Spectrophotometry

Spectrophotometry: is a device to measure the amount of light in the material used. this device consists of two parts: the first is the light source, and the second is the photometer . the work principle of this device : the liquid or material that we want to measure the elements inside is placed in a tub, this tube is then placed between the light source and the photometer . So that the amount of light passing through the sample is measured by photometer. when a photometer is exposed to light, it acquires or generates an electrical signal that changes with the amount of light absorbed by the liquid . this change in light absorption depends on the change in the concentration of the substance. the way work this device: this device it measures the absorption of light by liquid materials at different wavelengths, and thus can identify a number of unknown substances or calculate known concentrations of materials . Stepped flow Technique:is a rapid mixing device, to study the kinetics of quick chemical reactions in solutions . this device contains two reactants which are kept in separate reservoirs and are prevented from flowing freely . the interaction starts by installing the reactants in the device. these materials are then released to the mixing chamber, which mixes these interacting materials , the reaction is then monitored by observing the change in the absorption of the reaction solution . When the reaction progresses, it fills the â€Å"stop syringe† that expands until it reaches the point at which the interaction reaches a continuous flow , thus stopping flow or interaction sodium reacts strongly and quickly with water and produces a solution of sodium hydroxide and hydrogen gas, a colorless solution. During the reaction sodium can be heated and It may ignite and burn with an orange flame . Hydrogen gas released during the combustion process reacts with oxygen in the air . The resulting solution is basic because of the melting of sodium in the water. this interaction between sodium and water is an exothermic reaction. sodium reaction with water is the closest to explosion. Na +2 H2O ?2 NaOH + H2. this search used this interaction and because it is fast, it uses the stepped-flow techniques method to control it Rate=- (d[Na])/dt=-1/2 (d[H2O])/dt=1/2 (d[NaOH])/dt+(d[H2])/dt Spectrophotometry Spectrophotometry is a vary specific type of Spectroscopy which measures how much light is absorbed by measuring the intensity of the light beam that is not absorbed (transmittance).The word â€Å"Spectra† means the range of wavelength, â€Å"Photo† means light or photons and â€Å"Metry† is the measuring how much light a chemical substance absorbs which it calls the absorbance. But what we do is measure how much light of the original light beam gets through (transmittance). So, those are related to each other absorbance and percent transmittance mathematically.The basic way to works is the incident light which usually contain different kind of wavelength, for example when we see something have a red color that means the object is absorbs all colors' wavelength except red. It is helpful to know the color wheel because the color wheel will help you to understand or getting the idea of where in the visible spectrum you would except to see the best absorbance.The work principle of spectrophotometry in (Figure 1): Firstly, we have a light source typically white light contains all wavelengths. We want to collimate the light or make all the wavelengths parallel to one another so, the special collimator or lens can does that, then we pass the light beam through a prism to splits the light into its various wavelength so, for regular white light you get all the colors of the rainbow. Spectrophotometer does not just shine all that light at the sample, it shines a vary specific wavelength of light and we can choose that normally by moving a slit in the way of the one wavelength of light or color that we would like to shine through the sample. That particular color of light will then shine through the sample, some of it will be absorbed and some of it will be transmitted. (Io) is the incident light that is the first enters, and (It) is the amount of light that is transmitted through after some has been absorbed. The remaining light that gets through hits a photocell, photocell is a solid-state detector that picks up how much light, then it prints out on a digital display either absorbance how much was taken away or percent transmittance how much light go through and those two are related. Briefly,  you can determine the unknown concentration of the sample by using Beer-Lambert Law which states: there is a linear relationship between the absorbance and the concentration of a sample. Mathematical formula of Beer's Law is: A=?lcA  is the measure of absorbance.?  is the molar extinction coefficient or molar absorptivity.l  is the path length.-439528256528center842086600left221268Figure 100Figure 1c  is the concentration (which is required).There are special techniques for investigating fast reactions which have half-live less than a few secondsLet us take an example for the simplest fast reaction technique (the continuous flow method) which will be used to study the kinetics of the formation of the ferric thiocyanate complex FeSCN+22120900145742100 For the fast reaction between ferric and thiocyanate ions in an acid solution of constant pH, the observed behavior is consistent with the simple mechanism: center2191301Where kf is the bimolecular forward rate constant and kr is the unimolecular reverse rate constant. So, the rate law from this equation is:center27279960Recall that the equilibrium constant K is related to the rate constant by:15775923297435 Where the sign ? means the equilibrium (t=?) value:31439213903453641206384715300At any time (t), Using these relations, and then rewrite the equation in the form:1965852489141700To simplify the integration of this equation, we will choose the experimental conditions such that [Fe+3] ;; [SCN-]. This will allow us to assume that [Fe+3] is essentially constant during the reaction. The initial conditions are chosen so that [FeSCN+2]0= at t=0 we find:This an approximate solution which becomes exact only when [Fe+3] is constant. In real practice, [Fe+3]0 will be chosen to be ten times larger than [SCN-]0, so that [Fe+3] will be more by about 10 percent during the reaction.2803525690943500-569595690918400If a plot of ln](FeSCN+2)? – (FeSCN+2)[ versus t is linear, then the first order dependence on [SCN-] and [FeSCN+2] is confirmed. The rate dependence on [Fe+3] has been established as first order. -5779714625Schematic diagram of system for driving reactant solution.00Schematic diagram of system for driving reactant solution.452856889798Spectrophotometry setup00Spectrophotometry setupProcedure for an example of use Spectrophotometer technique in fast reaction: Firstly, turn on the spectrophotometer and leave it warm up before using. The wavelength setting should be 455 nm throughout the entire experiment. With both reagent stopcocks A and B and the vent stopcock V closed, slowly increase the gas pressure on the reagent solutions until Bourdon pressure gauge indicates about 500 Torr pressures above 1 atm. With the outlet stopcock C open, open and close the reagent stopcocks A and B several times to make sure that both solutions are flowing smoothly and to remove any air bubbles from the system. Use a beaker to catch the outflow from the capillary tube. Then set the capillary frame at the first fiducial mark which nearest to the mixing chamber, and carry out the three following steps:1- Open Stopcock A and allow the Fe+3 solution to flow for a sufficient time to remove from the capillary tube any solution containing FeSCN+2 species. Then close stopcock A and the outlet stopcock C.2- Open the outlet stopcock C then turn both stopcocks A and B to their fully open positions. Catch the outflow of solution from the capillary in a beaker until the flow becomes stable. Then quickly switch the outlet tube from the beaker to a volumetric flask and simultaneously start a timer. When It is full, stop the timer and record the time. Return the outlet tube to the beaker. Then carrying out the above flow rate measurement, you should determine the absorbance A of the reaction mixture and record that value together with the distance x from the mixing chamber. Work quickly to avoid any interference of the reagent solution.3- When both the flow and absorbance measurements are complete, close the outlet stopcock C and then close both stopcock A and B. This is a crucial step in the procedure. If A and B are left open, solution may siphon from one carboy to the other. After a few minutes, determine the absorbance again to obtain the infinite time value. Verify that this value does not change after one more minute. For the next run, move the capillary support frame so as to line up the second fiducial mark and repeat the first and third steps at this this new distance setting, be careful in moving the capillary support frame.Make two runs at each of the six or seven positions along the capillary tube. Use special care in making the absorbance readings at large values of x. If time permits, you should also take data at a different driving pressure. Either increase or decrease the gas pressure depending on weather you need more data at low percent reaction or at high, but it may not be safe to exceed about 700 torr overpressures.In this experiment, more of solution A will be used up than solution B if the Fe+3 solution is always used in the first step to make the zero adjustment of the spectrophotometer at each distance setting. The resulting change in the liquid level for A relative to that for solution B may change the relative flow rates of these solutions. This can be avoided by alternating the use of solution A and B for making the zero adjustments.References:1- Physical chemistry by  Gilbert William Castellan.2- msu.edu.3- Wiley online library. 4- UKessay.5- AliHayek.com Spectrophotometry 5448300-52387500-523875-53340000 Kinetics Chemistry Student Name:Saba Ahmad Bin Humaid Supervisor:Dr. Alia Abdulaziz Alfi Group Number: 41438-1439 Spectrophotometry is a technique which can be used for identifying reactants' concentrations.Spectrophotometry is an absorbance device which can measures the fraction of the incident light transmitted through a solution. More clearly, it is used to measure the amount of light that passes through particles of the sample and by differentiation of the initial intensity of light reaching the sample, it indirectly measures the amount of light absorbed by that sample. Spectrophotometers are made to transmit light of narrow wavelength ranges. A certain compound will not absorb all wavelengths evenly that's why things have different colours. Some compounds absorb only wavelengths outside of the visible light spectrum and that's why there are colourless solutions such as water. Because different compounds absorb light at different wavelengths, a spectrophotometer can be used to differentiate compounds by analyzing the type of wavelengths absorbed by a given sample. In addition of that, the amount of light absorbed is directly proportional to the concentration of absorbing compounds in that sample, so a spectrophotometer can also be used to determine concentrations of compounds in solution.To studying a compound in solution by spectrophotometry, you put it in a sample holder called a cuvette and place it in the spectrophotometer. Light of a specific wavelength passes through the solution inside the cuvette and the amount of light transmitted or absorbed by the solution is measured by a light meter. While a spectrophotometer can exhibit measurements as either transmittance or absorbance, in biological applications we are usually interested in the absorbance of a given sample. Because other compounds in a solution (or the solvent itself) may absorb the same wavelengths as the compound being analysed, we compare the absorbance of our test solution to a reference blank. The reference blank should contain everything found in the sample solution except the substance you are trying to analyse or measure.Briefly,-5143507591425003467100758190000  you can determine the unknown concentration of the sample by using Beer Lambert Law which states: there is a linear relationship between the absorbance and the concentration of a sample. Mathematical formula of Beer's Law is: A=?lcWhere:A  is the measure of absorbance.?  is the molar extinction coefficient or molar absorptivity.l  is the path length.c  is the concentration (which is required).There are special techniques for investigating fast reactions which have half-live less than a few secondsLet us take an example for the simplest fast reaction technique (the continuous flow method) which will be used to study the kinetics of the formation of the ferric thiocyanate complex FeSCN+22120900145742100 For the fast reaction between ferric and thiocyanate ions in an acid solution of constant pH, the observed behavior is consistent with the simple mechanism: center2191301Where kf is the bimolecular forward rate constant and kr is the unimolecular reverse rate constant. So, the rate law from this equation is:center27279960 Recall that the equilibrium constant K is related to the rate constant by:15775923297435Where the sign ? means the equilibrium (t=?) value:31439213903453641206384715300At any time (t), Using these relations, and then rewrite the equation in the form:1965852489141700To simplify the integration of this equation, we will choose the experimental conditions such that [Fe+3] ;; [SCN-]. This will allow us to assume that [Fe+3] is essentially constant during the reaction. The initial conditions are chosen so that [FeSCN+2]0= at t=0 we find:This an approximate solution which becomes exact only when [Fe+3] is constant. In real practice, [Fe+3]0 will be chosen to be ten times larger than [SCN-]0, so that [Fe+3] will be more by about 10 percent during the reaction.2803525690943500-569595690918400If a plot of ln](FeSCN+2)? – (FeSCN+2)[ versus t is linear, then the first order dependence on [SCN-] and [FeSCN+2] is confirmed. The rate dependence on [Fe+3] has been established as first order. -5779714625Schematic diagram of system for driving reactant solution.00Schematic diagram of system for driving reactant solution.452856889798Spectrophotometry setup00Spectrophotometry setupProcedure for an example of use Spectrophotometer technique in fast reaction: Firstly, turn on the spectrophotometer and leave it warm up before using. The wavelength setting should be 455 nm throughout the entire experiment. With both reagent stopcocks A and B and the vent stopcock V closed, slowly increase the gas pressure on the reagent solutions until Bourdon pressure gauge indicates about 500 Torr pressures above 1 atm. With the outlet stopcock C open, open and close the reagent stopcocks A and B several times to make sure that both solutions are flowing smoothly and to remove any air bubbles from the system. Use a beaker to catch the outflow from the capillary tube. Then set the capillary frame at the first fiducial mark which nearest to the mixing chamber, and carry out the three following steps:1- Open Stopcock A and allow the Fe+3 solution to flow for a sufficient time to remove from the capillary tube any solution containing FeSCN+2 species. Then close stopcock A and the outlet stopcock C.2- Open the outlet stopcock C then turn both stopcocks A and B to their fully open positions. Catch the outflow of solution from the capillary in a beaker until the flow becomes stable. Then quickly switch the outlet tube from the beaker to a volumetric flask and simultaneously start a timer. When It is full, stop the timer and record the time. Return the outlet tube to the beaker. Then carrying out the above flow rate measurement, you should determine the absorbance A of the reaction mixture and record that value together with the distance x from the mixing chamber. Work quickly to avoid any interference of the reagent solution. 3- When both the flow and absorbance measurements are complete, close the outlet stopcock C and then close both stopcock A and B. This is a crucial step in the procedure. If A and B are left open, solution may siphon from one carboy to the other. After a few minutes, determine the absorbance again to obtain the infinite time value. Verify that this value does not change after one more minute.For the next run, move the capillary support frame so as to line up the second fiducial mark and repeat the first and third steps at this this new distance setting, be careful in moving the capillary support frame.Make two runs at each of the six or seven positions along the capillary tube. Use special care in making the absorbance readings at large values of x. If time permits, you should also take data at a different driving pressure. Either increase or decrease the gas pressure depending on weather you need more data at low percent reaction or at high, but it may not be safe to exceed about 700 torr overpressures.In this experiment, more of solution A will be used up than solution B if the Fe+3 solution is always used in the first step to make the zero adjustment of the spectrophotometer at each distance setting. The resulting change in the liquid level for A relative to that for solution B may change the relative flow rates of these solutions. This can be avoided by alternating the use of solution A and B for making the zero adjustments.References:1- Physical chemistry by  Gilbert William Castellan.2- msu.edu.3- Wiley online library. 4- UKessay.5- AliHayek.com