The Determination of the Amount of Lead in Brass by Means of AAS

A. Example of a lab report

The determination of the amount of lead in brass by means of AAS

Name: John Student Experimental Date: 5th of January 2004 Report Date: 12th of January 2004 Class: H2chemistry Experiment: 24

Summary The amount of lead in brass was detemined by means of Atomic Absorption Spectrometry at (1.98 ± 0.04) m/m %. According to the reference value1 of about 2 m/m% the result meet the requirements set. 1. Introduction Atomic Absorption Spectrometry (AAS) is based upon the absorption of light by atoms. When a sample is atomised a substantial fraction of the metallic constituents are reduced to gaseous atoms, depending upon the temperature of the atomiser. In Atomic Absorption an instrument like figure 1 is used to determine the absorbance.

HCL FLAME MONOCHROMATOR DETECTOR Figure 1: diagram of an AAS

The HCL (Hollow Cathode Lamp) is the most common source for atomic absorption measurements. In this experiment the cathode of the HCL is constructed of lead and after it is turned on it emitts a specific emission spectrum of lead. The most important and critical part in the AAS is the flame and the introduction of the sample in the flame. The most common atomisation device for atomic spectroscopy consists of a nebuliser and a burner. The nebuliser converts a liquid sample into a fine spray or aerosol, which is then fed into the flame. The most common fuel- oxidiser combination is air-acetylene, which produces a flame temperature of 2400-2700 K. When a hotter flame is required to atomise high-boiling element (refractory elements), acetylene and nitrous oxide are usually used. Droplets entering the flame evaporate; the remaining solid vaporises and decomposes in atoms. Many elements form oxides and hydroxides as they rise through the outer cone of the flame. Molecules do not have the same spectra an as atoms so the atomic signal is lowered. If the flame is relatively rich in fuel, excess carbon tends to reduce metal oxides and hydroxides and thereby the sensitivity is increased. A “lean” flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis. The position in the flame at which maximum atomic absorption is observed depends on the element being measured as well as on the flow rates of sample, fuel and oxidiser. The atoms are able to absorb a specific wavelength from the HCL and the amount absorbed is detected by the detector. The monochromator selects a narrow band of wavelengths to pass on to the detector2,3. 2. Experimental section

2.1 Procedure The procedure was in accordance with experiment 24 in reader 068-021.

The amount of Pb in brass is about 2 m/m%. The sample amount is 100 mg so 100 mg contains 2 mg of Pb. The final volume is 250 ml so the final concentration in the sample is 8 mg Pb/L.

One of the criteria when using a calibration curve is that the sample should be in the middle of the concentration range of the standard solutions. The linear working range for Pb with AAS is 15 mg/L2,3

1 so a calibration curve of 0, 1, 2, 5, 10 and 15 mg/L was prepared from a 100 mg Pb/L stock solution.

2.2 Apparatus 1. AAS: Philips PU9000, HLO nr: 03549 2. Mettler Toledo Analytical Balance, HLO nr : 03685

3. Results and calculations

Absorbance Standard mg Pb/L 1 2 3 Mean 1 0 0.001 0.002 0.001 0.001 2 1 0.045 0.041 0.048 0.045 3 2 0.085 0.085 0.086 0.081 4 5 0.194 0.195 0.190 0.193 5 10 0.415 0.418 0.421 0.418 6 15 0.589 0.578 0.595 0.587 Sample 1 0.320 0.308 0.331 0.320 Sample 2 0.340 0.350 0.335 0.342 Table 1: results of the standard solutions and samples

In figure 2 the resulting calibration curve is shown.

2 Determination of the amount of lead in brass

y = 0.0395x + 0.0036 e c n a b r

o 0,5 s b A

0,0 0 5 10 15 20 mg Pb/L

Determination of the amount of lead in brass

y = 0,0395x + 0,0036 e c n a b

r 0,5 o s b A

0,0 0 5 10 15 20 mg Pb/L

Figure 2: calibration curve showing the concentration of Pb versus the absorbance y = 0.0395b1 + 0.0036

Sample 1: y = 0.320 so the concentration is 8.00 mg/L Sample 2: y = 0.341 so the concentration is 8.54 mg/L

The m/m% Pb is brass is in sample 1:

The final volume was 250 ml, so 2.00 mg/250 ml. The sample weight was 100.6 mg:

2.00 Sample 1: x100%  1.99 m / m% 100.6

3 2.14 Sample 2 : x100%  1.97 m / m% 108.5

The amount of Pb in brass is (1.98±0.01) m/m% Error calculation

6 Standard were prepared and the absorbance was measured. Requirements: 1. uncertainty in x –values << uncertainty in the y-values 2. standard deviation in the y-values is deviated normally 3. independent measurements

Verification of the requirements 1, 2 and 3

1. ● estimation of the % relative uncertainty in the absorbance: 3-5% ● estimation of the % relative uncertainty in a standard solution

Standard 5: 10 mg/L:

Concentration of the stock solution: 0.3% Pipette of 10 ml: 0.2% Volumetric flask of 100 ml: 0.1%

The percentage relative uncertainty in standard 5 is:

(0.3)2  (0.2)2  (0.1)2  0.37%

The % relative uncertainty in the standard << 3-5% in the absorbance so the first requirement fits.

2. The absorbance varies from 0.04 to 0.6. If the standard deviation in each number is about 0.003 the relative uncertainty in the smaller concentrations is larger then in larger concentration. The requirement does not completely fit but the error calculation will be continued.

3. Each standard was prepared from the stock solution of 100 mg/L so the measurements were independent.

S y 1 1 ( y  y ) 2 S  x (   0 mean ) x0 2 2 b1 m n b1 (xi  xmean )

From EXCEL (see appendix II)

Sy/x = 0.0115 b1 = 0.0395 m = 3 (number of measurements of the sample) n = 6 (number of standards) 2 (y0-ymean) = 0.00964 2 (xi-xmean) = 173.5

0.0115 1 1 S  (   0.0356)  0.213 mg / L x0 0.0395 3 6

4 So the absolute uncertainty in the final result is 0.213 mg/L.

The relative uncertainty is 0.215/8.00 = 0.027

The absolute uncertainty in the result is 0.027 * 1.98 m/m% = 0.05 m/m%.

The final result is (1.98 ± 0.05) m/m%.

4. Discussion and conclusion

The amount of lead in brass is (1.98 ± 0.05) m/m%. The reference value1 is about 2 m/m% so the result meets the requirements set. The repeatability of the experiment is good, due to the small uncertainty in the final result.

5. Questions 1. In this determination there is no interference from other metals in the brass because in the AAS a Hollow Cathode Lamp with lead cathode was used. The emission spectrum of this lamp is characteristic for lead and only lead atoms can absorb the specific radition. 2. The solutions can not be stored in glass for a week because the glass acts as an ion exchanger and the concentration of the lead solutions become smaller.

6. Literature

1. “Practical Analytical Chemistry”, Arts, T and Kraan, W.J.; 068-02 2. “Quantitative Chemical Analysis”, Harris, D.C.; fifth edtion; W.H. Freeman and Company, New York 3. “Principles of Instrumental Analysis”, Skoog, D.A.; third edition; Saunders College Publishing, New York Appendix I calibration curve showing the concentration of Pb versus the absorbance II calculation carried out in EXCEL

5 B. The lab report

Purpose of a lab report A lab report illustrates the processes and gains in knowledge and skills gained during work in the laboratory. Lab reports are typically aimed at an audience within the company, rather than the academic world in general. One the hand, this record shows the lab user what he/she has done over the past period, and what this work has resulted in academically. Important discoveries during the procedures that were carried out as well as results that were obtained may be used in scientific articles or Standard Operating Procedures (SOP). On the other hand, managers may use these reports to determine how effective and efficient their lab workers are. Lab reports illustrate the complexity of the work involved in answering a simple question, which may explain the height of the bill resulting from the lab work.

Reports on student practicals familiarize students with the techniques of reporting clearly, logically and scientifically. Writing practical reports in a format like that used in writing research papers provides valuable practice for a future vocation. Although research may take months, practical classes are usually limited to a few afternoons. This means that most of the design, planning, and preparation of reagents and equipment is done by the lab supervisors. Although this reduces the real-world effect, writing up a lab report contains most of the elements of scientific writing. The main parts are:

- Title - Introduction - The experiment - Materials and Method - Results/Calculations (derivation of the formula) - Error analysis - Discussion/Questions (finished if possible) - Literature

Criteria Any lab report has to meet the following criteria in order to meet the purposes illustrated above. A lab report must be verifiable: all steps and findings must be described in such detail that any other lab worker can determine how the results were obtained. In addition, lab reports must be complete and allow replicability: any lab worker with the same materials, set-up and apparatus must be able to repeat the same actions and come up with the same results as illustrated in the lab work. Finally, the position of the work in the literature must be illustrated, as well as the context of the work done. Lab reports therefore need to briefly state why the question underlying the lab work is an important and relevant issue. In addition, the report must briefly illustrate what previous work has been done in this area, and what the outcome of the lab work means in terms of related findings from other research.

Types Lab reports follow the same overall structure, but vary in details depending on companies and contexts. Nevertheless, some trends emerge across disciplines.

In chemical contexts readers want to know beforehand how much is used during titration in order to get a certain yield. This is done in a pre-calculation in which estimates are made.

6 Lab reports in organic chemistry contexts typically describe the reaction mechanism in the introduction, show in the Results section yield, melting point and infrared, and do not go into error calculation. These reports are typically molecule-oriented. Analytical lab reports, however, have as main aim the determination of certain materials. In these the procedure details are much more important in the text.

In Physics you need to describe/draw the setup to boost your theory section. The results section lists in tables the findings, and absolute errors are given. Report the type of multimeters used. Give one example of a calculation using the derived formula. Compare in the conclusions the theoretical values to the values you have found and speculate on causes.

In medical contexts, lab reports need to have a title that closely matches the goal of the lab work, must have a date or indication marking the sequence in which the work was done, an exact description of procedures. Often several reports together form an investigation of a tissue sample, and only the final in the sequence has conclusions and reference to literature.

C. Conventions

Title Puns or snappy titles may be attractive, but are less scientific to your readers. The top of a lab report usually has the looks of a memo. In educational contexts (i.e. student practicals), the lab reports has additional information like the number of the experiment in the lab syllabus that the student has conducted.

Summary In the summary the results of the experiment are written down (or in a table) including the mean and the error calculated at 3.3. Write this for readers who have little time but want to read about your aim, design, procedures and findings.

1. Introduction State briefly what you did, why and when you did it, and the outline of your report so that you readers know where they find what information.

1.1 The aim State here what you intended to measure, test or what you want your reader to get out of it. Depending on the instructions, reformulate the expectations (or hypotheses) about the outcome of the lab experiment. You show (using references) why you think a certain finding may occur, or present reactions and prior calculations.

1.2 Theory

Show your readers you have checked research to date and that you can avoid errors others made in the past. the In this part the pre-calculations are made, for example how many ml of titrant should be used or the weight of the sample. In such cases derive a formula. Use the number for weight, concentration, percentage or titrant in the procedure.

1.3 Hypothesis

7 Based on your research in the literature you expect a certain answer to your question formulated under Aim. State your expectation here.

2. The experiment Here you state what you have done, after having given an outline of the structure you use. If the instructions given were not clear to you, clarify the written material in your own version. Information from discussions with lab technicians or from ‘Prelab’ sessions may boost your report if they are incorporated logically in the Materials and Procedure section.

2.1 Materials Your readers should be able to replicate your experiment(s) without much trouble. Do not write down all the chemicals that you used in your experiment, refer to the number of the experiment and the dictation. Only write down the relevant information, like: - standard solutions (date, concentration) - sample (brand name, quality, purity)

When you use other materials always write down the reason why and the materials you used. Materials like glassware (pipettes, measuring cylinders, rods) should not be written down.

If the materials are listed in the logbook you can refer to that.

2.2 Procedure

How to carry out an experiment is written down in the procedure but it is recommended to make a work scheme (including time) so you can plan the experiment efficiently. For instance, when your solutions have to boil for 30 minutes you can prepare other solutions, dilute solutions, determine the melting point or turn on a spectrophotometer. In this work scheme of yours dilution schemes can be written down. Sometimes the lecturer makes changes in the procedure and in your report these change must be noted including the reason(s) why.

2.3 Apparatus

Always write down the apparatus you used: brand name, type, parameters and HLO number. For example: - spectrophotometer Hewlett Packard 8453, HLO nr. 1000, Quarz cuvet (lightpath = 1 cm) - pH/mV-meter, Schoot CG 840 with a combined glass and Ag/AgCl electrode system (Schott), HLO nr 1001. - microscope Olympus, type RN 1, HLO nr 1002. - Etc.

Write down the apparatus in your logbook and refer to that.

2.4 Experimental set-up

3. Results In this section you report what you came across during your experiment. Here too you help your reader by briefly stating the outline you have used. Be correct and faithful, but also

8 avoiding giving raw data or the notes from your laboratory notebook. Digest your findings and report results that directly relate to the hypothesis presented in the Introduction.

3.1 Data

Write down the measurements in a table and also describe the visual observations like changes in colour, gas etc. All measurements and observations should be written down in a data-sheet. The original sheet is an appendix to the report and the carbon copy is submitted at the end of the practical session to the lecturer. Data should not be copied to a new sheet.

3.2 Calculations

If you measure two/three times describe your complete calculation only once to avoid overburdoning your readers. Substitute the measurements into the derived formula. The result is only reported from the other measurements.

3.3 Error calculation

Calculate the mean and error of all results. Always report the right significant result here as well as in the summary. It is not worthwhile to go beyond the fifth decimal, so 0,01982 ± 0,00008 is right.

4. Discussion and conclusion In this penultimate section you state what the findings imply for your hypothesis and original aim. First you report what the findings may mean for the hypothesis in the Introduction. Then you relate this to the literature consulted.

4.1 Implications in terms of theory What are the implications of your findings for the studies carried out so far?

4.2 Error discussion and error calculation How trustworthy are your findings? In each experiment an error calculation should be carried out. This error is an indication of reliability of the results. In many cases statistical methods are used to make an estimation of the reliability. On the other hand only two or three results, like on your lab work, are too little to perform official statistical calculations on. Before you start the experiment you can make an estimate of the maximum theoretical error by the use of a so-called error table (examples of these kind of errors can be found in the error analysis reader). At the end of the experiment the calculated error is to be compared with the theoretical error. If the calculated error  theoretical error, you may conclude that the results are reproductive.

4.3 Final conclusions Having done all that work and all that thinking, what do you think the readers need to know.

5. Questions

Describe clearly how the experiment may have been done better, or whatpossible doubts may be raised about your findings. Questions should always be addressed and answered in an

9 addition to the report. Always include the questions as a part of your answer. Always refer to the literature and explain your answer. Example: What kind of indicator was used? Answer: The kind of indicator used was methyl red [1], because the end point was at pH 6.0

6. Literature

If you use any literature in your report (theory, questions etc.) always check the reference is accurate and refers to the citation you use in the text. In order for others to find the literature you refer to, stick to the following customs:

Book: Writer; -Title- press; City: Publisher; (year), page

1. Skoog, D.A. and West, D.M.; - Fundamentals of Analytical Chemistry- 4th ed; New York: Holt e.a. (1986), 374

Journal Writer; Title, Journal (abbrevation); Volume (year), page

2. Bruins, A.P. e.a.; 'Measuring Ph', Journal of Chromatography 271 (1983), 71.

Appendices Put here the printouts obtained that would interrupt the flow of reading too much in the main text. These prints often illustrate to other specialists what other findings you came across. Do not forget to indicate what the x-axis and y-axis indicate. Legends help improve the legibility of your report. Illustration should state the date, name of experiment.

D. Tips

Lab reports are not the same as ‘lab journals’ ‘bench notes’, ‘worksheets’ or ‘notebooks’: the former are formal, outlining your findings, whereas the latter merely list your personal notes you made during your work in class, a discussion of errors and learning objectives. They provide information for lab journals.

Practicals are usually very short, thus not giving the student many opportunities for two or three drafts of the report. Since precision, clarity and conciseness must be adhered to, editing your report before submission helps. Asking a class mate to do so may improve your own understanding.

Do not forget to write in an impersonal style: avoid using personal pronouns, and stick to an academic style that avoids colloqualisms and slangy terms.

The introduction and discussion need to be written in the present tense, thus indicating the immediacy of the work. The method and results found are written in the past tense.

Headings may not exceed three levels in order to avoid cluttering the page. Titles like “1.3.3.1 The set up” are therefore to be avoided.

10 Conventions indicated above may vary with the traditions and customs of each lab or lab manager. To avoid disappointment, always verify beforehand the specific conventions used in your discipline or lab. If you wish to publish in a journal, check the January issues for the specifications for authors.

Cover letters (transmittal letters) often accompany reports and inform readers of a report's context. Typically, the letter includes information not found in the report. For example, when turning in a lab report, you would include the report's title, what assignment it fulfils and the due date. Check with your instructor to determine whether or not you should attach a transmittal letter to your report

E. Links http://www.rpi.edu/dept/llc/writecenter/web/labs.html Lab reports handouts (chemical, physics and electrical engineering) http://writing.eng.vt.edu/handbook/constraints.html Writing constrains for lab reports (mechanical engineering, computer engineering), style, standard formats, a checklist prior to submission. http://owl.english.perdue.edu/handouts/general/index.html Perdue University On Line Writing provides examples and assistance on many writing issues. http://filebox.vt.edu/eng/mech/writing/index.html Writing for Engineering and Science Handbook on Lab reports, thesis, memos, letters, CVs, style, punctuation, etc. (including lesson plans). http://writing.colostate.edu/references/documents/celab/index.cfm Civil Engineering lab reports (internal, external, error calibration, style, formats) http://writing.colostate.edu/references/documents/eelab/index.cfm (Lab reports in Electrical Engineering) http://www.io.com/%7Ehcexres/tcm1603/acchtml/acctoc.html Contents of textbook on technical and business writing. http://mel.lib.mi.us/science/writ.html State of Michigan website hosting a wealth of online resources for scientific and technical writing. http://cscwww.cats.ohiou.edu/esl/project/science.html University of Ohio Science & Technology projects page. http://www.rpi.edu/dept/llc/writecenter/web/labs.html English Lab report http://www.vlib.org/ The Virtual library on scientific disciplines. http://www.harcourt.com/dictionary/ The Harcourt online Science and Technology dictionary. http://www.geocities.com/albaruthenia/AS/dictiona.html Scientific Press, Dictionaries and Encyclopedias

F. References

Alley, M, (1996). The craft of Scientific Writing. Berlin, Springer Verlag. Dean, J, Jones A., Holmes, D, Reed, R, Weyers, J & Jones, (1996). A. Practical Skills in Chemistry. Harlow, Pearson Education Ltd. Gibaldi, J. (1999). The MLA Handbook for Writers of Research Papers. 5th edition. New York: Modern Language Association of America. Lindsay, D. (1996). A Guide to Scientific Writing. Melbourne: Addison Wesley Longman Australia Pty. Porush, D. (1995). A Short Guide to Writing about Science. Harlow, Longman. Shortland, M. & Gregory, J. (1991). Communicating Science. A Handbook. Harlow, Longman. Silyn-Roberts, H. (1996). Writing for Science. A practical Handbook for Science, Engineering and Technology Students. Harlow, Longman. Williams, J.M. (1990). Style: towards Clarity and Grace. Chicago: Chicago UP.