CHE 450G: Practical Inorganic Chemistry
U of KY Dept of ChemistryThis handout explains how to use the Guoy Balance used in our laboratory. You will use this instrument to complete the laboratory exercise Mn(acac)3 as well as one or two others.
Please make sure you have done the appropriate background reading so that you understand the basic principle of operation.
Step by Step Instructions
- You should make sure that you have read the following before attempting to use the balance or follow this procedure. Knowing why you're doing these measurements will help you immensely!
- Angelici Experiment #5 (Use this notation in your report)
- Szafran, Chapter 5A (We are not using an apparatus of this type...)
- Jolly, Chapter 25
- Optional: West Chapter 16.1 and 16.2
- Sign in on the logbook. Failure to sign the log will result in a loss of points for lab technique.
- Remove any metallic objects such as watches, bracelets, necklaces etc. that might get near the magnet. Also keep magnetic charge cards away from the magnet.
- You should find the apparatus already assembled with a clean, dry and empty sample tube in position.
- You will need to make several measurements for each experiment. Each measurement must be performed exactly the same way. You will need to weigh the following items both with the magnetic field off and the magnetic field on:
- The dry, clean empty sample tube (determines delta)
- The sample tube filled to the line with distilled water (determines volume )
- The sample tube filled with HgCo(NCS)4 (calibration standard). PLEASE SAVE AND REUSE THE STANDARD. Both the standard and its disposal are expen$ive.
- Your unknown sample
- To perform your weighings:
THE BALANCE IS EXTREMELY SENSITIVE AND FRAGILE -- USE EXTREME CARE WHEN SWITCHING SAMPLES!!!
- Carefully disassemble the apparatus by removing the air jacket and unhooking the sample tube. Be careful not to snap the monofilament line that supports the sample tube.
- If your sample is a solid, grind it thoroughly and pack it tightly into the sample tube (Why?). Fill the tube to the mark.
- Reassemble the apparatus, again being careful not to snap the quartz fiber.
- Turn the balance on and record the weight of your sample with the field off.
- Turn on the magnet using the power switch on the lower left (A). Slowly turn the DC current knob (B) until the DC current meter (C) reads a value that is close to 5.0, but is below 5.0. It is important that you set the meter to this exact setting for ALL of your weighings!!!
- Record the weight of your sample in the magnetic field.
- Turn off the magnet by slowly dialing the DC current to zero and then switch off the power supply.
- Repeat these steps as necessary.
- When you are finished your runs, turn off the balance, remove your sample and then clean and dry the tube. Reassemble the apparatus and leave the instrument ready for use by the next group. Remember to sign out on the logbook!
As always, be sure to report any problems to your instructor immediately.
Tips and Tricks
- If the balance reading is unstable, it may be due to a draft. There are two pieces of cardboard under the left side of the Guoy table. Slide one around to the front and leave the other where it is to seal out most drafts.
- Instability can also occur if the wire, sample tube, or hanger is contacting the air jacket. If so, gently reposition the tube in the hanger. If you still get drift, consult your instructor.
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