4.2: Laboratory Equipment

A technologist is asked to weigh 80 g of sodium chloride (with an error of ± 1%) to prepare a 2M sodium chloride solution. Ten mL of this solution will be diluted to a final volume of 100 ml. Fifty-three mL of the diluted salt solution will be used to dissolve 10 mg (+ 1%) of albumin. It will take an overnight mixing to achieve solution of the albumin.

QUESTION

Which laboratory equipment discussed in Chapter 1 or shown In Figure 1-1 should be used for this preparation?

Questions to Consider

1. What are the different types of balances available, and how do they differ in their precision of weighing and use?
2. How are different types of glassware used to deliver different volumes of fluid?

Answer

Which laboratory equipment should be used for this experiment?

To weigh 80 g NaCl use:

To weigh 10 mg albumin use:

To deliver 10 mL of saline use:

Deliver 10 mL of saline for dilution into:

To deliver 53 mL of diluted saline use:

Dissolve albumin in a:

Answer

Answers to Questions to Consider

1. Balances differ in their useful range of operation, their achievable precision, and their ease of use (Table 1-11). Thus, precision mechanical balances are used to weigh gram quantities or greater when the reproducibility required is only in the 1-1,000 mg range. Mechanical trip balances are very easy to use for rapid, relatively precise weighings of 10-100 gm weights. Electronic microbalances, on the other hand, are only useful when weighing quantities <10 mg, but have reproducibility in the microgram range. The analytical balances have intermediary use and precision.
2. For delivery of an exact volume of fluid, volumetric pipets give the best accuracy and precision. However, not all pipet sizes are usually available in the laboratory. Mohr type graduated pipets are useful for delivery of volumes less than 25 mL. For volumes larger than 25 mL, one would need to use graduated cylinders. One should always use the smallest size of Mohr or graduated cylinder that will contain the desired volume. This will result in the smallest error of measurement (see Tables 1-6 and 1-7).

Answer

For weighing 80 g of NaCl, it would be time consuming and unnecessary to use the microbalances. A mechanical precision balance, would allow one to weigh the salt rapidly with an acceptable error of less than 1%. On the other hand, only the electronic microbalances would be useful for weighing as little as 10 mg of albumin with the desired accuracy.

To deliver exactly 10 mL of saline solution, one should use a 10 mL volumetric pipet. This should be delivered into a 100 mL volumetric flask for accurate. dilution. To deliver 53 mL of diluted saline, a 100 mL graduated cylinder should be used, since it is the smallest graduated cylinder that will contain the desired volume.

The albumin can be placed in either a 100 mL Erlenmeyer flask or a 100 mL beaker. For overnight stirring, an Erlenmeyer flask would be easier to cover to prevent evaporation and contamination and thus might be the most convenient flask to use.