Exercise 1 How sweet is that?

Transcription

1 CHM1011/1022 Student Laboratory Manual Exercise 1 - How sweet is that? Exercise 1 How sweet is that? The determination of sugar in lemonade by polarimetry and densitometry Aim To determine the amount of sucrose in lemonade samples using polarimetry and densitometry. After today's practical you should: 1. Have an understanding of: chirality, polarimetry, densitometry 2. Know how a polarimeter works and be able to use it to determine the concentration of sucrose in lemonade. 3. Be able to prepare a calibration curve and use this to determine an unknown concentration. Reading Chirality (Ch. 4) and carbohydrates (Ch. 16) W. H. Brown, in Introduction to Organic Chemistry, 2nd edn 2000 (Saunders College Publishing: Florida). Introduction Sugars Sugars are simple carbohydrates known as saccharides. Saccharides are classified according to their structure, from small single sugars (monosaccharides) up to long chains (polysaccharides). Sucrose (or table sugar) is a disaccharide (composed of glucose and fructose joined by a glycoside linkage see Figure 1) and is the most abundant disaccharide in the biological world. HO HO O glycoside linkage O O Figure 1 Sucrose (or table sugar) Sugars like sucrose are chiral molecules (they are not superimposable upon their mirror images see Figure 2) and they exhibit optical activity (they rotate plane polarised light). It is possible to measure optical activity using a polarimeter. Victorian Institute for Chemical Sciences Monash University

2 Exercise 1 - How sweet is that? Polarimetry Light can be thought of as waves vibrating in all directions perpendicular to the direction the light beam is travelling. A good visualisation is that a beam of light is like a wheel with infinite number of spokes (see Figure 3). If you pass light through a polarising filter (like a pair of polaroid sunglasses), the resulting polarised light is only vibrating in one direction. If another polarising filter was set at 90 to the first, no light would get through the second filter (this is why computer screens can look black when you're wearing sunglasses). Polarimeters use two polarising filters to quantify a chiral molecule's ability to rotate light (the specific rotation). CHM1011/1022 Student Laboratory Manual Figure 2 Polarised light Figure 3 Polarised light A polarimeter consists of a light source, two polarising filters, a sample tube and a detector (see Figure 4). The second polarising filter is able to rotate and is known as the analysing filter. If the sample tube is empty, light passes through the first polarising filter and will hit the analysing filter at the same angle. If the analysing filter is set at 90 to the polarising filter, no light will hit the detector. This position is set as 0 rotation (ie. no rotation is needed). Monash University 2 Victorian Institute for Chemical Sciences 2005

3 CHM1011/1022 Student Laboratory Manual Exercise 1 - How sweet is that? Figure 4 Polarimeter When an optically active solution is placed in the sample tube, the polarised light will be rotated by the solution and some light will hit the detector (the polarised light is not at 90 to analysing filter anymore). By rotating the analysing filter until no light hits the detector again, the observed rotation for the sample can be measured. If the analysing filter is moved clockwise, the compound is labelled dextrorotatory (from Latin dexter, on the right side) and the angle is written as positive. If the analysing filter is moved anti-clockwise, the compound is levorotatory (from Latin laevus, on the left side) and the angle is written as negative. The observed rotation for a compound depends on a number of things (concentration, the length of the sample tube, temperature, the solvent and the wavelength of light used). Specific rotation, [α], is defined as the observed rotation at a specific cell length (in decimetres) and sample concentration (in g/ml). The temperature, T (in C), and wavelength of light used, λ (in nm), are often recorded in super- and sub-script respectively. specific rotation = [ α ] T λ observed rotation (degrees) = length of sample tube (dm) concentration (g/ml) The specific rotation of aqueous solutions of sucrose at 20.0 C and a wavelength of 589 nm (the sodium "D" line, or the centre of the two yellow lines of the sodium lamp) is given by the ICUMSA (the International Commission for Uniform Methods of Sugar Analysis) as: [ ] α = ± Victorian Institute for Chemical Sciences Monash University

4 Exercise 1 - How sweet is that? CHM1011/1022 Student Laboratory Manual In the right conditions, it is possible to use this value (along with the observed rotation and the length of the sample tube) to determine an unknown concentration of a sucrose solution. An alternate method is to set up a calibration curve of known concentrations. Today we will generate a calibration curve using 5 known concentrations of sucrose, and use our curve to determine the amount of sucrose in lemonade. Densitometry Density (ρ, in kg/m 3 ) is a function of mass (m, in kg) and volume (V, in m 3 ): m ρ = V The density of distilled water at 25 C is 1.00 g/cm 3 (Note: non-si notation), whereas the density of a solution of sucrose is higher than this. For a constant volume, a calibration curve of the density of sucrose solutions can be prepared, and the concentration of sucrose in lemonade can be calculated. Pre-Lab Questions There are compulsory Pre-Lab questions. They are worth 5 marks and must be completed before your lab session. 1. Sucrose is made up of glucose and fructose joined by an α-1,2-gycoside bond. Draw the structures and Fischer diagrams of glucose and fructose. On the Fischer diagram, label any chiral centres with an asterisk (*). 2. Which of the following are chiral? screw, nail, bucket, foot, chair, propellor 3. If the observed rotation for a sucrose solution was found to be , what is the concentration of the solution? What assumptions have you made? 4. Could you determine the concentration of a solution of sucrose at 25 C if the observed rotation was found to be ? Why / why not? 5. In densitometry, why does it not matter if the inside of the sample vial is wet? Safety and the Environment The chemicals used in today's exercise are sucrose (or table sugar) and water. Along with lemonade (the test solution), all three of these substances are common household items. So how can you tell whether they're dangerous or not? The best way is to look up the material safety data sheet or MSDS. An MSDS is a form that contains data about a particular substance. An MSDS contains toxicity and hazard information such as health effects, handling and storage, first aid and emergency procedures, and disposal and spill management for the substance. Information about the physical and chemical properties of the material are also contained in the MSDS. The MSDS for a particular material focuses primarily on the hazards associated with working with the material in an occupational sense. For example, an MSDS for a Monash University 4 Victorian Institute for Chemical Sciences 2005

5 CHM1011/1022 Student Laboratory Manual Exercise 1 - How sweet is that? cleaning solution is not necessarily essential for someone who uses one bottle of cleaner per year, but it is extremely important for someone who uses the solution 35 hours a week in confined spaces. To show you what a material safety data sheet might look like, the MSDS of sucrose is included with your proforma. Use the MSDS to answer the following questions: 1. Before looking at the MSDS, would you consider sucrose (sugar) a hazardous substance? Why / why not? 2. In what circumstances is sucrose hazardous? 3. What would you do if you got sucrose in your eye? 4. How many grams of sucrose could you dissolve in 3 ml of water? Experimental Procedure Record all data on the provided proforma. Part A: Sucrose Standards 4. Label four 50 ml volumetric flasks with numbers 1 to Into separate flasks weigh approximately 2.5, 5.0, 7.5, and 10.0 g of sucrose. Record exactly how much you use (to 0.01 g) directly onto your proforma. 6. Dissolve the sucrose in distilled water, make each flask up to the mark and mix well. 7. Calculate the amount of sugar in each flask in g/100 ml. 8. Measure the temperature of one of your solutions. Part B: Lemonade Samples 1. Collect about 30 ml of lemonade. 2. Record the brand and the listed amount of sugar. 3. If the lemonade isn't flat, degas it by vigorously stirring it with a glass rod. Part C: Polarimetry 1. Collect a polarimetry sample tube and make sure it's clean. 2. Fill the tube with distilled water and place the sample tube into the instrument. Wait until the needle stabilises (5 10 s) and then record the observed rotation directly onto your proforma. This is the solvent blank. Note how much the needle fluctuates over 5 s this is the error. 3. Rinse the sample tube twice with 2 3 ml of solution 1 (into the sink). Fill the tube with solution 1 and measure the observed rotation using the same polarimeter. Record the value directly onto your proforma. 4. Empty the sample tube into a labelled beaker or conical flask. DO NOT throw out the solution you need it for densitometry. 5. Repeat steps 3 and 4 for remaining solutions (solutions 2 4 and lemonade samples). Note: Use the same polarimeter for all of your measurements! Victorian Institute for Chemical Sciences Monash University

6 Exercise 1 - How sweet is that? CHM1011/1022 Student Laboratory Manual 6. Plot concentration versus observed rotation for the standard solutions. 7. Using the observed rotation value obtained for the lemonade, determine the concentration of sucrose in the sample. Part D: Densitometry Note: Record all values directly onto your proforma. 1. Accurately weigh a sample vial with lid on the analytical balances (to four decimal places). 2. Carefully pipette 10 cm 3 of distilled water into the sample vial, put the lid on and accurately weigh the vial and solution. 3. Pipette another 10 cm 3 of distilled water into the same vial (do not empty). Put the lid on and accurately weigh the vial and solution on the same analytical balance. 4. Empty and rinse the sample vial. Repeat 1 4 for solutions 1, 2, 3 and 4 and the lemonade samples. Use the same analytical balance for all of your measurements! Note: Make sure you dry the outside of the sample vial before weighing. No need to dry the inside. 5. Estimate the errors for your measurements (10 cm 3 pipettes are volumetric glassware with 0.02 cm 3 uncertainty, whilst the uncertainty for an analytical balance is g). 6. Plot concentration versus mass for the standard solutions. 7. Using the density value obtained for the lemonade, determine the concentration of sucrose in the sample. Part E: Clean-up Thoroughly clean all your glassware with detergent and warm water. Discussion Questions 1. Compare the values obtained by polarimetry and densitometry for the amount of sucrose in lemonade. (eg. are they similar? different? which has larger errors?) 2. How does your value for the amount of sucrose (or sugar) in lemonade compare with the value listed on the bottle? Monash University 6 Victorian Institute for Chemical Sciences 2005