analytical (anal) lab final

Which of the following is an advantage to using the standard addition method?

-It accounts for any matrix affects. -Knowledge of the partition coefficient (K) is not needed. -There is no need to extract the total amount of ethanol.

What factors affect the separation of compounds in gas chromatography? (select all that apply)

-boiling point of compounds -flow rate of the carrier gas -Temperature of the column

Which of the following are true at the inflection point of the absorbance vs. pH plot?

-concentrations of acidic and basic forms are the same -pH = pKa

Which of the following statement(s) describe(s) buffer solutions? (select all that apply)

-they resist changes in ph -weak acid can resist strong base -weak base can reisst strong acid The pKa value(s) of the reagents selected and desired buffer pH, must be within +/- 1 pH unit of each other in order to prepare a proper buffer solution.

Describe how you would use the calibration curve (Figure 1) you created for your micropipette to correct your measurements. Also consider how you would use your calibration curve to adjust a measurement that falls in between two points. What would this assume?

-use the values in my "difference in volume measured vs volume contained" column (correction factor) -subtract the correction factor from relevant pipette reading. (For instance, if I pipetted 0.5 mL, I would subtract -0.014 mL.) find pipette reading and point on graph w cf, subtract from pipette volume. between 2 points: use the point on the graph between those points, or use the regression line to find a correction factor value.

exp 4 overview: preparation and investigation of buffers

-use theoretical (calculate w m1v1=m2v2 and H-H) and practical (add strong Acid/Base in increments while measuring pH) methods to create buffer solutions. use equations to analyze buffer capacity. create calibration curve and find correction factor...

There are many hazards associated with chemicals. Which of the following are the proper procedure to learn about the hazards you may encounter in lab (select all that apply).

. Your lab manual. B. Safety Data Sheet (SDS, for short) your ta

Calculate the partition coefficient if 1/3 of a 10% ethanol sample is extracted from the aqueous phase with 1-pentanol. (Hint: K = [CH3CH2OH]org / [CH3CH2OH]aq)

.5 pentanol : polar, aq ethanol: np, organic. 1/3 / 2/3 = .5

In one paragraph, describe how you would calibrate a 100-mL volumetric flask.

1. obtain the volumetric flask, rinse it with distilled water, and thoroughly dry 2. record the mass of the dry flask using an analytical balance. 3. fill the flask with distilled water, using an alcohol thermometer to monitor temperature and ensure it is constant. 4. read the meniscus on the flask to ensure I filled it to the correct volume. 5. weigh the full flask using the analytical balance and record the mass. I would repeat this process several times for accuracy, recording dry and full masses each time. The difference in masses between the dry flask and filled flask is the mass of water, & Using the known density of water (1g/mL), we can calculate the volume the flask contained.

You add 0.100 L of 0.20 M HCl to 0.100 L of 0.50 M formate (HCOO-). The pKa for HCOOH is 3.744. What is the pH of this solution?

3.92 log(.3/.2) + 3.744

Why did you perform a "gradient elution" in this experiment rather than simply running one mobile phase mixture?

A gradient elution was necessary in this experiment because we had to manually add in the three different mobile phases as the pigment elution progressed. This would be impossible using a single mobile phase, and then not all of our desired plant pigments would be able to elute from the column. Gradient elution is needed when there are a wide range of polarities of compounds that need to be eluted, which is why the composition of our three mobile phases gradually becomes more polar throughout the elution.

. In your own words, give an overview of how a pH meter operates. a. What does a pH probe directly measure? How? b. What must you do to convert your measured quantity to a pH?

A pH probe measures acidity or alkalinity of a solution. It directly measures the concentration of hydrogen ions by using reference and measurement electrodes. The reference electrode, an Ag rod coated with AgCl, uses a buffered solution that is adjunct to the liquid solution to produce a constant voltage. Meanwhile, the glass of the pH probe measures the voltage difference between the external solution and a neutral interior solution of KCl. you must calibrate the pH probe to convert the measured quantity to a pH

How did the color change of the bromothymol blue solution as the solution became more basic? How does this relate to the structure of the molecule?

As the solution became more basic, the color changed from yellow to teal and finally, to a blue that became more pronounced as the pH approached 10. This relates to the structure of bromothymol blue because its acidic form (HIn) is yellow, while the basic form (In-) is a royal blue color. When the pH becomes more basic and the acidic form is deprotonated, structural changes occur in the bonds of bromothymol blue (specifically, a 5-member ring is broken) causing a color change.

If chlorophyll a is the predominant pigment in spinach, explain why it is that its peak area is smaller than that of lutein, or even chlorophyll b, in your chromatogram at 450 nm? Hint: There is one major reason, what is it?

Chlorophyll a has two absorption peaks, one at 420 nm and one at 660 nm. The second peak at 660 nm is the largest difference from the detection wavelength at 450 nm out of all the pigments, so for this reason, the chlorophyll a peak area is smaller.

. Describe what you would expect to see in your chromatogram if a detection wavelength of 650 nm was used instead of 450 nm.

Fewer pigments would show prominent peaks, because most of the absorption peaks for the pigments were around 450 nm. Chlorophyll a and b would have distinct peaks, because their absorbance peaks are around 660 nm and 643 nm respectively (Table 3 of intro reading), but the rest of the pigments only peak around 400 nm, and therefore, would not be shown with a detection wavelength of 650 nm.

What are things that you should check when disposing of chemical waste (select all that apply)

For liquid waste, ensure the level of liquid is below the top of the bottle.Be sure that the lid of the waste container is completely closed, even if you know you will be returning to dispose of additional chemicals in a minute. open the funnel

What is happening to the concentrations of the acidic and basic forms of bromothymol blue as the pH increases? How do you know? Referencing your data, support your answer.

From Beer's Law, we know that Absorbance is directly proportional to concentration of a given substance. Based on Table 2, the absorbance (and therefore the concentration) of the acidic form is decreasing as pH increases, while the absorbance and concentration of the basic form increases with an increase in the pH.

You plan to analyze a beer sample for other alcohol impurities. You suspect the methanol and 1-propanol might be some of the contaminants. Including ethanol, water, and 1-pentanol, what would the elution order be for these five compounds? Support your answer with data and theory.

I believe that the elution order of these five compounds would be the following: methanol, ethanol, 1-propanol, water, and 1-pentanol. In Figure 9 of our introductory reading, we are provided with retention times for ethanol, water, and 1-pentanol. Using these known times and looking at the structure of methanol and 1-propanol, one can deduce the elution order of all five compounds. Methanol is very polar and not as sterically hindered as the other compounds, and has a lower boiling point, so it elutes first. Additionally, 1-propanol has a lower boiling point than 1-pentanol and water, so it will elute before they do. Compounds with higher boiling points take longer to reach the gas phase, so they elute more slowly than those with lower boiling points. We can also note that from Table 1 of our introductory reading, larger compounds will elute more slowly, and 1-propanol is smaller than 1-pentanol, so it makes sense for it to elute before 1-pentanol does.

In creating your calibration curve, you completed two trials. What would happen to the uncertainty if you increased the number of trials? There is a specific mathematical function that describes this. What is this function?

If we increased the number of trials performed, the uncertainty would decrease. due to the equation for uncertainty ( 2sd/n^(1/2)). so the uncertainty will decrease in proportion of 1/n^(1/2)

Generally speaking, describe what we needed to do to be able to determine the area under the peak for ethanol starting from an unprocessed chromatogram. Your descriptions do not need to include software specific information. What would happen if you did not take these steps and tried to determine the peak area? Support you answer.

In order to accurately determine the peak area for ethanol, beginning from an unprocessed chromatogram, we first had to use a baseline correction to ensure that our peak area aligned with y=0. We had to analyze the original graph and see how far off from the baseline our data was, and then use this number to add to our voltage values until we corrected the baseline to 0 V. If these steps were not taken before one tried to determine peak area, then the area of the peak would be inaccurate, because the software uses the x-axis to measure area, and additionally, some or all of the peak area would be negative.

When you are finished using a disposable glass Pasteur pipette, where should you dispose of it?

In the designated glass waste container in the lab room.

Excluding human error, consider one error that may have occurred during your experiments and describe how it would have affected your final result. Was this error random or systematic? For more information on error, see the Harris textbook.

It is possible that the pH probe had some degree of measurement error. This could have affected our final result by inaccurately measuring the pH of our buffer solution, which would cause us to add either too much or too little of our titrating agent in order to achieve our desired pH and would subsequently lead to inaccurate calculations for buffer capacity. This would be an example of a systematic error because it could have been calibrated for certain pH ranges, but could be less accurate for higher or lower ranges.

Compare and contrast the buffer capacity as a function of pH of the control experiments and the buffered solutions (Figure 2) over the pH range of 2-12.

Our buffered solutions produced S-shaped graphs as expected, with very little pH change at first, then a quick and drastic pH change. The buffer capacities for the control solutions were also generally larger than those of the theoretical solutions. For the control solutions, as the buffer capacity approached zero, the titration and accompanying pH change were also nearing completion. For our theoretical solutions, this occurred as well.

In one to two paragraphs, summarize the key results of the chromatograph. Points to consider: Were you able to see the separated pigments? What was the elution order?

Our chromatogram showed seven peaks, for each of the different pigment types. The pigments analyzed in this experiment included chlorophylls, carotenes and xanthophylls. Their order of elution was as expected for reversed-phase liquid chromatography: xanthophylls (most polar) eluted first, then chlorophylls, then carotenes (least polar). The peaks emerged well and had fairly good resolution and retention values, so our chromatogram was efficient, but separation could have been a bit better for a few of the peaks. We did encounter some errors while analyzing the data via our Vernier software, and had to obtain mock data because several of our peaks did not show at all. This could have been due to a variety of reasons, including poor calibration of the graphing software, the cuvette being dirty or improperly filled, issues with the spectroscopy instruments, or incorrect sample preparation. Next time, we could use greater care in preparing the sample, calibrating our equipment, and ensuring the cuvette is clean, adequately full, and free of air bubbles, as well as checking all of our equipment for any obvious issues or problems. We were a bit pressed for time, so this could also be a factor in why we encountered errors.

In a normal phase LC experiment, the stationary phase is _ and the mobile phase is _

SP: polar MP: nonpolar (reverse phase: SP nonpolar, MP polar)

You accidentally prick your finger with the end of a glass pipette, drawing a tiny drop of blood. What is the correct response?

Tell your TA immediately so they can fill out an accident report, then rinse your finger very well (15 minutes) with water, and put a band-aid on the injury.

If we switched the column to a polar column (normal-phase) AND reversed the order of the mobile phases (least- to-most polar), what would happen to the elution order? Why?

The elution order would be the same. Normal phase would be opposite elution order as our reversed-phase experiment (carotenes first, then chlorophylls, then xanthophylls), but if the order of mobile phases were also reversed, the elution order would once again return to xanthophylls first, then chlorophylls, and carotenes last. This is because with reversed order of the mobile phases, the mobile phases would become increasingly less polar, meaning that nonpolar pigments would elute first as well.'

What was the pKa you determined for bromothymol blue? How does your experimentally-determined pKa compare to the literature value? What was the percent error? What error(s) contributed to the discrepancies

The pKa value I determined for bromothymol blue is 7. I determined this from Figure 3, where absorbance versus pH at each form's λmax was graphed. The lines intersect at (7, .39), which means the absorbance is the same, therefore concentration of each species is the same, using first derivative plot is best way to determine pH. the pH at that point equals the pKa, which is 7. The literature value for the pKa of bromothymol blue is 7.1, which is fairly close to my experimental value (Sigma, 2021). The percent error is 1.41 percent, which is fairly low, but not perfect. Possible errors that could have contributed to this discrepancy include improper calibration of the Vernier tools, slightly inaccurate preparation of buffer solutions, etc.

Describe how the results obtained from the liquid-liquid extraction relate to the structure of the acidic and basic forms of bromothymol blue

The results show how the acidic and basic forms of bromothymol blue are structurally different, with different densities and solubilities. They are immiscible and formed distinct, separate layers, showing that each form of bromothymol blue has molecular differences. In the pH 5 sample, the top layer was colorless and the bottom layer was yellow, clearly separating the acidic form. In the pH 10 sample, the top layer was cloudy and colorless, and the bottom layer was a greenish blue. -layers different due to charge separation

1. Briefly summarize the sample preparation steps. What was the goal of this portion? What did you achieve in this step? Points to consider: What was being removed, generally speaking, during the extraction? You do not need to list the names of molecules removed, but rather a defining characteristic of the molecules removed. Why did you add acetonitrile after the extraction?

The sample was prepared by grinding up spinach extract along with acetone. Then, the spinach extract was incubated with acetone and filtered, which removed most of the plant pigments. This pigment extract is mixed with NaCl to create a two-phase system, with one aqueous layer and one organic layer. The NaCl serves to push the relatively nonpolar plant pigments into the organic phase for extraction. We added acetonitrile, a polar aprotic solvent, after the extraction in order to ensure that any remaining extract was flushed from the column.

In part 2 of this experiment, you created a standard addition calibration curve using beer sample A to determine the percent ethanol in beer sample A. You then used the same slope to determine the percent ethanol in samples B and C. Is this potentially problematic for the results? Why or why not? Hint: consider the reason to use standard addition versus a normal calibration curve.

Using the same slope from the standard addition calibration curve of beer sample A to determine percent ethanol in beer samples B and C is potentially problematic for our results . it is problematic because standard additions are used whenever matrix effects are high. Therefore, when we used to the same curve, we made an assumption that the matrix of the 3 beers is the same, which is not the case

Which of the following statements is/are correct:

When leaving the lab room with chemicals, I must place them on a cart, or in a secondary container to prevent chemicals from spilling in the hallways/elevator . When exiting the lab room I must first take off my gloves and dispose of them in the glove recycling containers provided on the same lab bench where I got my clean gloves.

Does the first derivative analysis (Figure 3) yield the same pKa value when using the absorbance values at the λmax of both the acidic and basic forms? Does this make sense? Why or why not?

Yes, the same pKa value is found using the absorbance values at λmax of the acidic and basic forms. Both of their inflection points occur near a pH of 7.3. This makes sense because a compound's pKa remains the same regardless if it is protonated or deprotonated, since the pKa is simply a measure of when a compound will accept or donate a proton.

You remember that you left your lab manual in your lab room yesterday after leaving Morehead. You stop by the lab room the next day before labs start and find that your lab room is open and no chemical work is being done. Which of the below statements are true?

You must find someone on the lab staff to accompany you into the lab room to pick up the manual that you left behind. D. You may email your TA and ask if they can go to the lab room and pick up your manual or meet you to escort you to the lab room.

f there is no chemical waste container in the main lab hood, what should you do?

You should immediately let your TA know that there is no waste container

Which of the following situations should you report to your TA if they occur in lab (select all that apply)?

You spill dilute acid on your arm. B. You or someone else in lab feels faint and/or has fainted C. Even though you were supposed to leave them on, you removed your goggles for a minute at your lab bench and in the process you think you got something in your eye. D. You burned your finger on a hot piece of glass

What is the correct way to mix water and acid?

add acid to water

If you are unsure about where to dispose of the waste from your experiment. What is the proper procedure you should follow?

ask TA

The eyewash in my lab is located:

at the front sink

Where should you place your backpack when you come to lab?

cabinets under whiteboard

__ is defined as the process of relating the actual physical quantity (such as volume) to the quantity indicated on the scale of an instrument.

calibration

Compound X is known to absorb at 510 nm. If you dilute a solution containing Compound X by 1/3, how does the absorbance change? The same cuvet is used for all measurements.

dec by 1/3

What are the forces that dictate the how the pigments will separate along the column?

dipole/dipole and electrostatic

While listening to your pre-lab lecture, you are not required to wear your safety goggles.

false

You are permitted to have a bottle of water outside your book bag in the lab room as long as it is securely capped or has a lid?

false

It's five minutes before your lab period begins and you realize that you are not properly dressed for lab. You could (choose all correct options):

go back home and change, buy scrubs at Morehead 102, aska friend to bring clothes

Molar absorptivity provides information about:

how much light a molecule absorbs at a particular wavelength

In which of the following locations are you required to have your lab goggles on and covering your eyes at all time?

in lab room and GC lab room

if a piece of glassware breaks during your experiment, the first thing you should do is:

inform TA

Where and when is it okay to sit in the floor in Morehead Labs? (Select all that apply)

none

Which of the following statements is correct about eating in the lab?

not allowed to eat anything in labroom

When obtaining reagents from the main lab hood for your experiment, which of the following indicated the proper technique?

pour a small amount into beaker or flask, cover, and bring back to lab bench

Two common parameters that are often used to describe the efficiency of a separation are ______ and _____.

resolution and relative retention time

In Figure 9 of the introductory reading, the retention times for ethanol, water, and 1-pentanol were 37 s, 65 s, and 180 s, respectively. Let's assume that you completed a chromatogram and you observe retention times of 30 s, 58 s, and 171 s for the same three peaks. What factor or factors could lead this result as it pertains to the instrument?

this could indicate several sources of error. One source could be a gas chromatograph column temperature that is slightly higher than expected, so the solvents reach their boiling points more quickly, thus eluting faster. Additionally, the stationary phase could be present in higher amounts than expected, so that the dipole-dipole interactions occur more quickly and the solvents elute faster. Retention times can also vary based on the flow rate of the Helium gas, and the individual column itself.

In reversed-phase liquid chromatography, you will begin your experiment with the most polar mobile phase and end with the least polar mobile phase.

true

My cell phone should remain put away at ALL times while in the lab room except when using it as a tool for the experiment OR using it to scan an assignment for submission.

true

The safety shower should only ever be used if a large volume of chemical is spilled on your skin.

true

Your lab coat must stay in the lab room the entire semester?

true

safety is always top priority

true

Separation in gas chromatography is based on:

volatility of components of a mixture and molecular interactions of compounds w the stationary phase due to differences in polarity

Was the experiment successful? What results were particularly significant? Were you able to calculate the pKa of bromothymol blue? Why is this important? Based on the error discussed, how could the experiment be improved?

yes, the experiment was successful. The experimental pKa value was especially significant, being very close to the theoretical value. Being able to calculate the pKa of bromothymol blue was important because similar applications of this technique can be used in technology identifying and classifying compounds solely based on their absorption spectra. To improve the experiment, you could always use more accurate and advanced lab equipment, thoroughly clean off the cuvette between uses, and use extra care in preparing the eight buffer solutions.

Mathematically buffer capacity is defined as:

β = - dCacid / dpH = dCbase / dpH