118

CHAPTER VI

GROWTH, SPECTRAL, STRUCTURAL AND MECHANICAL PROPERTIES OF

STRUVITE CRYSTAL GROWN IN PRESENCE OF NaF AND CaCl2

6.1 INTRODUCTION

Kidney stone formation (nephrolithiasis) is a common disease affecting all

geographical, cultural and racial groups worldwide [146]. The formation of urinary stones

in human body is a serious clinical problem that affects up to 20% of the population [147]

with the recurrence after treatment on the level of 50%. Urinary stones are located in the

kidneys, and only a small percentage is lodged in the urinary bladder and urethra. Urinary

stones form in the course of many physical and chemical processes. General conditions

that contribute to stone formation include a high concentration of salts in urine, retention

of these salts and crystals, pH, infection, and a decrease in the body‟s natural inhibitors of

crystal formation. Calculi are often heterogeneous, containing mainly oxalate, phosphate,

and uric acid crystals. A separate kind of urinary stones are the so-called infectious

stones related to urinary tract infection [148]. Magnesium ammonium phosphate

hexahydrate (MAP) or struvite stones are (MgNH4PO4·6H2O) also known as infection

stones, urease or triple-phosphate stones. Struvite kidney stones are called triple

phosphate stones because of their combination of magnesium, ammonium and

calcium/phosphate. They are potentiated by alkaline urine and high magnesium excretion

(high magnesium/plant-based diets). They are found predominantly in women with

chronic urinary infections causing to suffer from abnormal urinary tracts. Worldwide,

struvite forms 30% of all kidney stones in humans [149]. In contrast to humans, struvite

stones account for 75% of urinary calculi found in the animals like dogs and cats [150].

Upper urinary tract stones that involve the renal pelvis and extend into at least 2 calyces

are classified as staghorn calculi. Although all types of urinary stones can potentially

form staghorn calculi, approximately 75% are composed of a struvite-carbonate-apatite

matrix.

119

6.1.2 Properties of MAP crystals

Formula : MgNH4PO4·6H2O

Mineral name : Struvite

Colour : white or yellowish white, brownish white

Structure : orthorhombic

Space group : Pm21n

Lattice Parameters : a = 6.941 Å, b = 6.137 Å, c = 11.199 Å

Moh‟s hardness : 1.5-2

Specific gravity : 1.7 g/cm3

Magnesium ammonium phosphate hexahydrate is also known as “struvite,” a pet

name denoted by the German chemist George Ludwig Ulex who first acknowledged the

mineral and was a student of the Russian diplomat and naturalist Heinrich Christian

Gottfried Von Struve [151]. Hence the term “struvite stone” is often used interchangeably

with “infection stone.” The presence of urease-producing bacteria is obligatory to their

formation so it is also known as “urease stone”.

6.1.3 Growth of struvite Crystal

Abbona et al [152] investigated struvite crystal growth and crystal morphology.

They described the structure as regular PO43-

tetrahedra, Mg (H2O) 62+

octahedra and NH4

groups all held together by hydrogen bonding. They showed that crystal morphology was

associated with the rate of growth of crystals: very high supersaturations (i.e. high growth

kinetics where pH > 8.0) promoted formation of bidimensional or tridimensional twinned

crystals; whereas high supersaturation (pH about 7.0) promoted tabular crystals; and

decreasing supersaturation (pH 6.7) promoted crystals that were less tabular and more

elongated. In summing-up, their results showed that high growth kinetics promoted flat

crystals with large faces, whereas low growth kinetics promoted stick-like crystals, flat

and without faces. Bouropoulos et al [153] investigated the formation of struvite from

aqueous solutions, at 25 oC at pH 8.5. The effect of impurities on the morphology of

crystals was studied using gel growth technique [154]. The growth of platelet type

struvite crystals using gel growth technique was reported [155] and the effect of sunlight

on growth was also discussed [156]. The growth and characterization of the gel grown

struvite crystals was reported [157] and it was found that the struvite crystal morphology

as well as growth process were strongly dependent on growth parameters. The in vitro

120

growth inhibition studies of struvite by the juice of Citrus Medica Linn as well as the

herbal extracts of Commiphora Wightii, Boerhaavia Diffusa Linn and Rotula Aquatica

Lour were carried out [158] and found potent inhibitors of struvite.

6.1.4 Role of fluoride

Fluoride-related health hazards are painstaking major environmental problem in

many regions of the world. Chronic intake of excessive fluorine can lead to severe

permanent bone and joint deformations of skeletal fluorosis, which is irreversible and has

no treatment. Fluoride normally enters the environment and human body through water,

food, industrial exposure, drugs, cosmetics, etc. However, drinking water is the single

major source of daily intake. Taking health effects into consideration, the World Health

Organization has set a guideline value of 1.5 mg L−1

as the maximum permissible level of

fluoride in drinking waters. Role of fluoride on the urinary crystal was already discussed

in chapter IV. Urolithiasis was 4.6 times more common in an area with high fluoride (3.5

to 4.9 ppm) than in a similar area without high fluoride. Moreover, in the high fluoride

area, the prevalence of kidney stones was almost double in subjects with fluorosis than

without fluorosis [159].

6.1.5 Preparation of gel

Gel method is used to grow the struvite because the gel acts as a „three

dimensional crucible‟ which supports the crystals and at the same time yield to its growth

without exerting major forces upon it. This relative freedom from constriction is believed

to be an important factor in the achievement of high structural perfection. Urinary stones

grow in a gel like medium which provides simulation of synovial cartilage and other

biological fluids [160].

The source material, sodium metasilicate (SMS) of about 35 g was weighed and

dissolved in 100 ml of distilled water. The solution was stirred by using magnetic stirrer

for about half an hour. Then the solution was filtered by using whatman filter paper with

110 mm pore size. The SMS solution was taken in a measuring jar; and the density of the

solution was measured by using Hydrometer. The specific gravity of SMS is adjusted to

be 1.03 g/cm3

by adding distilled water. The SMS solution is added with the 1.5 M

ammonium dihydrogen phosphate, so that the pH of the solution was adjusted to 7.4.

These solutions were poured into test tubes and kept undisturbed for gelling. The

121

experiments were repeated with different concentrations (1.03 to 1.06 g/cm3) and

different pH values (7 - 8) of sodium metaslicate solution. At higher pH values (greater

than 8) and high concentrations (greater than 1.04 g/cm3) of acidified silica solution, the

time required for gelation was considerably higher (more than 5 weeks) and also the

resulting gel was unstable. The gel was set after 2 days

6.1.6 Growth of pure MAP

The supernatant solution of pure MAP was prepared by mixing 1.5 M of

magnesium acetate in double distilled water in a beaker. The contents of the beaker were

stirred for 15 minutes and the clear solution was then filtered. About 10 ml of the

magnesium acetate solution was poured carefully over the top of the gel medium without

disturbing the latter and the test tube was closed with airtight cover. The following

reaction is expected to occur in the gel between the two reactants [157]

NH4H2PO4.2H2O + (CH3COO)2Mg.4H2O → MgNH4PO4.6H2O+ 2CH3COOH

In the present work, tiny crystals appeared at the gel–liquid interface two days

after adding the supernatant solution. These crystals were found to be dendritic in nature.

In addition to this, small envelope and rectangular bar shaped crystals grew in the gel

medium after about eight days (Fig. 6.1a). The formation of acetic acid is maximum at the

gel-liquid interface due to high concentration gradients and faster reactions that take

place. But as one goes towards the bottom of the test tubes the diffusion of reactants is

comparatively less and the amount of acetic acid produced is also less, which does not

dissolve the growing crystals and hence a steady growth of crystals is observed. As the

depth of the gel column increases from the gel–liquid interface, the sizes of the grown

crystals became gradually smaller. The crystals were harvested after three weeks.

6.2 EFFECT OF FLUORIDE ON THE GROWTH OF MAP

The gel was prepared as in the case of section 6.2.2. Preparation of supernatant

solution was done by mixing magnesium acetate with water. Afterwards, NaF was mixed

in different concentration (0.1-0.5 M). At lower concentrations (0.1 & 0.2 M NaF), there

was no visible change in the crystallization behaviour. Further increase in fluoride content

(0.3 - 0.5 M) delayed the crystal growth and tiny crystals started appearing only after 4–5

days. The envelope shaped crystals appeared after tenth day. Number of such crystals

increased in the case of 0.3 M NaF and the size decreased as shown in Fig. 6. 1b-e. The

122

results indicate that the presence of NaF affects the nucleation and further growth of

struvite crystals resulting with different morphologies and poor yield. Impurities in

solution from which a compound may precipitate can affect the growth rate of crystals

due to blocking of active growth sites inhibiting the increase of crystal size [161]. In case

of struvite crystallization foreign compounds such as sodium fluoride could be adsorbed

on the crystal surface and retard struvite formation. Morphology of the crystals changed

along the test tube from top to bottom depending on the availability of the nutrients and

its diffusivity in the gel medium. Harvested crystals are shown in Fig. 6.2 a-f.

Fig 6. 1 MAP crystals in the presence of NaF; (a) 0, (b) 0.1, (c) 0.2 , (d) 0.3, (d) 0.4

and (e) 0.5 M

Fig 6.2 MAP crystals grown in the presence of NaF ; (a) 0, (b) 0.1, (c) 0.2, (d) 0.3,

(e) 0.4 and (f) 0.5 M

123

a b c Volume (Å3)

Reported

6.94

6.14

11.20

477.04

Pure MAP

6.95

6.14

11.20

478.1

MAP + 0.3 M NaF

6.93

6.13

11.20

476.3

MAP +0.5M NaF

6.93

6.13

11.21

476.1

a c

d b

6.2.1 RESULTS AND DISCUSSION

6.2.1.1 XRD analysis

The powder XRD pattern of pure and the doped MAP crystals are shown in Fig.

6.3 a-d. In general, all the XRD patterns agree well with the JCPDS (71-2089) value for

the struvite. The results confirm the phase purity and crystalline nature of the samples

(Fig. 6.3 a). However, intensities of many peaks of the doped samples showed significant

shifts as compared to that of pure samples (Fig.6.3 b-d). These discrepancies in the peak

intensities could be attributed to changes in preferred growth direction due to the presence

of excessive fluoride.

The MAP crystals are known to crystallize in the orthorhombic system (space

group Pm21n) with the lattice parameters, a = 6.941 Å, b = 6.137 Å, c = 11.199 Å.

Theoretically, the lattice parameters (a, b and c) for the orthorhombic crystal structure of

MAP, are determined by the equation

1 h 2 k 2 l 2

2 2 2 2

Volume V= abc

where a, b and c are the lattice parameters, λ is the wavelength (CuKα: λ = 1.5406 Ȧ) and

2θ is the diffraction angle. The lattice parameters from the single-crystal X-ray diffraction

data of the pure and doped samples are given in Table 6.1

Table 6.1 Lattice parameters of pure and fluoride grown MAP crystals

Samples

Lattice Parameters (Å)

124

Fig 6.3 XRD patterns of MAP crystals grown in the presence of NaF;

(a) 0, (b) 0.1, (c) 0.3 and (d) 0.5 M

6.2.1.2 Morphological studies

Among different types of crystals grown in each test tube, rectangular bar shaped

crystals (Fig. 6.4 a-c) were chosen for morphological and elemental analyses. Optical

microscope picture of the pure and fluoride doped struvite crystals are shown in Fig. 6.4.

It can be noticed that there is a systematic decrease in the surface area of the (1 0 1) plane

125

with the increase in the additive concentration and this plane finally disappears at the

concentration of 0.5 M NaF. Based on the computer model of the struvite crystal

morphology and experimental findings, the authors have concluded that the additive

phosphocitrate is not only having a strong affinity for the selective growing faces but also

an ability to get incorporated into the crystal lattice [162]. They also reported that the

adsorption of phosphocitrate on (101) plane of struvite is of primary importance to its

strong inhibitory activity. Similar results have been obtained for the addition of NaF.

Fig 6.4 Optical microscope study of MAP crystals grown in the presence of NaF;

(a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (d) 0.4 and (e) 0.5 M

6.2.1.3 SEM-EDAX analysis

The rectangular shaped crystals were chosen for morphological and elemental

analyses (Fig.6.5 a-c). Surface of the as grown pure crystal appeared clean and free from

major defects. There has been a significant change in the surface morphology of the

grown crystals due to the presence of sodium fluoride in the growth environment. Surface

of the as grown pure crystal appeared clean and free from major defects. At higher

126

fluoride concentrations (0.4 & 0.5 M), growth of certain planes are inhibited which could

be attributed to the restricted access of ionic species to the growing face of the crystal.

This result is in accordance with powder XRD results which indicated that the large area

plane changes with dopant concentration.

.

a

b c

Fig 6.5 SEM pictures of MAP crystals grown in the presence of NaF; (a) 0, (b) 0.4

and (c) 0.5 M

Fig. 6.6 a - c shows EDX spectrum of pure and fluoride doped struvite crystals.

From the results that are compiled in Table 6.4, we found that the magnesium content

in all the doped crystals was less than the stoichiometric value and there is a

simultaneous increase in the phosphorous content. The free fluoride ions may react with

magnesium forming magnesium fluoride (MgF2) [163]. The fluoride content in the

crystals increases with the increase in NaF concentration at the growth environment.

It can be noticed that there is a small amount of sodium in the struvite crystals

grown in the presence of NaF.

127

Fig 6.6 EDX spectra of MAP crystals grown in the presence of NaF; (a) 0,

(a) 0.2 and (c) 0.4 M

128

Table 6.4 EDX analyses of pure and fluoride grown MAP crystals

Element

Atomic percentage of elements present in the MAP

crystals grown in the presence of NaF

0 M 0.1 M 0.2 M 0.3 M 0.4 M

Mg 47.62 37.82 37.71 39.04 37.21

P 52.38 60.80 60.26 58.03 58.55

F - 1. 37 1.71 2.12 2.99

Na - - 0.32 0.81 1.25

Mg/P 0.909 0.622 0.626 0.673 0.636

6.2.1.4 FTIR studies

The recorded FTIR spectra of pure and the doped MAP crystals are shown in Fig.

6.7 a-d. The observed wave numbers, relative intensities obtained from the recorded

spectra and the assignments proposed for the crystals under investigation were found to

be in good agreement with the reported literature [164]. A broad asymmetric band in the

range between 3800-2200 cm-1

was attributed to O–H and N–H stretching vibrations

[165]. The absorptions occurring around 1650, 1570 and 1440 cm-1

have been attributed

to N–H bending vibrations.

The splitting of the band due to the υ mode (NH4) of struvite can be considered as

an indication of the rotation of the ammonium ions [164]. The shoulder at 1240 cm-1

in

the pure crystal is due to the existence of some kind of disorder of the ammonium ions

which is absent in the case of doped samples [165]. In all the investigated samples, υ3 and

υ4 modes of PO43-

ions were observed around 1050, 1003 and 569 cm-1

respectively. In

case of pure struvite, a shoulder appeared at 1050 cm-1

which got disappeared in the

fluoride doped samples. Strong band at 1003 cm-1

must originate from the components of

the υ3 (PO4) vibration. This peak got shifted to higher wavenumber side due to doping.

The shift may be small but is nevertheless real. The intensity of the bands appearing at

1006 and 569 cm-1

appreciably decreases on going from lower to higher concentrations of

NaF. Appearance of the band around 471 cm-1

could be attributed to the components of υ2

mode (PO4). This band is more predominant in the doped samples only. At the highest

129

concentration of 0.5 M NaF, the band at 887 cm-1

due to N–H rocking disappeared. Thus

the FTIR result indirectly confirms the doping of fluoride into struvite

Fig 6.7 FTIR spectra of MAP crystals grown in the presence of NaF; (a) 0, (b) 0.1,

(c) 0.3 and (d) 0.5 M

6.2.1.5 Vickers Microhardness Studies

Microhardness of a crystal is its capacity to resist indentation. Physically hardness

is the resistance offered by a material to the localized deformation by scratching or by

indentations. The study of mechanical properties of urinary calculi in real conditions

(in vivo) is difficult and possible only to a limited extent. So, the study of the mechanical

properties of urinary calculi in vitro will be useful in designing and optimizing the

parameters to develop a technique for the urinary stone fragmentation. Vickers hardness

indentations are made on the flat face of the crystals (0 0 1) plane at room temperature for

loads 25, 50, 100 and 200 g using Vickers hardness tester. The average value of the

130

diagonal lengths of the indentation mark for each load is used to calculate the hardness.

The Vickers microhardness number is determined from the relation

Hv = 1.8544 P/d2

Kg/mm2

where P is the load, d the length of the diagonal of the indentation impression and HV the

Vickers hardness. The variation of Hv with load is shown in Fig.6.8. It was observed that

in all the cases, the microhardness increased with increase in load. The pure crystal

withstands the load upto 200 g but in the case of fluoride doped samples crack initiation

and materials chipping become significant beyond 100 and 50 g for 0.1 M & 0.2 M NaF

doped crystals respectively. Hardness test could not be carried out above this load. The

plot log P vs log d is shown in Fig.6.9. According to Meyer‟s relation, the work hardening

coefficient is related to the diagonal length by

P = K dn

where K is an arbitrary constant and n is work hardening coefficient. The value of n is

determined from the slope of log P vs log d plot. HV increases with increase in P and „n‟

is estimated as 3.1987 for pure, 3.644 for 0.1 M and 4.923 for 0.2 M NaF doped MAP

crystals respectively. According to Onitsch [166], the Meyer‟s index in the range of

1.0 ≤ n ≤ 1.6 is for hard materials and n > 1.6 for soft materials. Hence, it is inferred that

MAP belongs to the soft category of materials and agrees well with the reported result

[167] i.e. the struvite is the softest naturally occurring stone. The addition of fluoride

further has softened the struvite crystal.

Fig 6.8 Variation of microhardness vs load for the MAP crystals grown in the

presence of NaF; (a) 0, (b) 0.1 and (c) 0.2 M

131

Fig 6.9 Variation of log d vs log P for the MAP crystals grown in the presence of

NaF; (a) 0, (b) 0.1 and (c) 0.2 M

6.2.1.6 Thermal analysis

Fig. 6.10 and 6.11 illustrate thermal behavior of pure and fluoride doped MAP

samples recorded in the temperature range between 30-1200ºC at the rate of 20ºC/min in

nitrogen atmosphere. In pure sample the weight loss occurs in two stages. The major

weight loss of about 53% occurs between 79ºC and 179ºC which indicates the elimination

of six water molecules and an ammonia molecule. The DTA shows the strong

endothermic peak at 136ºC corresponding to the weight loss. In the high temperature

region (179-730ºC), two molecules of MgHPO4 combine and result in the elimination of a

water molecule leading to the formation of magnesium pyrophosphate and nearly 36% of

the sample is stable. The following chemical reactions are expected to occur during the

dehydration and decomposition stages [168].

MgNH4PO4.6H2O MgHPO4

2MgHPO4 Mg2P2O7

132

Fig 6.10 TG – DTA curves of pure MAP crystals

Fig 6.11 TG – DTA curves of MAP crystal grown in presence of 0.3 M NaF

133

Nearly similar thermal behaviour occurrs in fluoride doped MAP also. The major

weight loss of about 51% occurs between 73ºC and 173ºC in the crystals grown with NaF

(0.5 M) addition. The mass loss corresponds well with the DTA results by the appearance

of an endothermic peak at 121ºC. Thus, there is a decrease in the peak temperature which

indicates a reduced thermal stability of MAP due to fluoride doping. In the second stage,

nearly 7% weight loss occurs and the rest of the sample (40%) is stable. The exothermic

peak around 710ºC in both pure and doped sample is due to the loss of one water

molecule resulting in the transition from the „amorphous‟ product to γ–crystalline form

[169].

6.3 INFLUENCE OF CALCIUM ON THE GROWTH OF STRUVITE

Patients with diabetes mellitus are at increased risk for struvite stone formation as

well [170]. Supersaturation depends on urinary pH, ionic strength and solute

concentration [171]. Increasing urinary alkalization results from ammonia being

generated by the infecting microorganism and together with the presence of the

magnesium and phosphate ion rapidly generates the large, relatively soft stone material.

Struvite stone can grow rapidly forming "staghorn-calculi", which is more painful

urological disorder and may involve the entire renal pelvis and calyces, which may lead

to the blockage of the urinary tract. Patients with a functionally complete cord transaction

are at the highest risk of developing a staghorn calculus. Patients with infected Staghorn

calculi who receive no treatment have about a 50% chance of losing the kidney [172].

Therefore, it is very much necessary to study the growth-inhibition of struvite crystals.

Struvite urolithiasis, which is a common disorder in cats, can be prevented and treated by

urinary acidification using ammonium chloride as urinary acidifier. But it (15 g)

enhanced urinary excretion of calcium in cats, which might have been accompanied by

loss of bone minerals. So calcium chloride was used either as supplement or substitute

and this could be used to prevent struvite urolithiasis [173]. Using precipitation method,

increasing the calcium concentration reduces the crystal size and inhibits the struvite

growth, or affects struvite crystallisation and leads to the formation of an amorphous

substance rather than crystalline struvite was reported [174]. In the present work, we have

studied the effect of calcium chloride on the growth and properties of struvite crystals

using gel method.

134

6.3.1 Influence of calcium on the growth of MAP crystals

The growth of the pure MAP crystal was already discussed in the section 6.2.3. In

the case of struvite crystals formed in presence of calcium, there was not much change in

the lower concentration of calcium (0.05 M). The tiny dendritic crystals started appearing

only after 7- 8 days in the case of 0.1- 0.2 M Ca. Further the envelope and rectangular

crystals appeared only after tenth day. The reduction in the size and the number of

crystals with the increase in calcium content was due to the presence of calcium in the

growth environment. Fig.6.12 a-c shows the as grown struvite crystals. The presence of

Ca ions can influence struvite formation, either by competing for phosphate ions or by

interfering with the crystallisation of struvite.

Fig 6.12 MAP crystals grown in the presence of NaF ; (a) 0, (b) 0.1 and (c) 0.2 M

6.3.3.1 Morphological studies

Optical microscope picture of the pure and fluoride doped struvite crystals

(rectangular bar shaped) are shown in Fig. 6.13. It can be noticed that there is a decrease

in the surface area of the (1 0 1) and (0 0 1) planes with the increase in the calcium

concentration. This may be the reason for the strong inhibitory activity of the struvite

crystal.

135

Fig. 6.13 Optical microscope study of MAP crystals grown in the presence of Ca;

(a) 0, (b) 0.05, (c) 0.1 and (d) 0.2M

6.3.3.2 SEM-EDAX analysis

Surface of the grown pure crystal (rectangular shaped) appeared clean and free

from major defects. Presence of agglomeration was seen in 0.1 M concentration (Fig.

6.14 b). At higher calcium concentration (0.2 M), growth of certain planes is inhibited

which could be attributed to the restricted access of ionic species to the growing face of

the crystal (Fig 6.14 c). The data generated by EDX analysis consists of spectra showing

peaks corresponding to the elements that are present in the sample. Fig. 6.15 and Fig.

6.16 shows EDX spectrum of pure and calcium doped struvite crystal. The results are

persented in Table 6.5. Magnesium content in the doped crystals was less than the

stoichiometric value and there is a simultaneous increase in the phosphorous content.

Table 6.5 EDX data of pure and Ca grown MAP cryatals

Element

Atomic percentage of elements present in the MAP crystals grown in the presence of Ca

0 M 0.2 M

Mg 47.62 37.33

P 52.38 54.33

Ca -

Mg/P =0.909

8.34

(Ca+Mg)/P = 0.842

136

Fig 6.14 SEM pictures of MAP crystals grown in presence of Ca; (a) 0, (b) 0.1 and

(c) 0.2 M

Fig 6.15 EDX spectrum of pure MAP crystals

137

Fig 6.16 EDX spectrum MAP crystals grown in the presence of 0.2 M Ca

6.3.3.3 XRD analysis

The powder XRD pattern of pure and the Ca doped MAP crystals are shown in

Fig. 6.17. In general, all the XRD patterns agree well with the JCPDS (71-2089) value for

the struvite. The MAP crystals are known to crystallize in the orthorhombic system (space

group Pm21n) with the lattice parameters, a = 6.941 Å, b = 6.137 Å, c = 11.199 Å. As Ca

concentration increases, the XRD results showed a decrease in intensity which is shown

in Fig. 6.17.

The unit cell parameters were determined from the single-crystal X-ray diffraction

data obtained with a four-circle Nonius CAD4 MACH3 diffractometer (MoKα, λ =

0.71073 Å). The results of the pure samples are in good agreement with the reported

values and are given in Table 6.6. The decrease in the „a‟ and „c‟ lattice parameters and

decrease in unit cell volume with increase in dopant concentration which suggests that the

calcium may be doped into MAP.

Table 6.6 Lattice Parameters of pure and Ca grown MAP crystals

Lattice Parameters (Å) Samples

a

B

c Volume (Å3)

Pure MAP

6.138

6.947

11.201

478.1

MAP+0.1M

Ca 6.128

6.940

11.199

476.1

MAP+0.2M

Ca 6.111

6.941

11.182

474.3

138

Fig 6.17 XRD patterns of MAP crystals grown in the presence of Ca (a) 0, (b) 0.1

and (c) 0.2 M

6.3.3.4 FTIR studies

The recorded FTIR spectra of pure and the doped MAP crystals are shown in Fig.

6.18 a-c. A broad asymmetric band between 3800-2200 cm-1

was attributed to O–H and

N–H stretching vibrations [165]. The absorptions occurring around 1650, 1570 & 1440

cm-1

have been attributed to N – H bending vibrations. The splitting of the band due to the

υ mode (NH4) of struvite can be considered as an indication of the rotation of the

ammonium ions. The shoulder at 1240 cm-1

in the pure crystal is due to the existence of

139

some kind of disorder of the ammonium ions which is absent in the case of doped

samples [176]. In all the investigated samples, υ3 and υ4 modes of PO43-

ions were

observed around 1050, 1003 and 569 cm-1

respectively. In case of pure struvite, a

shoulder appeared at 1050 cm-1

got disappeared in the calcium doped samples. Strong

band around 1000 cm-1

must originate from the components of the υ3 (PO4) vibration.

Appearance of the band around 460 cm-1

could be attributed to the components of υ2

mode (PO4). This band is more predominant in the doped samples (0.2 M) only. At the

highest concentration of 0.2 M Ca, the intensity of the band at 887 cm-1

due to N – H

rocking decreased.

Further, the spectral investigations strongly suggest that no major shifts or

additional peaks corresponding to the internal modes in the mid region have been

observed in this study. This recommends that the formation of any other complex

compound in calcium doped MAP crystal is impossible.

Fig 6.18 FTIR spectra of MAP crystals grown in the presence of Ca; (a) 0, (b) 0.1

and (c) 0.2 M

140

6.3.3.5 Thermal analysis

Thermal behavior of pure and calcium added MAP crystals were recorded in the

temperature range between 30-1200ºC at the rate of 20ºC/min in nitrogen atmosphere

(Fig.6.19, 6.20). In pure sample the weight loss occurs in two stages. The major weight

loss of about 53 % occurs between 79ºC and 179ºC which indicates the elimination of six

water molecules and an ammonia molecule. The DTA shows the strong endothermic peak

at 136ºC corresponding to the weight loss. In the high temperature region (179-730ºC),

two molecules of MgHPO4 combine and result in the elimination of a water molecule

leading to the formation of magnesium pyrophosphate and nearly 36% of the sample is

stable.

Nearly similar thermal behavior occurred in calcium doped MAP also. The major

weight loss of about 43 % occurs between 74ºC and 173ºC in the crystals grown with

CaCl2 (0.2 M) addition. The mass loss corresponds well with the DTA results by the

appearance of an endothermic peak at 122ºC. In the second stage, nearly 8 % weight loss

occurs and the rest of the sample (49%) is stable. The exothermic peak around 714ºC in

both pure and doped sample is due to the loss of one water molecule resulting in the

transition from the „amorphous‟ product to γ–crystalline form [24].

Fig 6.19 TG – DTA curves of pure MAP crystals

141

Fig 6.20 TG-DTA curves of MAP crystals grown in presence of 0.2 M Ca

6.3.3.6 Vickers Microhardness Studies

The mechanical properties of renal calculi was determined, for the better

understanding the efficiency of stone fragmentation during ESWL, Vickers hardness

measurements were done and is used to study the resisting capacity of a stone against a

penetrating load. Vickers hardness indentations are made on the flat face of the

crystals at room temperature for loads 25, 50, 100 and 200 g using Vickers hardness

tester. The average value of the diagonal lengths of the indentation mark for each load is

used to calculate the hardness. The Vickers microhardness number is determined from the

relation

Hv = 1.8544 P/d2

Kg/mm2

where P is the load, d the length of the diagonal of the indentation impression and HV the

Vickers hardness.