Analytical Operation Skills

1、Analytical Operation SkillsContentsWeighing Use Electronic balance to get the accurate weight of standard or sample. Pipetting (volumetric pipette) Use pipette to measure and transport an accurate volume of solution from one container to another.Why are the two skills important?ARs work to attain th

2、e degree of precision and accuracyEvery standard begins with an accurately weighing with balance, an accurate dilution with pipetteEven a quality, calibrated balance or pipette can be unreliable if used incorrectlyBad habits ruin many of your resultsElectronic BalanceWorking PrincipleElectromagnetic

3、 balancing type (电磁力平衡) Electromagnetic force generated from a magnet and coil Balances in AR LabAnalytical Balance calibration range: 20mg-200g Accurate weighing Range: 20mg-100g (d=0.01mg) 100mg-200g (d=0.1mg)Balances in AR LabTop Loading calibration range: 1g-600g Accurate weighing range: 8.2-600

4、gBalances in AR LabMicro-balanceCalibration range: 2mg-2gAccurate Weighing Range: 2mg-2gBalances in AR LabUltra micro-balanceCalibration range: 1mg-2gAccurate Weighing Range:1mg-2gWeighingBalance StatusProcedureWeighing MethodsConsiderationBalance StatusEnvironment dustproof, shockproof, damp-proof,

5、 no significant temperature fluctuation keep the balance away from direct sunlight Flat, stable surface tablePower on for equilibrating at least 1 hourBalance StatusAdjust anchor bolts to keep bubble in the centerWeighing ProcedureDaily Calibration According to target weight, choose Balance with sui

6、table weight span Top-Loading: 1g-600g Analytical Balance: 20mg-200g Micro-Balance: 2mg-2g Ultra Micro-Balance: 1mg-2gMake sure the surroundings are clean. Make sure the pan is clean.Wear mask and glovesWeighing ProcedureUse tweezers or gloves to place the empty container you will use for the substa

7、nce to be measured on the balance platform. Weighing ProcedurePress the Tare button to automatically deduct the weight of the container from future calculations. Weighing ProcedureCarefully add the substance to the container. Weighing ProcedureWait for a while when the digital display is stable and

8、print the mass.Weighing ProcedureClean the pan and surroundings after weighing and tare the balance. balance brush bulb suck roll paper + MeOH/ACN Weighing Methods Direct weighing method (no hygroscopicity) 1. a suitable clean dry container on the pan 2. add target amount of sample to the container

9、3. weigh sample Weight loss weighing method (hygroscopic, volatile) 1. weigh total weight of sample and container Wt 2. transport certain amount of sample to target container 3. weigh the residual sample and container Wr 4. the obtained sample weight W=Wt-WrSpecial WeighingViscous Standard (Vit E) w

10、eighing boatLiquid Impurity Standard capillary tubeWeighing ConsiderationTemperature (2 ppm/C )Humidity, Wind, Shaky surfaceWeighing rangeBoth the inside and the outside should be kept clean Fingerprints and other greases from your hands Avoid Corrosive Chemicalsdrifting phenomenon of balance value

11、in one directionStatic electricity (40%RH)Sample close to the panImproper Weighing PracticeKnock on the panWindow openOverloadingWeighing paper too largeSweep powder into pan gapPipetting Rubber bulb volumetric pipetteThe Calibration ToleranceStandard of Accuracy-The capacity of tolerances for volum

12、etric pipette is that accepted by the National Institute of Standards and Technology (Class A). Use Class A volumetric pipette unless otherwise specified in the individual monography.Designated volume, mL125102550Limit of error, mL0.0060.0060.010.020.030.05Limit of error, %0.600.300.200.200.120.10Vo

13、lumetric Pipette CalibrationThe volumetric pipette is calibrated at 20C. Most pipettes are calibrated “to deliver”.T.D.-to deliverPipette should be drained in a vertical position and then touched against the wall of the receiving vessel to drain the tips.T.C.-to containPipette is called for in speci

14、al cases, generally for measuring viscous fluids like syrups; however, a volumetric flask may be substituted for a “to contain” pipette. In such case, the pipette should be washed clean. After drain and the washings added to the measured portion.Source of Pipette ErrorVolatile liquid evaporatesHigh

15、density liquid sinksViscous liquid sticksImmersed tips carry over liquidsAngled pipette take more liquidLiquid drops hold on the inner wall of pipetteHot or cold liquidHuman factorHow to use a volumetric pipetteExamine tip for breakagePre-wet the pipette by drawing up and dispending the chosen solut

16、ion a minimum of 3 times.How to use a volumetric pipette If your suction device is a rubber bulb, it ought NOT to be placed so that it is attached to the mouth of the pipette. How to use a volumetric pipetteAspirating Solution Squeeze the bulb Press against the mouth of pipette How to use a volumetr

17、ic pipetteImmerse tip in the solution to be measured (90 degree) Dont press the tip against the bottom 1-2cm from bottom Release the bulb slowly How to use a volumetric pipetteDraw up the liquid past several centimeters above the inscribed markDo not allow the solution to reach the bulb.Tip remains

18、beneath the surface How to use a volumetric pipetteQuickly remove the bulb and put your index finger over the open hole of the pipette How to use a volumetric pipetteWipe the tip to remove excess liquid outside the tipGently release the seal made by your finger until the bottom of the meniscus coinc

19、ides with the calibration mark on the pipette.How to use a volumetric pipetteDispensing Solution Drain pipette in a vertical position and then touched against the wall of the receiving VF (held at 45 degree angle) to drain the tipHow to use a volumetric pipetteA volumetric pipet should not be blown out to eject all liquid at the