AbstractBi5O7Br needles and crystals of NaBi6O9Br were grown by means of chemical reaction in an aqueous solution. Series of solutions were prepared to obtain the desired concentration of the various solutions. After a period of one week crystals and needles crystallized out of the solution and using optical microscopy x-ray defractometer and the matchl software it was possible to identify the phases involved in the crystals and needles.

Table of ContentsAbstract..................................................................................................................................................1

1.0 Introduction................................................................................................................................2

2.0 Aim of Experiment.....................................................................................................................2

2.1 Theory..........................................................................................................................................2

2.1.1 Molarity.................................................................................................................................2

2.1.2 Normality:.............................................................................................................................3

2.1.3 Yield:.....................................................................................................................................3

3.0 Experimental Set Up........................................................................................................................3

3.1 Apparatus and Materials...............................................................................................................3

3.2 Experimental Procedure...............................................................................................................4

3.2.1 Preparation of NaOH Solution..............................................................................................4

3.2.2 The Preparation of the [BiBr4-].............................................................................................4

4.0 Results..............................................................................................................................................6

4.1 Optical Microscopic observations................................................................................................6

4.2 XRD analysis................................................................................................................................7

5. Conclusion........................................................................................................................................10

6. Reference.........................................................................................................................................10

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1.0 Introduction Crystallization from a saturated aqueous solution can occur either by gradually reducing the temperature of the solution for compounds whose solubility depend on temperature or by supersaturating the solution at constant temperature for compounds whose solubility are independent on the temperature. Crystal growth by chemical reaction is a solution growth technique whereby two soluble reactants yield a product which is less soluble at the same temperature. Therefore Crystallization by chemical reactions can be used when the chemical reaction yields a substance whose solubility is less than those of the reacting substances. In the present experiment small crystals are grown from aqueous solution at constant temperature and with increasing solvent amount by the mixing of two reactant partners. The reactions of these types are usually very fast and super saturation can be rapidly attained yielding very fine grained crystals. Other ways of mixing the reagents include using a membrane, gas-solution, and gel-sel. These methods ensure orderly transport conditions and to make it easier to control the rate of reaction

The amount of a substance formed per unit time is determined by the rate of supply of reagents to the reaction zone, that is, by the rate of mixing. In this experiment one of the reagents is filled in a growth vessel while the other one is added by a burette drop by drop. The advantages of this method include the simple apparatus and technique and the possibility of preparing special crystals of substances which have difficulty in growth by direct crystallization. However, the disadvantages are also distinct. The slowness of the process and the small size of the crystals restrain the application of this method.

2.0 Aim of ExperimentThe aim of this experiment is to grow crystals of NaBi6O9Br and needles of Bi5O7Br by means of chemical reaction in an aqueous solution.

2.1 TheoryThis experiment is based on chemical reactions and it will be wise to define or clarify some terms. Some properties in chemistry such as concentration can be expressed in various ways such as ppm, molarity, kg/l and so on. Even though these terms all describe the concentration, they are no exactly the same therefore a brief definition of the expressions used in this experiment are given below.

2.1.1 Molarity

Molarity is an expression of concentration and can be defined as the number of moles of solute dissolved per liter of solution. It units therefore are moles per liter of solution and it is denoted by capital M

Molarity[M]=moles of solute/liter of solution

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One liter of a solution usually contains either slightly more or slightly less than 1 liter of solvent because the process of dissolution causes volume of liquid to increase or decrease. [JIR06]

2.1.2 Normality:

In simple terms the normality of a solution is a multiple of the molarity and usually takes the factor of one, two, and three and in rare cases above four and up to seven. Normality is a very useful expression of concentration in situations where solutions have to be compared based on the concentration of specific ions or the charge of the ions.

In chemistry, the normality of a solution is defined as the molar concentration Ci divided by an equivalence factor fed

There are three common areas where normality is used as a measure of concentration in solution:

In acid-base, normality is used to express the concentration of protons (H+) or hydroxide ions (OH−) in a solution. Here, 1/ fed is an integer value. Each solute can produce one or more equivalents of reactive species when dissolved.

In reduction and oxidation reactions, the equivalence factor describes the number of electrons that an oxidizing or reducing agent can accept or donate. 1/ fed can have a

In precipitation reactions, the equivalence factor represents the number of ions which will precipitate in a given reaction. 1/ fed is an integer value too.

2.1.3 Yield:

Yield, also referred to as a reaction yield, is the amount of product obtained in a chemical reaction. The absolute yield can be given as the weight in grams or in moles (molar yield).

3.0 Experimental Set Up

3.1 Apparatus and MaterialsSeveral materials and apparatus were used for this experiment. These include

Bi2O3

2N HBr NaOH Distilled water Thermometer Plaster containers Weighing scale

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Plastic foil burettes Microscope X-ray defractometer

3.2 Experimental ProcedureThe experiment requires the preparation of the following solution

10 ml 6N NaOH-solution 10 ml 11N NaOH-solution 10 ml 0,01N [BiBr4-] solution 10 ml 0,02N [BiBr4-] solution

3.2.1 Preparation of NaOH Solution

As mentioned above a pair of NaOH solutions was prepared, one with 6N normality and the other with 11N normality. A calculated amount of solid NaOH was dissolved in a certain volume of water to give 10ml of solution.

The calculation for the amount of solid NaOH to give the various normality were done as follows;

6N NaOH solution

6N NaOH=6mol/L NaOH

M(NaOH)=40g/mol

V=10ml

m(NaoH)=M*n=M*c*V=40g/mol*6mol/L*0.01L=2.4g=2400mg

11N NaOH solution

11N NaOH=11mol/L NaOH

M(NaOH)=40g/mol

V=10ml

m(NaoH)=M*n=M*c*V=40g/mol*11mol/L*0.01L=4.4g=4400mg

3.2.2 The Preparation of the [BiBr4-]

The Preparation of the [BiBr4-] solution was don as follows;

[BiBr4-] is not available, so we use Bi2O3 which is dissolved in aqueous 2N HBr to get BiBr4-

The equation of the reaction is shown below:

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Bi2O3 + 8HBr = 2 BiBr4- + 2 H+ + 3H2O

M(HBr)=80.9g/mol

M(Bi2O3)=466g/mol

For 2N HBr, we need 0.25N Bi2O3 according to the equation above.

0.25N=0.25mol/L

V=10ml

M(Bi2O3)=466g/mol

m(Bi2O3)=0.25*M(Bi2O3)*V=0.25mol/L*466g/mol*10ml=1165mg

After mixing HBr and Bi2O3 we get 0.5 mole BiBr4- , so we still have to lower the concentration of BiBr4- to certain value.

0.01N of [BiBr4 - ] solution

0.5mol/0.01N=50

m(Bi2O3)=1165mg/50=23.3mg

0.02N of [BiBr4 - ] solution

0.5mol/0.02N=25

m(Bi2O3)=1165mg/25=46.6mg

the prepared solutions were mixed as follows:

10 ml 6N NaOH-solution with 10 ml 0,01N [BiBr4-] solution 10 ml 11N NaOH-solution with 10 ml 0,02N [BiBr4-] solution

The [BiBr4-] solutions were put in two plastic cups while the two NaOH solutions were poured into the burettes above the plastic containers and the NaOH solutions from the burettes were made to leak out drop by drop into the [BiBr4-] solutions. The drop rate was set to about 1.5drops per minute. During this phase of the experiment, measures were put in place to minimize evaporation as this might lead to super saturation. One of such measure was to cover the containers with a plastic foil.

The equations for these reactions are as follow;

10 BiBr4-+ 14 OH- = 2 Bi5O7Br+38 Br-+14 H+

6 BiBr4-+ 9 OH- +Na+ = NaBi6O9Br+23 Br-+9 H+

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4.0 ResultsAfter a week of reaction time, it was possible to observe some needles and crystals with the naked eye. Most of them were concentrated along the container walls, but a microscope was need for proper observation.

4.1 Optical Microscopic observationsWhiskers or needles were observed in the first solution as confirmed by the microscopic observations. Crystals were completing lacking. Figure 4.1.1 shows a picture of the needle-shaped crystals. The yield of the needles or whiskers was 18mg.

Fig. 1 Picture of needle-shaped crystals formed in solution

The second solution consisted entirely of crystals as revealed by optical microscopy. These crystals are ordered and possessing symmetry elements. It is visible that crystals are cubic rather than hexagonal (see Fig 4.1.2.2). Their diagonal length is about 0.15mm and the yield was 38mg.

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Fig. 2 picture of crystals of sample 2.

Fig.3 cubic crystals seen from diagonal direction

4.2 XRD analysisThe two samples, crystal and needles were ground into fine powder size and analyzed using the X-ray powder diffractometer shown below.

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Fig.4 X-ray powder diffractometer

The XRD machine was calibrated using pure silicon. After the calibration, our samples were introduced to the x-ray defractometer . A set step of 0.5°/sec and scan from 5°to 85°, were used and required almost 11 hours. After XRD measuring, we get our sample data.

The data gotten from the XRD analysis was matched with spectra from a data base using software called “matchl powder diffraction”. Matchl powder diffraction software can be used for phase identification from powder diffraction.

First the crystals were analysed. The background was removed and the curve was smoothened. Then the matchl data base was search to see if there are substances with the same spectrum like ours. By checking the FOM (form of merit value with bigger values better or closer to the analysed sample) it was identify some phases. A FOM value of one means the sample analysed is exactly the same as that in the matchl data base. If the FOM is less than one we proceed telling the software which elements are in the sample. In this experiment we used xpow and compared data with that of Mr.Keller who happens to have made a spectrum for this element

The analysis can be briefly concluded into several steps as follows:

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subtract the background smoth the curve search for diffraction peak

Two options:

direct match restrain optional element as Na,Br,Bi,O,N,H as H20 and N2 may effect the result. match the data to certain compound

At last, we define sample 1 as Bi5O7Br whose spectrum is shown below

Fig.5 spectral lines from matchl confirming sample one as Bi5O7Br

Sample 2 as NaBi6O9Br, which coincide what we expected.

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Fig.6 spectral lines from matchl confirming sample two as NaBi6O9Br

5. Conclusion Through the process of the experiment, we get a better understanding about the basic concepts of chemical reaction and basic experimental operation. This experiment exposed us to an intimate knowledge and experience of the X-ray diffraction analysis. Also, we applied crystal structure knowledge to the measurement of our production, which enhanced the impression of both.

6. Reference

[JIR06] Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "amount concentration, c". IUPAC Compendium of Chemical Terminology.

[KEL12] E. Keller, Skript „Experiment Z6: Crystal growth by chemical reaction in aqueous solution“, 2012, Crystallography Institute of Geoscience, Albert-Ludwigs-Universität Freiburg

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