IMAGE. Close-up of a buret.
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Before beginning to record a reading, examine the buret closely.
On other types of glassware, such as the graduated cylinder, zero was at the bottom of the scale, with values increasing going up the cylinder. However, a buret has zero at the top with values increasing going down the scale. In examples where values on the scale increased going up, the digits in the reading were recorded by looking at the last graduation below the meniscus. However,
in examples where the scale values increase going down, record digits by looking at the last graduation above the meniscus (see Steps 2 and 3).
Step 1: Determine the scale increment: To find the scale increment, subtract the values of any two adjacent labeled graduations and divide by the number of intervals between them.
 What is the scale increment?
Answer
In the buret above, first subtract 25 mL - 24 mL = 1 mL. Next, count that ten intervals are between the labeled graduations. Therefore, the scale increment is 1 mL/10 graduations = 0.1 mL/graduation. Step 2: Use the graduations to find all certain digits: Use the labeled graduations and the scale increment to find
the certain digits in the measurement.
What are the certain digits?
Answer
The first digits are 24, since the last labeled (major) graduation above the meniscus is 24 (remember that the buret has values increasing going down the scale). Next, use the scale increment. There are two unlabeled (minor) graduations above the meniscus (and seven below), and each graduation represents 0.1 mL, so the certain digits of the reading are 24.2 mL. Step 3: Estimate the uncertain digit and obtain a reading: Estimate the distance that the meniscus lies between the two
graduations in fifths. i.e. estimate to 0.02 of a mL.
What is the volume you should record?
Answer
Above, the meniscus is right on a minor graduation, so we estimate that the uncertain digit is zero. The uncertain digit in the reading is always the last digit, so the volume measurement in the image is 24.20 mL. |
