Analytical Chemistry Lab Manual, Lab Reports of Chemistry

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DEPARTMENT OF CHEMISTRY & BIOCHEMISTRY

Analytical Chemistry Lab

Manual

Spring 201 8

2018 M. Prushan

Analytical Chemistry Lab

Lab Information

Before each lab session, you should prepare by reading the lab manual and the textbook required reading. We expect you to have a good understanding of the purpose, details of the procedure, the use of all chemicals and any significant hazards, and the underlying science of the experiment when you come to lab.

Each Pre-lab recitation will have a prelab. Although your lab notebooks will not be checked before lab drawing up tables in your lab notebook for your data and carrying out the preparation calculations (e.g., mass needed to make up solutions of given molarities) will help you finish quickly and efficiently.

Report Format Although you are not English majors, it is still very important to be able to communicate your science to others. Writing poor enough to be unclear or confusing will affect your grade, as will lack of regard for grammar, punctuation, and spelling. Learn to use the spell-check facility that is built into your word processing software. Here are some general comments to help you with your scientific writing. For more detail, we recommend the first chapter of the ACS Style Guide. Scientific writing is not literary writing. You should aim to be brief, precise, and unambiguous. The reader should clearly understand what you are trying to say. Try to keep your verb tense consistent and appropriate. You may use either passive or active voice, but try to be consistent. Avoid using jargon or slang and use full sentences. It is rare that you would need to use first person; i.e., try not to refer to “I”, “we”, “our”, “us”, nor should you speak about yourself, e.g., “the student”.

Title

The Title should describe the experiment in an appropriate, adequate and concise manner.

Abstract

Is a summary of the goals, objectives and the results of the experiment. It should convey a sense of the full report in a concise and effective manner. Remember, a reader should be able to read the abstract and get a full sense of what was accomplished in an experiment.

Introduction

Connects the basic scientific concepts being tested, or utilized in an experiment with the objectives and purpose of the experiment. Hypotheses and logical reasoning should be given.

using a line-of-best fit to data, report your correlation coefficient. If you Q-test away one of your three replicates, then you cannot report a standard deviation. That’s ok, just talk about it in your conclusion.

Questions

Write numbered, coherent answers to all questions found in the lab manual. Use complete sentences and be sure your answer demonstrates understanding of the material. Like the rest of your report, the discussion section must be in your own words. Plagiarism will not be tolerated.

Conclusions

The most scientifically important part of an experiment is your interpretation of your observations. For an analytical chemist, the accuracy and precision of your results is also important. These priorities are reflected in the allocation of some of your lab report grade to these topics. It may seem difficult for you to imagine what more there is to discuss once you have determined something like the % content of ascorbic acid in a Vitamin C tablet; however, here are some guidelines to help you:

Summarize the point of the experiment. (What did you do? Why did you do it?) Discuss your results. (Can you compare to literature values or values given by a manufacturer? Do they seem reasonable? Why or why not?) In the cases where values are available for comparison, you should quantify the difference between your experimental values and the expected values. Rather than simply saying you were “way off”, a comment such as your value was 15% larger than the literature value would be more appropriate. List the possible sources of error in your experiment, being as specific as you can in your error descriptions; e.g., rather than saying “operator error”, describe exactly what aspect of the experiment you may have done incorrectly. Discuss any problems encountered and suggest ways around them if the experiment was repeated. Evaluate possible sources of error (operator, random, systematic, etc.) in the experiment. Discuss the most significant contributions to the error in this experiment and describe specifically how each of these significant errors would have affected your results(i.e., increased or decreased your answers). Give reasonable methods to eliminate or reduce these errors. Consider the experiment in terms of accuracy, precision, reproducibility, selectivity, and analysis time. Suggest ways to improve your procedure if you were to perform it again.

References Whenever you use material written by other people to help you create your lab report, you should reference your literature source; e.g., when you look up literature values, when you use reference information from the library, or when you read some information in a textbook to help you answer a discussion question. You should reference this lab manual and your textbook whenever appropriate. All facts, formulas, drawings, graphs, etc. used in the report that you did not create yourself require a citation of their source. Failure to reference this material has an implicit implication that they represent your original work. You should read the Scholastic Dishonesty section of this manual to be sure you are clear the policies. Rarely, if ever, will you need to directly quote material. I grade your understanding of what you have read. The ability to

find an appropriate quote is only half the battle, you must then frame it in your own words, applied to the particular experiment you have performed.

Your references should be numbered in consecutive series in order of mention in your report. If a reference is repeated, do not give it a new number; use the original reference number. Reference numbers in the text should be superscripted outside the punctuation of the sentence or phrase to which the citations applies, or after the name of the author if referred to in the text; e.g.,

Quantitation of chlordane in freshwater fish has been reported previously.^1 When compared to previous reports of chlordane in fish,1 the results reported here …… Work of Doe et al.^1 has shown that …

The complete list of literature citations should be placed at the end of your report, after the conclusions, but before the appendices. List references in numerical order and use the following formats. More information on good reference formatting is explained in the ACS Style Guide, available at the library reference desk. Journal references must include the author names, abbreviated journal title, year of publication, volume number (if applicable), and the range of pages of the cited article; e.g.,

(1) Smith, A. D.; Moini, M. Anal. Chem. 2001, 73 , 240-246.

Book references must include the author or editor’s name, book title, publisher, city of publication, and year of publication; e.g.,

(2) Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis , 5th ed.; Saunders College: USA, 1998; Chapter 7.

Website references must include the author (if known), title of site from browser window title bar, URL, and date accessed; e.g.,

(3) U.S. Environmental Protection Agency Home Page, http://www.epa.gov/ (accessed July 1996).

digits. When quoting your answers , you want to show all the certain digits, and only the first uncertain digit, thus you should quote each value such that the last significant figure of your experimental value is of the same order of magnitude as the absolute uncertainty in that value. If your uncertainty has two significant digits (i.e., the first digit is a “1”), then you should quote the answer such that the last significant figure of your experimental value is of the same order of magnitude as the second significant digit of your uncertainty. When giving your results in a lab report, we would expect you to quote the original experimental data with appropriate significant figures, and to use a reasonable number of significant figures when quoting your final results. Never truncate or round numbers during your calculations , even if some of the intermediate numbers were rounded to significant figures for reporting purposes along the way. Some experiments are treacherously susceptible to error if this rule is not followed.

Introduction to the use of Analytical Equipment

Electronic Balances

Unless accuracy is not critical to the procedural step being performed, all weighing in analytical lab is

performed using a clean dry weighing bottle. Chemicals must not be weighed directly on the pan of any

balance. Only weighing bottles, weigh boats and weighing paper should contact the pan. Any chemical

that falls on the pan will be weighed along with the chemical in the container but will not be transferred

to your experiment, resulting in errors. Never try to weigh an object heavier than the capacity of your

balance (160 grams for our Mettler balances).

Temperature can affect the weight of an object. To be weighed accurately, all objects should be at room

temperature. A warm weighing bottle will set up convection currents inside the balance case, which will

buoy up the balance pan reducing the apparent weight. In addition to the weighing bottle and its

contents, a balance also measures the weight of the column of air above the weighing bottle and this air

is less dense above a hot object. Hot objects, such as crucibles, should always be cooled to room

temperature in a desiccator (to prevent adsorption of water) prior to weighing.

Weighing by Difference

All weighing in analytical lab is performed by difference. We do not use the TARE option on the electronic balances. This procedure explains how to create triplicate samples of the same solid using weighing by difference.

1. Keep your balance clean. Remove dust from the pan and floor of the balance with a brush before weighing your sample.
2. Weigh your weighing bottle with lid to get an approximate idea of its weight. Do not touch the

weighing bottle with your hands — wear gloves, or hold it using a collar of paper around the neck of the bottle. Close all doors on the balance before making a weight determination. Any air movement can cause variation in weight in the mg range.

3. Many solids increase their weight by taking up water vapor or carbon dioxide from the atmosphere. Some solids are provided predried in a desiccator. Remove the chemical from the desiccator. If you try and transfer chemical from the container while still in the desiccator you are likely to spill chemical into the desiccator. Keep the lid on the desiccator except when actually taking things in or out of it. Do not add chemical to the weighing bottle while it is still on the balance. Place the weighing bottle on the counter beside the balance, and use the spatula provided to transfer sample from its container to your weighing bottle. When you are finished return the chemical container to the desiccator.
4. The weighing bottle lid minimizes absorption of water vapor or carbon dioxide from the air during the weighing process. Use it. Weigh the weighing bottle along with its contents, including the lid, recording the weight using all significant figures. If the difference between this weight and the bottle weight recorded in Step 2 is not the quantity you require (roughly three times a single sample), remove the weighing bottle from the balance and add or remove chemical as appropriate. Do not return unused chemical back into the stock container. Put it in the solid waste.
5. Remove the weighing bottle from the balance and transfer approximately one-third of its contents without loss directly into the first container to be used in your experiment. To transfer the solid, tip the weighing bottle sideways over the container, and use the lid of the weighing bottle to tap gently on the side of the weighing bottle, until an appropriate amount of solid is transferred. Transfer should be made into a container with a large enough mouth (e.g., Erlenmeyer flask) that there is no danger of losing solid during the transfer.
6. Weigh the weighing bottle and its remaining contents, including the lid, recording the weight using all significant figures. The weight of your first transferred sample is the difference between this weight and the weight you measured in Step 4.
7. Remove the weighing bottle from the balance and transfer approximately one-half of the remaining contents without loss directly into the second container to be used in your experiment.
8. Weigh the weighing bottle and its remaining contents, including the lid, recording the weight using all significant figures. The weight of your second transferred sample is the difference between this weight and the weight you measured in Step 6.
9. Remove the weighing bottle from the balance and transfer the remaining contents without loss directly into the third container to be used in your experiment.
10. Weigh the weighing bottle and any residual solid, including the lid, recording the weight using all significant figures. The weight of your third transferred sample is the difference between this weight and the weight you measured in Step 8.
11. When you have finished using the balance, make sure that all weights are recorded and clearly labeled in your notebook, no objects are left on the balance pan, the balance and surrounding bench

surface of the barrel. A long buret brush may be used in the barrel. Some burets allow you to remove

the valve and buret tip to get easier access to both ends of the barrel. The buret is clean when there are

no obvious deposits of material and if the buret drains cleanly without leaving droplets of solution on its

inner walls. Clean the tip and stopcock by repeated rinsing. If the tip or stopcock becomes clogged, they

may be unclogged by carefully pushing a piece of small diameter wire through the openings. If

applicable, after cleaning, reassemble your buret. Care should be taken to assure that the stopcock is

firmly seated or leakage will occur. Test for leaks by closing the stopcock, half filling the buret with

distilled water, and watching for any liquid seepage.

Rinsing and Filling a Buret

Once your buret is clean and doesn't leak, it is ready for use. Solutions used in burets are usually standardized (i.e., their concentrations are precisely known), so in order to avoid dilution, you must rinse your buret with your solution before filling; thus:

1. Lower the buret in its holder so the top of the barrel is within easy reach. Using a funnel, pour ~5 mL into the buret from a convenient vessel (a beaker is much better than a 1 liter bottle).
2. Remove your buret from its holder; tip and rotate the barrel so that the solution is brought into contact with the entire inner surface of the buret.
3. Allow the solution to drain completely out through the tip.
4. Repeat with a second 5 mL wash.
5. Close the stopcock and fill the buret to within 1 cm of the top.
6. Let the solution sit in the barrel with the stopcock closed. Make sure that there are no leaks.
7. Make sure that there are no air bubbles in either the barrel or the tip. Tap the barrel lightly to dislodge air bubbles (this is an indication of a dirty buret). To remove air from the tip, open and close the stopcock fully to force the air out. You may have to do this several times.
8. Allow the solution to drain until it is within the graduated portion of the barrel.
9. Label your buret with its contents, so you don't get your burets confused.
10. Remove the funnel before taking any readings, or else it may drip additional solution into your barrel, destroying your volume measurement. Record the initial level of the solution in the buret. There is no reason to attempt to start each titration with the solution exactly at 0.00 mL; this practice wastes time and introduces operator bias.

Once a buret has been rinsed and filled, it may be refilled as many times as necessary that same day

with the same solution without repeating the above routine. Each day and with each change of solution,

the buret should be thoroughly washed with soap and water, rinsed with distilled water, rinsed with the

standard solution and filled as above. Burets are stored upside down so that material does not collect in

their tips.

Reading the Buret

Determinations using a buret involve an initial and final reading; the volume used is the difference

between these two readings. Readings must be made correctly and in the same way each time. Remove

any partial drops on the buret tip by touching the tip to the inner side of your titration flask. If you have

been delivering solution rapidly, with the stopcock fully opened, wait a few seconds for all the liquid to

run down the inside walls of the barrel before taking your reading. Readings are made from the bottom

of the meniscus (curvature of the liquid in the barrel) for most solutions. With highly colored solutions,

you may not see the meniscus and then any other consistent point of reference may be used. All

readings with a buret or other volumetric glassware must be made at eye level. If you have to look up

or down, there will be a serious parallax error involved. Raise or lower your buret in its holder to move

the meniscus to your eye level. Your buret is graduated in 0.1 mL intervals. Read it to the nearest 0.

mL. A plain white index card held behind the buret will make it easier to read. It is also a good idea to

place a white piece of paper under your titration flask if the endpoint is being detected by an indicator

colour change.

Dispensing Solutions

Always hold the valve joint when turning the stopcock , so the buret will not break and so that you do not pull the valve and tip out of your buret by holding it too tensely. A technique that permits speed, accuracy, and control is as follows: Face your buret with the stopcock handle on the right side of the barrel. Operate the stopcock with your left hand from behind the buret, while swirling your solution with your right hand. Your left thumb and forefinger grasp the stopcock handle, while the other fingers of that hand hold the valve joint and tip steady. ( Reverse for left-handers-like your professor!)

Treat your first titration as a rough measurement to learn the approximate volume of titrant that will need to be added to reach the endpoint. For subsequent titrations, you can fully open the stopcock at first, allowing a fast stream of solution to enter the titration flask. As the endpoint is approached, the solution should be added more slowly, until it is added dropwise to reach a "perfect" endpoint (each extra drop added results in ~ 0.05 mL of error). Careful stopcock control allows dropwise addition. Be careful to ensure that each drop enters the sample solution. If you touch off a partial drop from the buret tip onto the inner wall of your sample flask, remember to rinse the partial drop down into your sample solution using a wash bottle.