Basic-tools - Basic tools/apparatus in Analytical Chemistry PDF

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Basic tools/apparatus in Analytical Chemistry...

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Basic tools/apparatus in Analytical Chemistry 1.

The Laboratory Notebook

Laboratory notebooks are records of analysis and show the raw data and any observation while the analysis is being done. It is a proof that an analysis was carried out by an analyst at a certain date and time and was reviewed by the head of the laboratory. The notebook, therefore, must have the signature of the analyst and of the head after each performance of a procedure. Date and time of receipt of sample must be reflected in the notebook. Computations may be shown in the notebook. Page number is an important feature of an organized laboratory notebook. This important document should stand court scrutiny in case of disputes and legal matters. Writings should stay on pages if ever liquids spill on the it – writing with “sign pens” should be avoided. Laboratory report is a separate document. It is the output of the analysis where the constituents or components of the sample are shown (for qualitative) and with appropriate units (for quantitative analysis). It is best to show the standard, highlighting those parameters that deviated from the standard. Retention time of these records should be determined by the organization. 2.

Volumetric Glasswares

From left to right: Burette, volumetric pipette, measuring pipette, volumetric flask, graduated cylinder

DISCUSSION: General guidelines for using volumetric glasswares: 1.

The liquid in the volumetric glasswares should be read as follows:

a. Ensure that the glassware containing the liquid is at your eye-level to avoid parallax error. Parallax is a deceptive change of the position of an object which is observed while the position of the observer changes. b.

For colorless liquid, read at the lower meniscus.

Meniscus is the curve seen at the top of the liquid in response to its container, caused by the property of liquid - surface tension. c.

For colored liquid, read at the upper meniscus.

2. Always rinse the glassware with the the liquid to be measured at least twice before using it. 3. Pipette (or pipet) DISCUSSION: Pipette is used to transfer a particular volume of liquid. Two types of Pipettes: 1.

Volumetric or transfer pipette

2.

Measuring or graduated pipette.

a.

Clinical or serological pipette

Other classification of Pipettes: 1. Blowout: Usually those pipettes with double line on the top are blowout pipettes.

2.

Ordinary pipette – no blowing out is necessary.

In our laboratory, we have at least two types of 10mL pipettes. Please see the pictures below. Pay close attention to the tip of the pipettes.

Compare pipette A and pipette B. When you fill pipette A with liquid until 10 mL and you will dispense only 4 mL, you are to release the liquid until the 6mL mark. On the other hand, when you fill pipette B with liquid until the 0 mark and you will dispense only 4 mL, you are to release the liquid until the 4mL mark. Note that pipette A is easier to use when you will dispense liquid that are less than 10mL. Lesser error will be committed. Now, take a look at pipette C. If you are going to dispense a particular amount of liquid, you are not to empty the pipette, because the calibration stops at 10mL. From 10 mL until the tip of the pipette, there is no calibration anymore. So, when you will dispense a particular amount of liquid, say 10 mL, you are to fill the pipette until the 0 mark, release the liquid and stop releasing until the 10mL mark.

4.

Burette (or buret)

DISCUSSION: Burette is a laboratory glassware that dispenses certain amount of liquid accurately. It is often used in analytical chemistry for titrimetric analysis. Description of titration: Titration is the technique for determining the concentration of the unknown solution by a slow addition of solution of known concentration. The analyte (or titrand) is substance being measured during titration. The sample is the the solution of unknown concentration, containing the analyte. On the other hand, the titrant (also called titrator) is the prepared solution of known concentration and is usually placed in the burette. The volume of titrant reacted is called titre. An indicator (usually phenolphthalein for acid-base titration) is added to the sample. When the indicator changed its color, the end-point is reached. The equivalence point in a titration is the point at which the added titrant is chemically equivalent completely to the analyte in the sample. This is the point where the chemical reaction is completed stoichiometrically. At this point, the sample is neutral. End-point of titration is when the indicator changed its color. Equivalent points comes before the end-point.

Rules for using burette: 1. Make sure that the stopcock can be easily be rotated. Lubricate when necessary. 2. Always test the burette for leaks in the stopcock before using it. A leaking burette will not give an accurate result. 3.

Rinse the burette twice with the titrant twice before using.

PROCEDURE: Practice in using the burette 1. Run the titrant through the burette. Ensure that the stopcock is closed while filling it with the titrant. Use water as titrant for practice. Discard the liquid. 2.

Fill the burette with the titrant until above “0” mark.

3. Open the stopcock and run through the liquid at the tip of the burette. Ensure that air is absent at the lower part of the burette. 4.

Get the initial reading and record on the laboratory notebook.

5.

Dispense about 10 mL of the liquid in an Erlenmeyer flask.

6.

Get the final reading and record.

7.

Repeat procedures 3 to 5, but this time, dispense 15 mL.

8.

Repeat procedures 3 to 5, but this time, dispense 5 mL.

Practice for titration: The teacher will demonstrate the technique. The student must do the titration.

1.

Fill the burette with 0.1 N NaOH. Record the initial reading.

2.

Pipette out about 5 mL of vinegar. Record as volume of analyte.

3.

Place 2 or 3 drops of phenolphthalein.

4. Titrate, adding NaOH slowly, shaking the analyte vigorously after each addition, until a permanent pale pink color is achieved. Record the final reading.

5. Subtract the initial reading from the final reading. Record the difference as volume of titrant. (Knowing the volume and the concentration of the titrant, and the volume of the sample, the approximate normality of the vinegar which can be calculated by this formula: C 1 V 1 = C 2V 2 where C1 = approximate concentration of NaOH V1 = volume of the titrant or NaOH V2 = volume of the vinegar C2 = approximate concentration of the vinegar (acetic acid) (the term “approximate” is used because NaOH and acetic acid was not standardized. Standardization of solution is discussed on Experiment 2) 1.

The Analytical Balance

Digital, top-loading balance for rough weighing

DISCUSSION: Weighing is the process of getting the mass of any object or substance. Mass is the amount of matter in an object while weight is the force exerted by the gravitational attraction on the object. There are two types of weighing being done in analytical chemistry: 1.

Rough weighing

2.

Accurate weighing

Rough weighing is normally used when the amount of substance to be weighed need only to be known to within a few percent. Examples are reagents to be dissolved and standardized later against a known standard. On the other hand, accurate weighing is done for samples that need high accuracy and is done on analytical balance. Examples are samples that are obtained for gravimetric analysis, those samples that are being dried to constant weight. The value from accurate weighing usually has more significant figures than the value obtained from rough weighing.

Types of electronic balance: 1.

The maximum mass that they can handle.

2.

The degree of accuracy with which they measure.

One type of electronic balance measures with an accuracy of 0.001g. This type of balance is sometimes referred to as milligram balance. This balance have glass doors to prevent air draft from affecting the accuracy of the weight. Other type has an accuracy of 0.1g and sometimes referred to as decigram balance. The analyst need to determine what type of balance to use. The accuracy is usually stated in the procedure. When using any balance, hard objects can be placed directly on the pan. For powders and liquids, a separate container should be used. When a container is used, the analyst can “tare-out” the weight of the container and the balance will give a reading of “0”. This process is called taring. When the analyst does not want to tare-out the weight of the container, weighing by difference is used. General Rules for Weighing 1. Never handle objects to be weighed with the fingers. A piece of clean paper or tongs should be used. 2.

Weigh at room temperature, avoiding air convection currents.

3. Never place chemicals directly on the pan, but weigh them in a vessel (weighing bottle, weighing dish) or on powder paper. Always brush spilled chemicals off immediately with a soft brush or dry tissue paper. 4. Always close the balance case door before making the weighing. Air currents will cause the balance to be steady. Important notes while weighing: Avoid weighing in excess by placing small amount of the substance and adding gradually by tapping the spatula. Have a separate container for the reagent being weighed where excess in weight will be placed. Do not return the excess substance to the container. Place the excess amount to a separate container. PROCEDURE: Practice in using the analytical balance: Operation: Calibrate the balance against a known standard.

a. Some analytical balances has a self-calibration features. Let the selfcalibration be finished before using. (Refer to the operation manual of the balance). Taring: Weighing solids:

1.

Place the weighing vessel on the pan of the

balance and tare. 2.

Add about 2.0 g of table salt. Record the exact mass.

3.

Transfer the weighed salt to a beaker.

4.

Repeat procedure 1 to 3, but this time, measure about 2.2 g.

5.

Repeat procedures 1 to 3 again. This time, measure about 2.5g.

Weighing liquid:

1.

Place a beaker on the pan of the balance and tare.

2.

By using a pipette, transfer about 5 mL of water

to the beaker. Use the top opening of the balance during transfer of liquid. 3.

Record the exact mass.

4.

Discard the water from the beaker and wipe dry the beaker.

5.

Repeat 1 to 4, this time, transfer 7 mL of water.

6.

Repeat 4, then 1 to 3, this time, transfer 9 mL of water....