6: Significant Figures and Standard Mass

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Chapter Six

Significant figures and Standard mass

Today, we're diving into the intricacies of measurements, focusing specifically on significant figures, the accuracy of measuring tools, and troubleshooting techniques to ensure precision in our scientific endeavors. Let's start with significant figures, which are crucial for accurately representing the precision of our measurements. They tell us the number of reliably known digits in a measurement. Consider using a ruler to measure an object where the true value lies between 1.5 cm and 1.6 cm (Figure 6.1). If our measurement falls between these two hash marks, the significant figures would include the digit assumed between them, ranging from 1.51 cm to 1.59 cm. Each of these digits represents a reliably known part of the measurement, emphasizing the importance of understanding significant figures to capture precision accurately.

When it comes to reading volume, especially with tools like a graduated cylinder or serological pipette, we encounter the meniscus (Figure 6.2). The meniscus, the bottom of the concave shape formed by the liquid, is considered the true volume. This practice accounts for the curvature caused by the cohesion and adhesion properties of water, providing a reliable significant figure for volume measurement.

Moving on to the accuracy of measuring tools, let's consider a micropipette. Such a tool offers more significant figures and precision compared to a serological pipette or a graduated cylinder. However, it's important to note that relying solely on the device may not guarantee accurate measurements. Human error and equipment limitations can impact the consistency of our measurements. Imagine using a micropipette with high precision to measure 1.000 mL. If there's a human error or a malfunction in the pipette, the volume measured might be 1.005 mL.

An effective method for ensuring equipment reliability involves the verification of instruments using a standard mass—a known and accurate weight. Let's consider an example with an electronic balance: if you weigh a standard mass of 100g and the balance reads 100.650g, the absolute error is calculated as (100.000g - 100.650g), resulting in a deviation of -0.650g from the expected value. Subsequently, let's say you measure 4.3 mL of water, but the balance reads 4.85 grams. Without applying the absolute error, the percent error is calculated as (4.85g - 4.3g)/4.3g * 100, resulting in a high 13% error. This high percentage might initially lead to attributing the error to human error or the inaccuracy of the micropipette. However, if you account for the absolute error by subtracting it, the actual weight becomes 4.20g. Now, with the adjusted value, the percent error is recalculated as (4.3g - 4.20g)/4.3g * 100, yielding a lower 2% error. This adjustment emphasizes the importance of considering absolute error, as it can significantly impact the perceived accuracy of measurements and guide decisions on whether adjustments or calibration are necessary for the device to ensure precise and reliable results.

LAB ACTIVITY - PART 2 - Verification of Analyical Scale

In order to ensure accurate measurements during your practical work, it's crucial to verify the absolute value of your analytical scales. Follow these steps to assess the accuracy of each scale using standard masses:

1. Assign a unique letter code to each analytical scale. This code will be used to identify and record the measurements in your lab notebook.
2. Obtain standard masses of 1g, 5g, 10g, 50g, 100g, and 200g. These masses will serve as reference points for calibration.
3. Place each standard mass individually on the designated scales. Record the displayed mass reading for each standard weight on the analytical scale.
4. Subtract the theoretical standard mass by the recorded mass reading for each measurements. The absolute value represents the difference between the displayed value and the standard mass.
5. Create a dedicated section in your lab notebook for each scale's verification. Clearly document the letter code of the scale, the standard mass used, the recorded mass reading, and the calculated absolute value.
6. (Optional) Repeat the process for each standard mass on the same scale to ensure precision. If there are discrepancies, note them down and consider repeating the verification.

These information will be valuable for future assessments, quizzes, or practical work where you can choose the scale that aligns with your personal preference.

LAB ACTIVITY - PART 3 - Serological pipetting

Serological pipettes play a crucial role in precisely measuring and dispensing liquids in various biotechnological applications. Serological pipettes come in two main types: "to contain" (TC) and "to deliver" (TD). TC pipettes are designed to hold a specified volume when filled to the capacity mark. However, due to liquid adherence, they may not deliver that exact amount when poured out. On the other hand, TD pipettes are marked differently to ensure precise delivery of the specified amount under specific conditions. Choosing the correct pipette, whether TC or TD, is critical for accurate liquid handling.

One fascinating technique we use with serological pipettes is called "Reverse Pipetting." Unlike conventional methods where liquid is expelled by the pipette's blow-out mechanism, reverse pipetting involves aspirating more liquid than needed and then dispensing the excess. This technique is invaluable when precision is paramount and avoiding the introduction of contaminants is crucial.

Now, let's walk through the steps of using serological pipettes in the lab:

1. Selecting the Appropriate Serological Pipette and Tips:
   1. Choose the appropriate serological pipette based on the desired volume to be dispensed. Also, ensure that compatible 25mL serological pipette tips are available.
2. Aspirating the Liquid to the Desired Volume:
   1. TD Delivery: Draw up exactly 5mL of liquid using the serological pipette. Dispense the 5mL volume into three different cups.
   2. TC Delivery: Draw up 6mL of liquid using the serological pipette. Dispense 5mL the 5mL volume into three different cups.
   3. Reverse Pipetting: Aspirate up to 20mL of liquid using the serological pipette. Dispense 5mL at a time into three different cups.

Challenge:

1. Place an empty cup on the balance and tare it to zero.
2. Applying TD, TC, or Reverse Pipetting Method:
3. Using the appropriate method, deliver the following solutions into the tared cup
   1. 3mL water
   2. 5mL water
   3. 4mL water
   4. 10mL water
4. Record the time taken for each delivery and determine your percent error
   1. %Error = ((Measured Volume - Expected Volume) / Expected Volume) * 100%
5. Continue timing yourself with different volumes of water and determine the percent error.

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