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Philippine Science High School Main Campus Chemistry 2 ~ Laboratory Techniques

H....U... ,.lleI Chemica"

• Read the label very carefully to be sure that you get the chemical that you want (Ex: Na2S04 and

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• Ta~ your container to the distribution station, rather than ta~ing the supply bottle to your wor~ area.

• Do not place the cap or stopper of a.supply bottle on the des~ in such a way that the inside of the cap touches the des~. This prevents contaminating the inside of the bottle from a dirty des~ when the cap is retumed to the bottle.

• If you are transferring a closely controlled quantity of chemical, you can regulate the flow of solid from your scoop by holding the scoop over the receiver container and tapping your hand gently.

• Ifyou have removed too much chemica/' do not return the excess to the bottle, but throw it away; It follows that you should judge your requirement carefully and ta~ no more chemical than you need.

• Be sure to return the cap or stopper to the supply bottle when you are finished.

• If any solid has been spilled on the des~, clean it up before leaving the distribution station.

HcmcIIlng Lt...eI Chemica..

• The general procedures for handling solid chemicals apply to liqUids, too.

• To control the flow of liquid when pouring down from the source bottle (or vessel) to a beaher (or vessel) use stirring rod.

• When using a dropper for removing the liquid, be sure to hold the dropper vertically with the rubber bulb at the top so that the liquid does not drain into the bulb and become contaminated.

• If you are required to ta~e a small quant.lty of liquidfrom a bottle not fitted with a dropper and you wish to us~ your own, do not place your dropper into the supply bottle. The proper procedure is to pour some of the liquld'into small bea~rand then used your dropper to transfer the liquid from the beal:ler to your container.

• Excess liquid should be thrown away. Estimate your needs carefully to the excess can be hept to a minimum

• When liquids used in the laboratory are flammable, releasing harmful vapors, and other dangerous properties, it is best to worh in fume hood.

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• Volumetric laboratory equipment is calibrated to measure volume by sighting to the bottom of the meniscus.

• It is essential that the line of sight be perpendicular to the calibrated vessel if you are to read it accurately •

.........." .......... (MLGA - M.... Volv., Light, ,bl••, Ga., At.)

• The proper lighting procedure is to stri~e the match, open the gas valve at the laboratory desh . completely, and then move the match flame to the burner just below the tip of the barrel, letting the top of the flame creep over the top of the barrel to light the gas, control the amount of gas, then a~just the amount of air.

• Crucibles and test tubes may be heated directly in the flame

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• When you are heating a liquid in a beai:?er or flasR, the vessel should be placed on a wire screen with an asbestos center, which is mounted on a ring stand or tripod.

• Never, under any circumstances, heat calibrated volumetric glassware, such as graduated cylinders.

Usln. Pipette (RLDR - Rlntln" Loaclln" Deloaclln" Rlntln,)

• Avoid self-contamination! o . Never allow the liquid in a pipette to rise up to the bulb or aspirator. o For the same reason, do not rest the pipette on a dirty surface.

• Avoid cross-contamination! There are two sources of pipette cross';contamination: o A pipette that is used to eXtract one liquid must never be used to extract another! Doing so will

contaminate pipette and liquid. For example, never use a pipette to extract Mercuric thiocyanate Solution and then use it to extract Ferric Ion Solution.

o Watch that tip! When using a pipette to add liquid Xto liquid Y, do not allow the pipette tip to touch liquid Y or its container.

• For the most accurate results, avoid using the last milliliter of the pipette (ie. the bottom inch or so). As you have probably noticed, several drops of liquid get sturn in this tip, shortchanging your measurement. Therefore, a good rule of thumb is to fill the pipette with 1mL more than you need. For example, if you only need to measure 2mL of liquid, fill the pipette with 3mL, and then drain out 2 mL This way, you avoid the problem of the tip.

• For many jobs, it may be faster to pipette with your thumb, rather than with a bulb. In this case, the Simplest way to release liquid is to grasp the pipette in your fist. and use your thumb to firmly cover the top of the pipette. .

• Remember that the concept of meniscus applies to pipettes, too! • There are several good reasons not to use your mouth to suc!:? liquid into a pipette.

Me.turln, M... - Wel,hln,

• Chemicals are never weighed directly on the pan of a laboratory balance:The mass is determined by the process i:?nown as wel,hln, ., cllfference. A suitable container - a small beai:?er, or perhaps a test

tube that is used in the experiment- is wei.gked empty on a balance. The desired chemical is added to the container, the total mass of the combination is determined. By subtracting the mass of the empty container from the mass of the container plus the chemical, you find the mass of the chemical.

• Most common is the practice of placing a piece of paper on the pan of a balance, transferring the required quantity of chemical to the paper, and then transferring it to the vessel to be used in the experiment. If you use this technique to obtain a measured mass of the chemi~al, your first weighing should be of the paper with the chemical on it. Then transfer the chemical, and bring the paper bac!:? for {] second weighing.

• DO 0 . Allow hot objects to cool to room temperature before weighing

0 Close the side doors or hood of a milligram balance before weighing

0 Record all the digits allowed by the accuracy of the balance used, even if the last digit happens to be a zero on the right side of the decimal point.

• DON'T 0 Weigh objects that are warm or hot

0 Weigh objects that are wet 0 Weigh volatile liquids in uncovered vessels

0 Touch the object with your hand if you are using a milligram or analytical balance: your

fingerprints have weight. too! 0 Forget to checR the zero on a milligram balance after weighing 0 Forget to record the mass to as many digits as the accuracy of the balance allows- and no more.

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