experiment: Gravimetric determination of moisture in fertilizer samples
1. What are the advantages of weighing by difference to direct weighing when using an analytical balance?
2. Why are the samples grinded before drying in the oven?
Weighing by difference minimizes errors in the balance, weights, and procedure. For example, suppose a 1 g weight actually had a mass of 2 grams. I know that won't happen but suppose it did. The mass you determine might be 1.500 but you would determine it as 2.500. Then suppose the next weighing is 2.5500 (actual mass is 1.550). Subracting 2.550 - 2.500 = 0.050 g and subtracting 1.550 - 1.500 = 0.05 g. That is you get the same answer although the mass of the weight was not correct. Solid samples are ground before drying in order to increase the surface area of the sample so that moisture is more readily driven out of the sample.
To determine the advantages of weighing by difference over direct weighing when using an analytical balance, let's break it down:
1. Weighing by difference: This method involves weighing a container (such as a weighing boat) without the sample, then adding the sample and weighing again. The difference between the two measurements gives you the mass of the sample.
Advantages:
- Minimizes errors: By subtracting the initial mass of the container, the errors associated with the container's mass are eliminated. Any inaccuracies in the container's weight would not affect the final result.
- Reduces handling errors: Since the sample is not directly weighed on the balance, there is less risk of accidentally spilling, dropping, or contaminating the sample during the weighing process.
- Allows for better precision: Weighing by difference allows for more accurate and precise measurements, as any variations in the container's weight are accounted for and not included in the final result.
2. Grinding samples before drying in the oven: In the gravimetric determination of moisture, it is important to remove all the moisture from the fertilizer sample to obtain accurate results. Grinding the samples before drying serves to increase the surface area of the sample, enabling more efficient moisture removal.
Reasons for grinding:
- Increased surface area: By grinding the solid sample, you break it into smaller particles, exposing a larger surface area to the heat in the oven. This increased surface area allows for better contact between the sample and the heated air, facilitating the evaporation of moisture.
- Faster drying time: With a larger surface area, moisture is more readily driven out of the sample, leading to a faster and more efficient drying process.
- Consistent and homogeneous drying: Grinding ensures that the moisture content is evenly distributed throughout the sample, resulting in more consistent and accurate drying. Uneven distribution could lead to incomplete drying or variations in the moisture content within the sample.
Overall, grinding the fertilizer samples before drying aids in achieving more reliable and precise results in the gravimetric determination of moisture.