Calibrating Lab Instruments : Short useful guide

Calibrating Lab Instruments : Short useful guide, Bio Data Technology: Bioinformatics and Biotechnology directory

tives

Your performance will be satisfactory when you are able to

  • Calibrate a pH meter using standard pH buffers and an SOP

  • Use a pH meter to correctly determine the pH of an unknown solution

  • Calibrate an electronic balance using standard weights and an SOP

  • Use an electronic balance to obtain a desired mass of a substance

  • Compare the measuring accuracy of glassware using an electronic balance

  • LEAVE A CLEAN LAB AREA

Introduction

Different pieces of lab equipment are designed to measure properties such as temperature, pH, mass, and volume to varying degrees of accuracy. If the temperature markings on the side of a thermometer are not set accurately, the instrument’s measurements will not be accurate. The accuracy of these markings is due to the calibration or standardization of the thermometer. The standards used to calibrate a thermometer are freezing and boiling water.

Some equipment must be periodically recalibrated because the settings are not as immovable as lines on a graduated cylinder or thermometer. The calibration of instruments such as pH meters, electronic balances, and micropipetters can be rendered inaccurate by factors such as movement, humidity, electrical field changes, and many others.

In this lab, you will calibrate and use pH meters and electronic balances. Both of these instruments should be calibrated each time they are turned on. If they are left on for long periods of time or used frequently during a day, they should be periodically recalibrated during that time as well.

Materials

Each group: Class shares:

large waste beaker pH meter with SOP

25 mL of a solution of unknown pH electronic balance with SOP

10 mL graduated cylinder 200 g standard mass for balance calibration

50-mL beaker with 10-mL graduations pH standard buffers for meter calibration

100- or 150-mL beaker

5 mL pipette

10 mL pipette

Pipette bulb or filler

Beral pipettes



Procedure

A. Calibrating a pH meter

Several factors may affect the accuracy of a pH meter. First, most meters will only give accurate readings for solutions between -5 and 60 degrees Celsius. Second, a pH meter usually cannot measure a pH of 12 or greater accurately, and these high pH solutions can sometimes damage the electrode. Finally, solutions that have high sodium ion concentrations generally give erroneous results. While an electrode is designed to allow H+ ions to pass through its walls, Na+ ions may also pass through. The Na+ ions affect the electrical potential measured by the electrode, which causes inaccurate pH readings.

  1. Select a pH meter to calibrate and record its model number. Turn on and calibrate the pH meter using the SOP provided. You will be using pH 7, pH 10 and pH 4 standard buffers. The manufacturer often adds color to help identify the solutions; become familiar with these characteristic colors. Buffers are used because they have very stable pH value. Remember to always rinse the pH electrode with a wash bottle of distilled water or with the next solution before using it, catching the rinse liquid in a waste beaker labeled as such. Never rinse into the electrode storage solution or buffer.

  1. After calibration is complete, use the meter to measure the pH of an unknown solution. Swirl the solution slightly to ensure that it makes good contact with the electrode before recording a reading. Record the pH in your notebook and have the instructor check your answer before going to the next section.

B. Calibrating and Using an Electronic Balance

The standards used to calibrate electronic balances are objects of known mass. For balances that measure to ± 0.01g, the standard is usually a 200-gram weight. These balances are used to measure amounts over 0.05 g. When you place the 200-gram weight on the balance in calibration mode, the balance recognizes the weight as 200 grams, and will then use that information to measure other masses.

  1. Dispense 50 mL of orange stock solution into a 100- or 150-mL beaker.

GLP Tip: Never pipet from a stock solution. Rather, pour the approximate amount that you plan to use from a stock solution, and never return unused portions to the stock solution. This will prevent contamination.

  1. Select a balance and record the model number. Follow the instructions in the SOP provided for the balance model to calibrate the balance. Have the instructor look at the live readout with the 200-gram weight on the balance before moving to the next section.

  1. Place a weigh boat on the balance pan and press the tare button. This subtracts the weight of the weigh boat so that you are only weighing what is put inside it.

  1. Draw up 4 mL of orange water using a 5 mL pipette and pipette filler or bulb and deliver it into the weigh boat. You might need to practice drawing up and delivering the liquid back into the flask until you can do it smoothly. If these are blowout pipets, to get the most accurate measure you must blow out the last drop (see the lab equipment exercise for ways to distinguish between blowout and to deliver pipettes). If you have a pipette filler, roll the wheel up and down quickly several times; if you have a pipette bulb, squeeze the bulb after emptying the pipette. Another option is to measure 5 mL and then deliver only down to the 1 mL mark. Remember to discard the remaining liquid in your waste beaker, not back into the stock solution.

GLP Tip: When adjusting the volume of a pipette, the bottom of the meniscus should be even with the marking to which you are adjusting. Make sure that your eye is level with the marking and meniscus in order to make sure that they are lined up correctly. Keep your pipet perpendicular to the benchtop. Make sure that there are no droplets of liquid on the outside of the pipet before you transfer it, and touch the pipet tip to the side wall of the container as you are dispensing the liquid.

  1. Record the mass of the orange water in your notebook and in the analysis section. At sea level, 1.00 mL of water weighs 1.00 g. This should help you decide if your measurement was accurate.

  1. Have each member of the group repeat step 4 and add 4 mL of orange water to the weigh boat (do not empty the weigh boat between measurements). Divide the total mass by the number of people in the group to find the average mass. Show calculations in your notebook.



C. Measuring Accuracy of Glassware





GLP Tip: Always inspect glassware before use. This is especially important if you are handling hazardous materials or heating the glassware. Discard chipped or cracked glassware in a specially designated glass disposal box. Never throw away glassware in the trash, and never dispose of plastic pipettes or other non-glass items in the glass disposal box.



  1. Measure 10 mL of water in a 50 mL beaker using the lines on the beaker for your measurement. Tare a weigh boat on the balance. Pour the water in the weigh boat and record the mass in your lab notebook and on the analysis page. Each member of the group should repeat this step and record each measurement. Calculate the average mass of 10 mL of water measured with a beaker.

  1. Repeat step 1 using a 10 mL graduated cylinder and repeat step 1 again with a 50 mL graduated Cylinder.



  1. Repeat step 1 using a 10 mL pipette.

  1. Repeat step 1 using a 50 mL flask.



  1. Clean up your lab station. Pour the contents of the waste beaker down the sink with plenty of water, and return dirty glassware to the cart. Wipe off your lab bench with a wet paper towel. Make sure that the balances, pH meters, and the areas around them are clean and dry, and turn off the equipment.



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