Lab TechniquesMichael ChoeBB69/20/07

Abstract

Three principle techniques were explored: quantitative transfer, dilutions, and volumetric

pipettes. In addition to these key techniques, the fundamentals of experimental chemistry

were also explored, such as reading the meniscus properly, proper laboratory notebook

documentation, and use of precision with Pasteur pipettes. Indicators were employed as

evaluating the precision of the chemist’s technique. In exploring the quantitative transfer

method, AgNO3 was used as an indicator and when added to the residual water no

cloudiness formed indicating that the quantitative transfer was successful. Accurate and

careful measurements are critical for the outcome of a chemical experiment, and these

techniques ensure greater accuracy and essentially more precise results.

Introduction

Reproduction and precision is the integral basis of the experiments performed

within the scientific community. The procedure of a well crafted experiment must be

specific and direct enough so that any scientist within the same community could repeat

the steps in order to produce similar, if not, exact results. Likewise, the procedure should

be carried out with near exact precision in order to prevent variances between the results

produced by a protocol and a repeated experiment.

This experiment is essentially an introduction to proper laboratory techniques. It

acts as an opportunity to practice correct documentation of laboratory results and to

establish proper format of a laboratory note book. In this experiment, several key

procedures were outlined as they include: quantitative transfer, dilutions, and volumetric

pipetting. In addition to these procedures proper techniques were introduced to ensure for

exact measurements.

Quantitative transfer is a method of precision in the sense that the chemist is

removing as much of the solution from one beaker and transferring it into another flask ,

beaker, or container using repeated rinses. However, in order to carry out this procedure,

the chemist should also use proper measuring techniques. For example, the water level

should be read at the meniscus with a white piece of paper behind the glass. This ensures

that the water level measurement is standard and accurate when documenting.

In verifying that the proper method for quantitative transfer was employed, a test

was carried out with 3 droplets of AgNO3 into a solution of KCl. Tests similar to this are

essential for experiments, as the provide verification of a successful experiment. The

follow reaction illustrates the formation of the white cloudiness in the presence of Cl-

ions.

Cl- + Ag+ => AgCl(s)

Dilutions are also important in experimental procedures. If a chemist wanted to

use a proper number of moles of a solute within a certain range or magnitude, a small

amount of solution would be drawn from a concentrated stock and diluted to obtain a

proper concentration. These techniques can be used frequently in acid and base

experiments but certainly not limited to them.

Volumetric pipettes are used to transfer precise amounts of solution from one

container to another. Proper use of a volumetric is simple in concept, but difficult to

master. These techniques are essential in producing accurate experiment results, because

if the incorrect amount is drawn, the results are going to produce numbers artificially

incorrect.

Quantitative transfer, dilutions, and volumetric pipetting are nearly ubiquitous in

chemistry experiments. For example, in a chemistry experiment titled “On the Acid-Base

Properties of Humic Acid in Soil”, the experiment dealt with an acid-base titration that

required dilutions, but more importantly precision. The experiments were carried out with

using stock solutions as the experiment stated “the acid-washed soils were stored as stock

suspensions in a refrigerator” pages 466-467 (1). Although the use of a volumetric

pipettes were not employed, it is clear that the volume of liquid used needed to be

accurately recorded and transferred as the figures of experiment were in the magnitudes

of mM (10-3 moles).

Experimental

The procedures of this experiment were conducted following the procedure of

“An Introduction to Chemical Systems in the Laboratory” pages 3-8 (2)

In weighing approximately 2.00g grams KCl, the actual weight of the KCl

measured was 2.02g

Results

Quantitative Transfer:

Adding AgNO3, produced no visible solid, nor did any cloudiness appear.

For the purposes of this lab the following equation describes the formation of the

white precipitate:

Cl- + Ag+ => AgCl(s)

Dilutions:

The first time adding the water droplets from the Pasteur pipettes into the

volumetric flask, the meniscus was below the line. After reexamining the water

level, an additional two drops of water were added. The meniscus was then visible

right above the line at eye level

Error Analysis

No mathematical analysis was completed

Discuss and Reason

This lab experiment was carried out as an opportunity to learn the fundamentals

of chemistry laboratory techniques and to exercise precision. Beakers, volumetric, flasks,

policeman stirring rods, graduated cylinders, Pasteur pipettes, and volumetric pipettes

were used to carryout basic quantitative transfers, dilutions, and proper use of a

volumetric pipet. Accuracy tests were employed to ensure the techniques were properly

executed. AgNO3 was added to the rinse succeeding the quantitative transfer to test of

residual KCl. White pieces of paper were place behind that glass of either a volumetric

flash or a graduated cylinder to test for the precise level of the meniscus.

In chemistry, every measurement should be exact or as accurate as possible.

Calculations are to be carried out with accordance to the accuracy of the experiment.

Therefore, to produce correct results and draw solid conclusions, good technique must be

utilized within the actual experiment. All observations, figures, and procedures are

essential and therefore, should be documented in a laboratory notebook. This lab is an

introduction to the fundamentals of good techniques in the lab.

Three main techniques were explored. Quantitative transfer was not a procedure

that produced quantitative results. However, a test could be used as an indication of

success or failure producing qualitative results. When adding water to KCl, the salt

dissolves in the separating into K+ and Cl- ions. In quantitative transfer, the objective was

to remove and transfer the solution from a beaker and into a volumetric flask. Rinsing the

beaker multiple times suspends residual ions from the beaker, stirring rod, and funnel and

allows for complete transfer of all the ions. However, if this procedure was performed

correctly, residual K+ and Cl- ions would remain in the suspension within the 150 ml

beaker after the preceding rinse. However, these ions are invisible to the naked eye so an

indicator must be added to test for residual ions. AgNO3 is used as an indicator that also

dissolves in water forming Ag+ and NO3- ions. When Ag+ comes in proximity of Cl- ions,

AgCl(s), a visible white cloudy substance will form. In experiment the performed, no

visible white precipitate formed. However, in the case of precipitate forming, AgNO3

indicates that residual Cl-. The possible cause of a failed quantitative transfer is a direct

result of failing to remove all Cl- ions. Perhaps the beaker, rod, and funnel were not

rinsed thoroughly enough. This experiment did not require the documentation of any

quantitative data. However, it should also be noted that 2.02 g of KCl was used.

Differences in mass may include the unevenness of weight distribution on the scale, but

are subjected more to the amount of KCl used.

When the quantitative transfer was completed, the dissolved KCl should have

been completely transferred in into a 250ml volumetric flask. However, for this

experiment, no numerical concentration was to be obtained. Therefore, this experiment

had no experimental quantitative data and thus analysis of error is inapplicable to this

experiment. However, for future experiments when considering the concentration of a

solution diluted, it is important to note that the volumes added to the flask are being done

in a drop wise fashion with Pasteur pipettes. As accurate as 250 ml volumetric flask was

the amount of water was not exact, because the addition volumes of water in a drop wise

fashion adds water in varying amounts for each drop. Therefore, the water level was

approximate so that the meniscus of the water level would not be exactly above the line

every time. The error on the part of the chemist, however, may include an impaired

perception. The meniscus must be read at eye level with a white piece of paper behind the

glass. The chemist may be unable to come to exact eye level with the meniscus, and thus

skews the perception. Any height difference while reading the meniscus results in a

variance of measurements and can ultimately lead to a variance in results.

The volumetric pipeting marked the last of the three experiments performed.

Volumetric pipettes were linked with a drawer to transfer 25ml of the KCl dilute three

times into a clean flask. This procedure required dexterity as the water level would drop

quickly when the drawer was detached from the tube. For the purposes of this

experiment, no data was collected. However, it should be noted that these pipettes were

far more accurate than the flasks as the pipettes were calibrated to at least near tenth of a

milliliter. In contrast, the flasks seem to be merely estimates. When water 25 ml of water

was drawn three times and placed into a 150 ml flask, the water level was well below the

75 ml mark where it should have been.

The essence of these exercises imparts a good understanding of proper technique

and precision to the chemist. The quantitative transfer gave a good understanding of the

importance of thorough transferring. The dilutions illustrated the importance of accuracy

in the volume of liquid added. The use of a volumetric pipette helps the chemist gain the

experience to accurately.

However, some questions remain. In the quantitative transfer, even if no visible

precipitate was formed, there must have been some amount of Cl- ions present. To what

extent can we consider the quantitative transfer accurate? For example, if there were

thirty Cl- ions present, surely no visible AgCl will form. However, if there were maybe

10,000 Cl- ions, visible precipitate might form. To what extent is this indicator accurate?

In other words, to what extent is this method of quantitative transfer accurate? It must be

accurate for the experiments conducted in this course. However, will it be accurate

enough to conduct research outside this course? In the bigger scheme of this lab, how

accurate are these methods that were taught in this lab? Although the answers to these

questions are unclear, following these techniques along with proper documentation of

laboratory observations will ensure greater accuracy in the results produced within the

scope of future chemistry experiments.

References

(1) Cooke, J. D.; Hamilton-Taylor, J.; Tipping, E. Environ. Sci. Technol. ACS. 2007; 41(2); 465-470.

(2) Chemistry 203/205 “An Introduction to Chemical Systems in the Laboratory”, Stipes Publishing

Company, Champaign, IL .2007-2009; 3-8