Lab Report (7) 1200 - Lecture notes 1-6 PDF

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Title: Identification of Unknowns (Part 7) Aim: By using both quantitative and qualitative techniques, the aim of this experiment was to identify both an unknown solid and an unknown liquid using IR and NMR techniques. The aim is to both understand and be capable of utilising IR and NMR techniques as well as the Karl Fischer titration and be able to interpret spectra to identify unknowns Materials and Methods: Chemical Acetone

CAS No. 67-64-1

Hazard Statements H225: Highly Flammable liquid and vapour H319: Causes serious eye irritation

Ammonium

1336-21-6

Hydroxide

H336: May Cause Drowsiness or dizziness H314: Causes severe skin burns and eye damage H400: Very toxic to aquatic life

Deuterium Chloroform 865-49-6 2,4 119-26-6

H228: Flammable Solid

dinitrophenylhydrazin

H302: Harmful if swallowed

e Potassium Bromide

2139626

H315: Causes skin irritation H319: Causes serious eye irritation

Siver Nitrate

7761-88-8

H335: May cause respiratory irritation H272: May Intensify fire; oxidizer H314: Causes severe skin burns and eye damage H410: Very toxic to aquatic life with lasting

Sodium Hydroxide Pure Hydranal Solvent

1310-73-2

effects H314: Causes severe skin burns and eye damage H225: Highly Flammable liquid and vapour H301: Toxic if swallowed H311: Toxic in contact with skin H331: Toxic if inhaled H314: Causes severe skin burns and eye damage H370: Causes damage to organs H360: May damage fertility or the unborn

child H225: Highly Flammable liquid and vapour

Hydranal Titrant 5

H301: Toxic if swallowed H311: Toxic in contact with skin H331: Toxic if inhaled Methanol

67-56-1

H370: Causes damage to organs H225: Highly Flammable liquid and vapour H301: Toxic if swallowed H311: Toxic in contact with skin H331: Toxic if inhaled H370: Causes damage to organs

The methods were carried out in accordance with laboratory manual CS555, chapter 7 Machines Used: Mettler-Toledo v20 volumetric KF titrator (Karl-Fischer) Shimadzu IR Affinity 1 Fourier Transform IR Spectrophotometer (IR) Introduction: For this experiment a number of different techniques were to be used in order to determine the unknown species of both the solid and liquid unknowns that we were given. To do this the water content of the samples had to be determined via the Karl Fischer technique. Water content is very important in determining chemical stability, reactivity and is a skill often used in the food industry which helps to determine shelf life and product quality. Titration via the Karl Fischer technique is widely used and is universally recognised as it is extremely accurate in determining trace amounts of water in substances. The automated reaction which takes place is a quantitative titration between water, a lower alcohol which was methanol in our case, an organic base, iodine and the sample. The reaction can be seen below. H2O+I2+SO2+3C5H5N --> 2(C5H5N+H)IC5H5N*SO3 + CH3OH -

+ C5H5N•SO3

(C5H5N+H)O-SO2•OCH3•

The reagent Hydranal is a noxious pyridine free reagent which was used to replace the organic base, noxious pyridine, and improve Karl Fischer titrations. Hydranal solvent allows

for high speed titrations with accurate results with stable end points. Hydranal also has a longer shelf-life than other organic bases and is safer to use. The machine can be used either as a volumetric titration method or as a coulometric titration method(1).The volumetric method uses a Karl Fischer solution that contains iodine being added until the first trace of excess iodine is detected. The water content is then determined by the amount of iodine that has been consumed. For the coulometric method, the iodine used in the reaction is generated in the titration vessel by electrochemical oxidation until again a trace amount of unreacted iodine is detected. For this method the electric charge consumed during the reaction is used to calculate the water content of the sample. Below is an image of the Karl Fischer titration machine which we used during the experiment. On top of the machine is the vessel where the titration occurs and solvents are mixed. Below there is a touch- screen where the mass of the unknown is entered and the resulting data is shown.

To add the unknown sample to the Karl Fischer a back-weighing technique was used. This involves weighing the substance in a small pipette or weighing boat and zeroing the scales. Then the substance is added to the titration vessel and the weighing boat or pipette is then weighed. The scales now show the weight that has been dispensed into the titration. Melting points are a physical property of solids that can be used to help identify a substance. For this experiment the melting point technique was used for additional information to help identify the solid unknown. Usually substances melt over a range of temperatures. This means that when the temperature range is narrow, the substance can be assumed to be

relatively pure. This also applies for the contrary with a wide temperature melting range suggesting an impure compound. (2) Infrared or IR spectroscopy is based on the absorption of radiation in the infrared section of the electromagnetic spectrum, from wavelengths 2500-16000 nm. This is at the low energy side of the scale with the radiation not having enough energy to excite the electrons but it does vibrate, rotate and stretch the molecules. The motional changes that the molecules experience are specific to different atoms which allow functional groups and compounds being tested to be identified by the spectrophotometer. Each spectrophotometer is made up of many different components, the main of which being a radiation source, beam splitter or chopper, monochromator and a finally a detector. Light is emitted from the source and is split into 2 beams. One beam is directed towards the sample cell and the other towards the reference cell. The absorbance then is measured via the detector and the difference found between the reference and sample cell. Below is a typical pathway for an IR Spectrophotometer.

Nuclear Magnetic Resonance or NMR is a very common method nowadays used for the identification of organic molecules. NMR uses the fact that atoms behave like magnets when place in a magnetic field and the spin of the nucleus can alter (3). Most elements that are tested in the NMR have a spin or I of ½. When the samples are under a magnetic field, the elements with a nuclear spin of ½ align with or against the magnetic field. The elements that align with the field have a low energy and a positive spin. The elements that align against the field are higher in energy with a negative spin. Radio waves are then applied which causes the lower energy nuclei to jump to the higher energy state. Then as the nuclei fall back

towards the lower energy state, the release in energy is measured. This data then can be used to identify the samples being tested. (3)

Discussion and Calculations: Part A For this part of the experiment the machine had been prepared for use upon our entry to the lab. Pre-titration had been completed so that the sample titrations were able to take place immediately. Firstly methanol was weighed out and titrated to find the water content percentage, results are shown below. Water Content of Methanol by Karl Fischer Titration Titration Number 1 2 3 4

Mass (g) 0.3168 0.3158 0.2901 0.2337

Water Content (%) 0.84 1.21 0.63 0.62

The second titration we carried out had much larger water content. We put this down to experimental error as we left the methanol exposed to the air for a much longer time than the other samples. This probably aloud the water content in the air to be absorbed which likely affected experimental accuracy. This is why we ran the titration again the fourth time to ensure we had three similar and reliable data results. Other than this data point the water content in the methanol was very low, averaging less than 0.7%.

Water Content of Solid Table Salt (NaCl) by Karl Fischer Titration Titration Number 1 2 3

Mass (g) 0.0675 0.2264 0.2824

Water Content (%) 0.55 0.09 0.05

Water content of salt was low as expected with a percentage well below 1% as predicted. Water Content of vanilla essence by Karl Fischer Titration Titration Number 1 2 3

Mass (g) 0.2086 0.1331 0.1815

Water Content (%) 8.31 7.93 8.89

For the vanilla extract titration a one in twenty dilution was taken. This was due to the extract being very concentrated and the possibility of this extending the run time of the titration. Part B Water Content of deuterated chloroform by Karl Fischer Titration Titration Number 1 2 3

Mass (g) 0.4428 0.4329 0.5009

Water Content (%) 0.12 0.18 0.14

Deuterated chloroform was used for this part of experiment instead of normal chloroform. Deuterated chloroform is an isotope of chloroform with the deuterium atom replacing the hydrogen atom. This form of chloroform is used mostly in NMR because of the reduction in interference caused by this isotope. Normal chloroform can exhibit a large peak during proton NMR, whereas the Deuterated chloroform does not interfere as much.

Water Content for KBr by Karl Fischer Titration Titration Number 1 2 3

Mass (g) 0.1618 0.1604 0.1955

Water Content (%) 0.09 0.08 0.04

While using the KBr it was ensure to try to restrict the KBr exposure to open air as it is highly susceptible to absorbing water. KBr

Part C IR Determination of Solid and Liquid unknown The melting point of C5 (solid Unknown) was 59-59.5 degrees Celsius and this showed it was of pure nature due to the solid melting over a small range of maybe half a degree. A background scan was also done so that interference could be disregarded. For the liquid unknown (A1), salt plates were used to find an IR spectra. For the unknown solid C5 spectra were found using KBr Discs. As this proved difficult to gain a high percentage of clarity the method then involved using nujol gel. Spectra were found with just the nujol gel and also running the nujol gel as background with the unknown C5 to reduce interference and get clear peaks for the unknowns. (See Bottom for Spectra)

Part D

NMR Analysis Spectra for unknown A1(above)

NMR Analysis Spectra for Unknown C5(Below)

A lack of personal knowledge on interpreting spectra’s led to us not being able to interpret the NMR spectra+ Part E Confirmation Tests Brady’s Test A1:

Yellow and cloudy => Cloudy Orange

This was a positive result for the liquid being either a ketone or an aldehyde C5:

Yellow => No change

This is a negative result for the solid being either a ketone or an aldehyde Tollens test (Silver mirror test) A1:

Turned a dullish grey with a slight mirror formed

This was classed as a positive result C5:

-

Personal Reflection: In my opinion this lab was enjoyable and the aims were met as I now fully understand how to work a Karl Fischer titration and IR spectrophotometer and also know the techniques associated with these machines such as making KBr disks and so forth. However my knowledge of identifying spectra’s was not enough and if I was to repeat this experiment I would spend more time researching these spectra’s and possibly ask for more assistance interpreting the graphs as this lack of knowledge prevented me from identifying the unknowns. Conclusion: For this experiment the Karl Fischer titrations all worked well with all water content percentages as expected. KBr had the lowest water content with an average percentage of 0.07% and the vanilla extract having the highest water content with an average percentage of 8.38%. This machine worked very accurately and efficiently and all results were within the expected range bar experimental error on our behalf.

Identifying our unknowns proved difficult. The solid unknown C5 has a melting point of 59 degrees Celsius and is not a keytone or aldehyde. The unknown liquid is an aldehyde but its exact structure is still not known. More personal knowledge of interpreting both IR and NMR spectra is needed to gain a structure from both unknowns.

References: 1. http://us.mt.com/us/en/home/supportive_content/application_editorials/Moisture_dete

rmination_by_Karl_Fischer_1.html 2. http://www.chem.umass.edu/~samal/269/mp.pdf

3. http://www.rsc.org/learn-chemistry/wiki/Introduction_to_NMR_spectroscopy Questions: 1. KBr discs are often chosen over nujol gel as a mineral oil used in the nujol preparation can cause interference as it has its own specific spectrum and can alter the results of the desired sample

2. Ordinary solvents with a H bond instead of a deuterium can cause large solvent absorption and interference with the NMR spectrum. This is why deuterated solvents are used as this isotope does not interfere as much.

3. Mass Spectroscopy could be also used to identify the structure of both unknowns. Mass spectroscopy measures the mass to charge ratio of ions and produces a mass spectrum which is then compared to many different structures so that a match may be found for the unknowns being tested. 4. An advantage of 13C-NMR is that it has a broader spectrum than 1H-NMR which simplifies the identification of unknowns. Carbon NMR has a spectrum of 0-200 ppm whereas 1HNMR has a spectrum of 0-12 ppm. A higher resolution can also be obtained using the carbon NMR however it may lead to longer scanning times and lower sensitivities due to the gamma radiation.

4. The optimum pH range when using the Karl fischer titration method is from 5 to 7. When the pH is lower there are less products formed and the reaction can take much longer to complete. If the pH is much higher the iodine is consumed and the end points may not be detected.

Appendices

IR Spectra for A1 unknown

IR spectra for C5 unknown and Nujol gel...