S.) on SHIMADZU FT-IR 8400 spectrophotometer.

The IR spectra of these gel grown Strontium Oxalate crystals were obtained in the wave number range 400-4000 cm-1 for KBr line, accumulation 70, resolution 4 cm-1, gain-auto (128), scanning speed -auto (2mm/sec.), apodization - cosine. The IR spectra obtained for the grown crystals in hydro silica gel is shown in fig. 3.6 and in agar- agar gel is shown in fig. 3.7.

IR studies on various oxalates have been carried out by several investigators [6-8, 20, 21, 47-51, 90].

IR spectra for undoped strontium oxalate crystals by hydro silica gel

In the IR spectrum of undoped strontium oxalate crystals, the absorption at 3428.81 cm-1 is due to O-H stretching mode and water stretch. Few bands on the small wave number side of these bands represent overtones and combination tones occurring at smaller wave number.

A strong asymmetrical band at 1788.65 cm-1 is attributed to the C=O stretch of carbonyl group.

The peak 1351.86 cm-1 is due to of plane bending O-H stretch, which established the presence of water molecule associated with crystal.

Around the peak 1071.26 cm-1 corresponds to the asymmetric stretching mode of C-O bond.

The sharp IR peak at 719.318 cm-1 may be attributed to presence of metal-oxygen (Sr-O mode) bond.

The absorption in between 610.36 to 562.148 cm-1 is due to presence of water of crystallization.

Table - 3.3 Spectral assignments of the IR peaks

Wave number (cm-1)

Assignment

3428.81

O-H Stretching

1788.65

C=O Stretching

1351.86

O-H bending

1071.26

C-O bond

719.318

metal - oxygen bond

610.36 to 562.148

Water of crystallization

IR spectra for undoped strontium oxalate crystals by agar- agar gel

In the IR spectrum of strontium oxalate crystals, the absorption at 3301.54 cm-1 is due to O-H stretching mode and water stretch. Few bands on the small wave number side of these bands represent overtones and combination tones occurring at smaller wave number.

A strong asymmetrical band at 1608.34 cm-1 is attributed to the C=O

stretch of carbonyl group.

The peak 1316.18 cm-1 is due to of plane bending O-H stretch, which

established the presence of water molecule associated with crystal.

The peak 1009.55 cm-1 is due to C-O stretching.

The sharp IR band observed at 859.132 cm-1 contains absorptions caused

by C-C stretching.

The sharp IR peak at 740.317 cm-1 may be attributed to presence of metal-

oxygen (Sr-O mode) bond.

The absorption from 668.214 cm-1 is due to presence of water of

crystallization.

Table - 3.4 Spectral assignments of the IR peaks

Wave number (cm-1)

Assignment

3301.54

O-H Stretching

1608.34

C=O Stretching

1316.18

O-H bending

1009.55

C-O bond

740.317

metal - oxygen bond

668.214

Water of crystallization

Therefore from above discussion the structure of undoped strontium oxalate may be

O = C - O

Sr xH2O (x - unknown)

O = C - O

3.12.3 Thermal analysis

Thermal studies of certain oxalates have been reported by several investigators [3-5, 7-9, 12, 20, 91].

In the present work, TGA and DTA of undoped strontium oxalate was carried out 30oC to 600oC and 30oC to 1000oC at a heating rate of 10oC / min in an atmosphere of air and the sample undoped strontium oxalate hold for 1 minute at 30oC. DSC of undoped strontium oxalate was carried out 30oC to 400oC at a heating rate of 10oC / min and cooled from 400oC to 30oC at the rate of 10oC / min in nitrogen. The sample undoped strontium oxalate was hold for 1 minute at 30oC.

3.12.3.1 Thermo-Gravimetric Analysis (TGA)

TGA was carried out on Diamond TG / DTA Perkin Elmer instrument at National Chemical Laboratory, Pune, (M. S.) and at University Department of Chemical Technology, North Maharashtra University, Jalgaon, (M. S.) on SHIMADZU DSC 600, Japan.

TGA curves are shown in fig. 3.8 for undoped strontium oxalate by hydro silica gel and in fig. 3.9 for undoped strontium oxalate by agar-agar gel.

From the thermo gram of undoped strontium oxalate by hydro silica gel one can observe that

The compound is stable up to 130.88oC.

8.887% weight loss in temperature range 130.88oC to 191.28oC may be due to dehydration of 0.85 water molecule and there is no further weight loss up to 261.21oC.

18.253% weight loss in temperature range 261.21oC to 301.56oC from the dehydrated compound corresponds to loss of CO.

12.260% weight loss in temperature range 365.98oC to 498.55oC corresponds to loss of CO2.

The residue remains stable from 498.55oC.

TGA data indicates that the grown crystals contains 0.85 water molecule, which is lost up to 261.21oC and dehydrated compound decomposed by loosing CO up to 301.56oC. Again the compound decomposed by loosing CO2 up to 498.55oC and after that the remaining compound remains stable. These results can be interpreted by the following thermo chemical reactions.

130.88oC - 191.28oC

SrC2O4, 0.85 H2O SrC2O4 + 0.85H2O

Stage -I

261.21oC - 301.56oC

SrC2O4 SrCO3 + CO

Stage -II

365.98oC - 498.55oC

SrCO3 SrO + CO2

Stage -III

Percentage of weight loss in the different stages of decomposition of undoped strontium oxalate crystals are observed as mention in the table- 3.5.

Table -- 3.5 Percentage of weight loss of undoped strontium oxalate crystals

by hydro-silica gel

Stage

Temperature

(oC)

Loss of material

Observed weight loss (%)

Calculated weight loss (%)

I

130.88 to 191.28

0.85 H2O

8.887

7.929

II

261.21 to 301.56

CO

18.253

18.784

III

365.98 to 498.55

CO2

12.260

11.816

Thus the strontium carbonate finally turns into strontium oxide at 498.55oC for undoped sample which is confirmed by residual weight up to end of analysis 60.600% of SrO, which is in agreement with calculated residual weight 61.471%.

From the thermo gram of undoped strontium oxalate by agar-agar gel one can observe that

The compound is stable up to 121oC.

7.933% weight loss in temperature range 121oC to 240.47oC may be due to

dehydration of 0.85 water molecule and there is no further weight loss up

to 251.19oC.

18.795% weight loss in temperature range 251.19oC to 356.87oC from the

dehydrated compound corresponds to loss of CO.

11.83% weight loss in temperature range 359.94oC to 497.74oC corresponds to loss of CO2.

The residue remains stable from 497.74oC.

TGA data indicates that the grown crystals contains 0.85 water molecule, which is lost up to 240.47oC and dehydrated compound decomposed by loosing CO up to 356.87oC. Again the compound decomposed by loosing CO2 up to 497.74oC and after that the remaining compound remains stable. These results can be interpreted by the following thermo chemical reactions.

121oC-240.47oC

SrC2O4, 0.85 H2O SrC2O4 + 0.85H2O

Stage -I

251.19oC-356.87oC

SrC2O4 SrCO3 + CO

Stage -II

359.94oC-497.74oC

SrCO3 SrO + CO2

Stage -III

Percentage of weight loss in the different stages of decomposition of undoped strontium oxalate crystals are observed as mention in the table- 3.6.

Table -- 3.6 Percentage of weight loss of undoped strontium oxalate crystals

by agar-agar gel

Stage

Temperature

(oC)

Loss of material

Observed weight loss (%)

Calculated weight loss (%)

I

121.00 to 240.47

0.85 H2O

7.933

7.929

II

251.19 to 356.87

CO

18.795

18.784

III

359.94 to 497.74

CO2

11.830

11.816

Thus the strontium carbonate finally turns into strontium oxide at 497.74oC for undoped sample which is confirmed by residual weight up to end of analysis 61.442% of SrO, which is in agreement with calculated residual weight 61.471% as shown in table- 3.6.

3.12.3.2 Differential Thermal Analysis (DTA)

DTA was carried out on Diamond TG / DTA Perkin Elmer instrument at National Chemical Laboratory, Pune, (M. S.).

DTA curves are shown in fig. 3.10 for undoped strontium oxalate by hydro silica gel and in fig. 3.11 for undoped strontium oxalate by agar-agar gel.

From DTA curve of undoped strontium oxalate by hydro silica gel one can observe that the loss of bulk of water of crystallization in a single peaked endothermic at 185.99oC. Complete dehydration is only on the onset of oxalate decomposition as observed. DTA curve at the peak 293.34oC and 505.02oC are characterized by an exothermic peak which shows the oxalate decomposition.

Loss of weight at the temperature relates to the loss of water of crystallization (endothermic reaction) at the range 101.04oC to 211.49oC.

Loss of weight at the temperature 293.34oC relates to release of CO and loss of weight at the temperature 505.02oC relates to release of CO2 which are exothermic in character. That means the weight loss with respect to temperature of the grown crystals was further supported by DTA results. DTA data is shown in table- 3.7.

Table -- 3.7 DTA data of undoped strontium oxalate crystals by hydro silica gel

On set

(oC)

Peaks Recorded

(oC)

Peak Height

(v)

Area

(v.s)

Nature

166.37

185.99

-23.029

5058.205

Endothermic

276.15

293.34

10.773

-22847.144

Exothermic

499.74

505.53

20.350

-7846.491

Exothermic

From DTA curve of undoped strontium oxalate by agar-agar gel one can observe that the loss of bulk of water of crystallization in a single peaked endothermic at 172.47oC. Complete dehydration is only on the onset of oxalate decomposition as observed. DTA curve at the peak 315.91oC and 481.36oC are characterized by an exothermic peak which shows the oxalate decomposition.

Loss of weight at the temperature relates to the loss of water of crystallization (endothermic reaction) at the range 111.04oC to 243.49oC.

Loss of weight at the temperature 315.91oC relates to release of CO and loss of weight at the temperature 481.36oC relates to release of CO2 which are exothermic in character. That means the weight loss with respect to temperature of the grown crystals was further supported by DTA results. DTA data is shown in table- 3.8.

Table -- 3.8 DTA data of undoped strontium oxalate crystals by agar-agar gel

On set

(oC)

Peaks Recorded

(oC)

Peak Height

(v)

Area

(v.s)

Nature

163.25

172.47

-99.791

19241.244

Endothermic

309.02

315.91

37.272

-1023.200

Exothermic

426.63

481.36

519.138

-34084.412

Exothermic

3.12.3.3 Differential Scanning Calorimetry (DSC)

DSC was carried out on Perkin Elmer instrument Pyris 6 DSC at National Chemical Laboratory, Pune, (M. S.) and at University Department of Chemical Technology, North Maharashtra University, Jalgaon, (M. S.) on SHIMADZU DSC 600, Japan.

DSC curves are shown in fig. 3.12 for undoped strontium oxalate by hydro silica gel and in fig. 3.13 for undoped strontium oxalate by agar-agar gel.

From DSC curve of undoped strontium oxalate by hydro silica gel one can

observe that:

Step-I

The initiation temperature is 132.50oC and equilibrium temperature is 220oC. At 132.50oC initiation of phase change start and phase change is completed at peak endo-down temperature 169.45oC. The temperature at which the sample and reference come to the thermal equilibrium by thermal diffusion appears to be at 220oC.

Area under the curve is 4460.46 mJ.

Heat of transition ΔH i.e. enthalpy change of transition is 220.0645 J/gm; this is 0.2201 KJ/mole. Since molecular weight is 1 gm/mole.

Therefore, ΔHtr = ΔHf

Hence heat of phase formation is also 0.2201 KJ/mole.

Where ΔHf is enthalpy change of new phase formation or it is called heat of phase formation.

Step-II

At 230.10oC initiation of phase change start and phase change is completed at peak endo-down temperature 299.10oC.

In the DSC study the two endothermic stages were obtained at 169.45oC and 299.10oC respectively. The result of DSC measurement is presented in the table- 3.9.

Table -- 3.9 DSC measurements of undoped strontium oxalate crystals by hydro-silica gel

Sample

Wt of the sample

Change in enthalpy ΔHf

Transition temperature

Undoped Strontium Oxalate

7.900 mg

0.2201 KJ/mole

169.45oC

From DSC curve of undoped strontium oxalate by agar-agar gel one can observe that:

Step-I

The initiation temperature is 131.46oC and equilibrium temperature is 219.36oC. At 131.46oC initiation of phase change start and phase change is completed at peak endo-down temperature 171.56oC. The temperature at which the sample and reference come to the thermal equilibrium by thermal diffusion appears to be at 219.36oC.

Area under the curve is 4452.803 mJ.

Heat of transition ΔH i.e. enthalpy change of transition is 214.0771 J/gm; this is 0.2141 KJ/mole. Since molecular weight is 1 gm/mole.

Therefore, ΔHtr = ΔHf

Hence heat of phase formation is also 0.2141 KJ/mole.

Where ΔHf is enthalpy change of new phase formation or it is called heat of phase formation.

Step-II

The initiation temperature is 229.07oC and equilibrium temperature is 352.53oC. At 229.07oC initiation of phase change start and phase change is completed at peak endo-down temperature 304.26oC. The temperature at which the sample and reference come to the thermal equilibrium by thermal diffusion appears to be at 352.53oC.

Area under the curve is 3964.438 mJ.

Heat of transition ΔH i.e. enthalpy change of transition is 190.5980 J/gm; this is 0.1906 KJ/mole. Since molecular weight is 1 gm/mole.

Therefore, ΔHtr = ΔHf

Hence heat of phase formation is also 0.1906 KJ/mole.

Where ΔHf is enthalpy change of new phase formation or it is called heat of phase formation.

In the DSC study the two endothermic stages were obtained at 171.56oC and 304.26oC respectively. The result of DSC measurement is presented in the table- 3.10.

Table -- 3.10 DSC measurements of undoped strontium oxalate crystals by

agar- agar gel

Sample

Wt of the sample

Change in enthalpy ΔHf

Transition temperature

Undoped Strontium Oxalate

20.800 mg

0.2141 KJ/mole

171.56oC

0.1906 KJ/mole

304.26oC

Chemical analysis

Chemical analysis was carried out at Department of chemistry, Shri Shivaji Vidya Prasarak Santha's Bapusaheb Shivajirao Deore College of Engineering, Dhule, (M. S.).

1. Gravimetric method

Strontium is quantitatively estimated as strontium sulphate, from the grown crystals of undoped strontium oxalate. 1 gm of crystals were dissolved in hydrochloric acid and diluted to 100 ml with distilled water. In a boiling solution slight excess of hot 0.5 M sulphuric acid solution was added slowly with constant stirring and precipitate was filtered in a weighted porcelain- filter crucible, washed with hot acidified water and then with warm distilled water . It was dried in electric muffle furnace at 600oC and allowed to cool and take the weight.

The estimated amount of strontium was found 65.45 % in the grown crystals by gravimetric analysis is in agreement with the calculated amount of strontium, (64.40%) in SrC2O4, 0.85H2O.

2. Volumetric Analysis

Strontium is quantitatively estimated volumetrically using standard EDTA solutions. 0.0135 gm of strontium oxalate crystals were dissolved in 100 ml de-ionized water with few drops of HCL (0.01N solution). 25 ml of this solution was pipette in a titration flask, 12.1 pH was adjusted by the addition of 1M sodium hydroxide solution in it and Eriochrome black T was added as an indicator. It was titrated with 0.01 N (standard) EDTA solutions until the colour changed from blue to gray.

1mole EDTA = 1 mole Sr2+

i.e. 1000 ml 1 mole EDTA contain 87.63 gm strontium

As 25 ml pipette solution required 24.7 ml 0.01N, EDTA

100 ml solution required = 98.8, 0.01 N EDTA

As 1000 ml 0.01N EDTA = 0.08763 gm Sr2+

98.8, 0.01N EDTA = 0.008657 gm Sr2+

As 0.0135 gm of strontium oxalate solution = 0.008657 gm strontium

100 gm of strontium oxalate solution contains = 64.13 gm strontium

Thus the volumetric estimation of strontium in the grown crystals was found to be 64.13%. This is matched with the calculated amount (64.40%) of strontium in SrC2O4, 0.85 H2O.

Energy Dispersive Analysis by X-rays (EDAX)

Elemental analysis was carried out at Sophisticated Instrumentation Centre for

Applied Research and Testing (SICART), Sardar Patel Centre for Science and Technology, Aanand, Gujarat.

The graph of the sample undoped strontium oxalate grown in hydro-silica gel

obtained after EDAX is shown in fig. 3.14 and the data obtained is given in table- 3.11, which shows mass (wt) % of different elements in the sample. The presence of Sr metal is confirmed from EDAX. The observed mass (wt) % is in agreement with calculated one.

Table -- 3.11 EDAX data of undoped strontium oxalate crystals by hydro-silica gel

Element

Content as measured by EDAX

wt %

at %

C

7.83

23.85

O

20.13

46.05

Sr

72.05

30.10

The graph of the sample undoped strontium oxalate grown in agar-agar gel obtained after EDAX is shown in fig. 3.15 and the data obtained is given in table- 3.12, which shows mass (wt) % of different elements in the sample. The presence of Sr metal is confirmed from EDAX. The observed mass (wt) % is in agreement with calculated one.

Table -- 3.12 EDAX data of undoped strontium oxalate crystals by agar-agar gel

Element

Content as measured by EDAX

wt %

at %

C

12.28

32.57

O

21.84

43.48

Sr

65.88

23.95

3.12.6 Scanning Electron Microscope (SEM)

In present work Scanning Electron Microscope (SEM) studies of gel grown undoped strontium oxalate crystals by hydro silica gel and agar-agar gel are done by using latest computerized scanning electron microscope Quanta 200 3D at National Chemical Laboratory, Pune, (M. S.). The successive photographs were taken at magnification of 250-, 500- and 2500- at width 15.1 mm, 14.9 mm respectively and at high voltage 20 KV for undoped strontium oxalate crystals by hydro silica gel as shown in fig.3.16 (a, b, c). While the successive photographs were taken at magnification of 250-, 500-, 1000- and 2000-, at common width 14.7 mm and at high voltage 20 KV for undoped strontium oxalate crystals by agar-agar gel as shown in fig. 3.17 (d, e, f, g).

Fig. (a) shows the surface of undoped strontium oxalate crystals grown in hydro silica gel. The whole surface is dark but in within the surface one can see white regular geometrical images of different shape and size. In general within the surface there are white images on the dark background. The geometrical shapes of the white images are mostly pentagon but they are irregularly oriented. If the portion A of fig. (a) at magnification 250- is observed at higher magnification it is shown by portion A1 in fig. (b). All the four figures on the portion A are well isolated in portion A1. One of the figure in A1 is regular hexagonal while the second is irregular pentagon, the third is regular square and fourth is without any shape. But all the four figures in portion A1 have small grains on the surface.

If the portion B on A1 in fig. (b) that is square and is further magnified its geometrical nature and grains on the surface is clearly seen in portion B1 in fig. (c) at higher magnification 2500-.

If the surface of undoped strontium oxalate crystals grown in agar-agar gel observed at magnification 250- it can be represented by fig. (d) and it shows in general pentagon. The growth layers forming the structure of pentagon are clearly visible. The growth layers of different faces are parallel to each other. As the growth layers forming the hexagonal hillock are clearly seen. It can be stated that the growth of the hillock in particular and growth of the crystal in general takes place by two dimensional mechanisms. Portion A of fig. (d) is magnified and shown by portion A1 in fig. (e) at magnification 500-. The portion A1 shows the well defined edges of the geometrical figures.

The portion B in fig. (e) which is a part of A1 is further magnified, it is shown by the portion B1 in fig. (f) at magnification 1000-. The portion B1 shows clearly formation of growth hillocks by growth layers that is growth by two dimensional mechanisms.

The geometrical nature of the portion C, which is a part of B1 of fig. (f) is shown by the portion C1 which is highly magnified as shown in fig. (g) at magnification 2000-. Because of higher magnification of fig (g) the intensity of the figure in general is reduced, but the edges of the geometrical figure are quite sharp and clearly visible.

Conclusions

From the above discussion, the following conclusions can be derived:

Gel technique can be successfully employed for growth of undoped strontium oxalate crystals. Single diffusion method is convenient for growth of the crystals. Micro crystals of strontium oxalate were observed in hydro-silica gel at 4.0 pH value with gel density (1.04gm/cc), white transparent, tiny, shining crystals were obtained in agar gel, i.e. size of the crystals improved in agar- agar gel.

Gel aging period reduces the nucleation centers without affecting the quality of crystals.

Nucleation density was increased with the concentration of reactants. Concentration of reactants has pronounced effect on the habit, quality and size of the crystals. Rate of diffusion of supernatant has great effect on the quality of crystals.

From the study of XRD, IR, TGA, DTA, DSC, EDAX and SEM of undoped sample of strontium oxalate following conclusions can be predicated:

From XRD the unit cell parameters of grown undoped strontium oxalate crystals satisfy the conditions for triclinic system.

From FT-IR first four bands are on lower wave number, metal-oxygen bond and water of crystallization on higher wave number in agar-agar gel.

For the decomposition of grown crystals the temperature range in hydro-silica gel is slightly more as compared to agar-agar gel hence the thermal stability of grown crystals is same.

Chemical compositions of the grown crystals by EDAX are approximately same with the theoretical calculation. The percentage of strontium in the crystals grown by hydro-silica gel is more as compared to that is grown by agar-agar gel.

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