Measurements
Experimental Errors
Weighing a sample on a triple beam balance can weigh it to a tenth of a gram
It weighs 1.1 grams
Now let us weigh it on a top loading balance which measures to a 100th of a gram
Now there are three numbers after the decimal point in this weight
Now we can weigh the sample on an analytical balance to 0.1 milligrams
The door is closed to eliminate effects due to air currents
It weighs 1.1362 grams
To be sure the weighing is accurate the calibration of the balance must be checked
This is the standard kilogram for the United States. It is in a locked Safe in Washington DC
Another use of a standard involves a spectrograph to measure concentrations
First water is used to set 100% transmission
100% transmission or zero absorbance
Then the absorbance of standard solution is measured
The absorbance is determined at a concentration
A point is place
Accurate dilutions of the standard with water can be made
A calibration curve can be created, and used to measure the concentration of unknown
After measuring the absorbance the concentration is read from the graph
Usually the average of three determinations are used to obtain a results
When reporting a results, always give some idea of reliability
PPT Slide
ERROR
Types of Experimental Measurements
TYPES OF EXPERIMENTAL ERRORS
SYSTEMATIC ERROR
RANDOM ERROR
ACCURACY AND PRECISION
A set of measurements is accurate if a set of values is close to the true vale
Precision is a measure of the tightness of the values, a measurement can be precise but not accurate
A measurement can be accurate but not precise
ANALYTICAL BALANCE
VOLUMETRIC FLASK
PIPETTE
NBS TOLERANCES FOR VOLUMETRIC GLASSWARE MAXIMUM ERROR TOTAL OR PARTIAL CAPACITY
TOLERANCE
Email: Grow@adm.njit.edu
Home Page: http://www-ec.njit.edu/~grow/statisti.htm
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