The Laboratory Notebook, Laboratory Reports

The Laboratory Notebook, Laboratory Reports

The Laboratory Notebook, Laboratory Reports,

and General Laboratory Reference

AnSOP-001/3

Scientific work is usually recorded in two ways: first in a notebook, then in a report. Fundamentally, the notebook is the investigator's personal record, while the report is the means of communicating results to others.

The Notebook: Although a personal, working record, the notebook contains the most immediate and detailed description of the work. It is thus the last resort in cases of doubt or conflict, such as differing reports by others and questions of priority or patents. For this reason, certain practices are standard and will be expected in this course.

a.A Chemistry Notebook available in the bookstore. It has “carbon” pages. Use the removable periodic table and use it as a backing to prevent print-through.

b.Notes are made directly in the notebook in INK. Observations must be recorded at the time the experiment is carried out.

c.Nothing, once entered, is removed, erased, or obliterated. A single line is drawn through words or figures to indicate that they are invalid. If space is available, the new entry may be written directly above the old. If a whole section is to be marked out, use a single diagonal line. The reason for any extensive change should be given in a marginal note. Your change should also be initialed and dated.

d.The notes should be as brief as is consistent with clarity. They should be organized as far as foresight permits.

e.The record includes the date and also the name of any person with whom the work is done. Notebook pages should also we countersigned and dated.

The Report: The laboratory report is an organized presentation of the study that has been made. The following format should help:

A.FORMAL LAB REPORTS

Every report should include a title page that includes: 1) YOUR NAME, 2) DATE(S) experiment was performed, 3) LAB PARTNER (if any), 4) TITLE of the experiment, 5) LAB SECTION.

Formal reports ought to consist of six major sections:

I.Abstract

II.Introduction

III.Experimental / Procedure

IV.Data

V.Calculations

VI.Results and Discussion

I. Abstract

The abstract should be prepared last but presented first in your report. The abstract should be a brief summary of the key results of the work. It should state the methods used and any numerical results (with error) obtained. It is not a purpose or reason statement. It should also mention the unknown number if one was determined in the lab. There should be sufficient information in this section to assign a grade to your quantitative result. An abstract should tell the reader what was set out to accomplished, how it was done and the results.

II.Introduction

a.Should be concise, but there is not a specific length.

b.Should be a specific discussion of what will follow, including the purpose of the experiment and the approach.

c.Should include all non-trivial mathematicalformulas used in the experiment.

d.Should include balanced equations of chemical reactions, when appropriate.

e.Should not include any discussion of data.

III.Experimental

a.No specific length is recommended, but we suggest brevity.

b.Include a reference to any written, external procedures (NEVER RECOPY A WRITTEN PROCEDURE) and make note of any revision of the procedure, when necessary.

c.Your description should be qualitative, not quantitative. Try to keep "numbers" out of the procedure and in the "DATA" section.

IV.Data

a.Include all data in a logical manner so that it is readily accessible.

b.Tables are most useful, and if used, they should include a complete title, and all data presented must be well-labeled. Table legends should be at the top of the table and labeled in sequence using Roman Numerals. Figures, information presented in graphical or diagram format should have their legends at the bottom and be numbered in Arabic numerals. In fact, it cannot be over-emphasized that in any chemistry notebook or report, a dimensionless number in the absence of some explanation has no meaning. Use units!

c.Omit extraneous numbers/digits.

d.Observe the rules concerning significant figures. (Section C)

e.Numbers received from someone else should be included in final form only. However, credit must be plainly given to this person.

V.Calculations

a.Show all calculations.

b.Do a sample calculation, then summarize via tables.

c.Do not show arithmetic.

d.Use only significant figures; don't make answer too long or too short.

e.When you calculate an average, make sure that you are averaging the "end-product" of the calculations and not some intermediate piece of data.

VI.Results and Discussion

This component is your section. It should summarize the results of the calculations with enlightened remarks about the significance of your efforts with regard to Chemical Principles. Please don't rehash your introduction; evaluate results as to their reasonableness in view of experimentally known facts, such as consistency in a series of runs (standard deviation), and reference values. It should also discuss any sources of error you might encounter.

Notes

Reports are due one (1) week after completion of the experiment. In general, we suggest you keep them short and readable. Be sure to give the source of any reference values. You should submit your reports in a word processed format.

If the quality of the usage of the English language (grammar, spelling, construction) is not at an acceptable level, the report will be returned ungraded in order to be brought up to an acceptable standard.

B.GRAPHS AND GRAPHICAL ANALYSIS

Why is a graphical representation of data often useful? Graphs, accurately prepared, will reveal maxima, minima, inflection points, and the presence of irregular data. Other significant mathematical information is also easily derived.[a]

Surprisingly, people often ruin a good data set with poorly drawn or poorly composed graphs. A few suggestions may help you.

1.Choosing Graph Format

This depends on the nature of the data and the functional relation between dependent and independent variables. In most cases you will be required to use spreadsheets for analysis and graphical presentation. Excel is generally available to most students and is a powerful tool for presentation of plots. In almost all cases you will use XY (Scatter) format. (See Harris, Section 2-11 for procedures)

2.Preparing a graph

a.The scale for the independent variable should be plotted along the abscissa (X-axis).

b.The plotting function in Excel will usually scale properly. Since you are presenting data your plot should be free of non-information. As such, your background should be white and generally grid lines should be suppressed. (This is not the default condition in Excel)

c.Your data should be presented as points and a best-fit line should be presented as a line. You may use the trendline option for this but care should be taken with the default fit equation since the number of significant figures will often be incorrect. Also it is often necessary to have you line extrapolate to an axis intercept.

d.Obviously, each coordinate is to be labeled with the names and dimensions of the quantity being represented along the axis.

e.Finally, you should prepare a descriptive caption (25 words or fewer) that includes a more or less complete description of what the graph is intended to show. This caption should appear at the bottom of the figure. A title may appear at the top of the graph. If the data were shared or gathered by someone else, indicate so.

C.Significant Figures

The use of significant figures is one way of expressing the uncertainty or reliability of data. In this method, all digits that are certain and one additional uncertain (estimated ) digit are used. Many digital devices allow you to assume that the last digit is uncertain. To express the 3.1097 g on a scale that can weigh to one ten-thousandth of a gram, four digits, 3.109, are certain but the fifth digit is not (it is estimated). Thus, 3.1097 contains five significant figures. The number of significant digits has nothing to do with the magnitude of the number: 0.2065, 2.065, and 2065 each contain four significant figures.

Three general rules for determining significant figures are described below:

1.Zeros that merely indicate the magnitude of the measurement or the position of the decimal point are not significant. For example, in the number 0.00342, the three zeros are taken as not significant. This number could also be written 3.42 x 10-3. Zeros inserted between non zero digits are significant. If a zero is the last digit(s) of a number and is certain or estimated, then it is significant. Thus 1.0000 has five significant digits if measured on an instrument capable of measuring to one ten-thousandth. A general rule of thumb is that zeros which lie to the right of the decimal point and to the left of non-zero digits are not significant, e.g., 0.00342 = 3.42 x 10-3. These zeros simply indicate the order of magnitude of the number (position of the decimal point). Zeros to the right of non-zero digits are ambiguous. It is usually helpful to use exponential notation to indicate the number of significant figures (1 x 106 has one significant figure, but 1.000 x 106 has four significant figures).

2.When multiplying or dividing, the number of significant digits in the result must be rounded off to equal the number of significant digits in the number with the fewest number of significant digits used in the operation.

For example: 1.055 x 2.01 = 2.12055 = 2.12

3.When adding or subtracting, however, the precision of the result can only be as good as the least precise position in the numbers used in the operation.

For example:5.056 - 3.15 = 1.906 = 1.91

but,5.056 - 0.315 = 4.741

Since, in the first case, 3.15 is known only to two places beyond the decimal point, the result cannot be more precise than this. In the second case, both numbers are known to three places beyond the decimal point so the result is also known to that precision.

D.Evaluation of Data

1.Some common terms.

Accuracy vs. Precision. Precision refers to how well sets of values agree with each other. Accuracy is a measure of how well an experimental value agrees with the accepted (true) value.

Median vs. Mean. The mean (average) is the sum of a set of values divided by the number of values. (There are i separate values.  = "the sum of", " x = actual values.)


The median is the result about which the others are equally distributed and is seldom used in chemical analysis.

2.Precision. One can describe some of the standard methods of illustrating precision by considering the data on the following table:

DeviationDeviation

Sample%Cl-from meanfrom median

124.39 0.10 0.11

224.20-0.09-0.08

324.28 (median) 0.01 0.00

______

mean 24.29 0.07 0.06

a.Deviation is the numerical difference between any one value and the mean (or median).

b.Range is the numerical difference between the highest and lowest values. For %Cl-, the range is 0.19.

c.Relative precision. For sample 1, the relative deviation from the mean is calculated as follows:

0.10 x 1000= 4.1 parts per thousand

24.29

3.Accuracy

a.Absolute error is the difference between the observed (24.29) and the accepted (24.34) values.

24.29 - 24.34 = -0.05

(The negative sign indicates that the experimental value is low.)

b.Relative error is calculated as follows:

-0.05 x 100 = -0.2%

24.34

4.Types of Errors

a.Determinate errors. These errors can, in principle, be eliminated. Typical determinate errors include personal errors and instrument malfunction. Determinate errors can be proportional or constant. Proportional errors are dependent on sample size while constant errors will result in the same absolute error independent of amount of sample. Common methods for checking for determinate errors include varying sample size, analyzing a standard (known) sample, and comparing results with an independent method of analysis.

b.Indeterminate errors are unavoidable random fluctuations. These are always present in any measurement, and they tend to average out to some extent.

5.Distribution of Indeterminate Errors

a.Normal error curve (reflects indeterminate errors).

These errors will distribute in a “normal” fashion. That is if many determinations are made then the values will distribute around this mean value and the probability of values far removed from the mean are less likely. This results in the familiar bell curve.

b.Standard deviation.

1.The standard deviation, , is a statistically defined section of the distribution curve such that 68.3% of the results lie within + 1; 95.5% within + 2 and 97.3% within + 3. Thus,results deviating by 3 or more arevery improbable.

  1. The standard deviation is an important measure of the scatter or uncertainty of an experimental measurement. Its formal definition is that the

In simpler terms, the difference between each value in the series and the calculated mean

(xi - x) is computed and squared. The sum of these squares is then taken (not the sum of the sum) and is divided by n-1. The square root of this result is equal to the standard deviation for that series.

E.Volumetric Glassware

Many of the experiments will call for the use of various pieces of accurately calibrated glassware designed to measure a given volume of a liquid or solution. Such glassware is therefore called volumetric glassware. The calibration on a graduated cylinder is not of sufficient accuracy to be used in any quantitative analysis.

1.Volumetric Flask. The volumetric flask is commonly used for the preparation of solutions of known concentration or for making accurate dilutions. First place your sample (solid or solution) in the flask and fill with solvent to within two centimeters of the calibration line. (If attempting to dissolve a compound that goes slowly into solution it is sometime helpful to fill the flask only half full so you get the benefit of maximum turbulence in mixing. Then fill to the two centimeter mark). Stopper and invert several times until mixing is complete. Remove the stopper and add solvent, drop-wise, until the bottom of the meniscus of the liquid is up to the calibration line. After addition of all the solution, stopper and invert again.

In order to observe the liquid level clearly, draw a thin dark line on a piece of white paper (or index card) and hold it behind the neck of the flask in alignment with the calibration line.

2.Pipets. A pipet allows transfer of specific volume of solution. In order to draw the liquid up into the pipet, attach a pipet bulb to the top end, expel the air from the bulb and insert the pipet into the solution or liquid in question. Draw the solution well above the calibration line (but not into the pipet bulb). Have the forefinger of one hand at the base of the bulb, and as you remove the bulb, simultaneously slide your finger over the top of the pipet. Now by a simple release of pressure from your finger you may allow the liquid level to drop until it reaches the calibration line. The liquid flow is stopped by reapplication of the full pressure of your finger. You may wish to have your lab instructor demonstrate this technique.

NEVER PIPET BY MOUTH

3.Burets. The buret allows a controlled release of variable amounts of liquid or solution. In order to read the initial and final volume on the buret, use a piece of paper in back of the buret to make the meniscus easier to see. Proper titration technique requires development of a certain amount of motor skill. You should control the buret stopcock with one hand while swirling the flask to receive the solution with the other hand (in order to achieve continuous mixing). You should ask your lab instructor to demonstrate this technique.

AnSOP001/3 Release Date 16Nov 2005 1 of 6

[a] Experimental Physical Chemistry, Daniels et al., 6th Ed., McGraw-Hill, 1962, pp. 408-9.