For example, a 10.0 mL water sample was determined to contain 0.3 ppm iron.
To a second 10.0 mL sample, 0.1 mL of 50 ppm iron standard was added. The
concentration of iron due to the “spike” was (0.10 mL x 50 ppm)/10.0 mL =
0.50 ppm. The concentration of iron determined in the spiked sample should be
0.3 + 0.5 = 0.8 ppm iron.
(Note: any error due to the increased volume from the “spike” is negligible).
LaMotte offers a line of calibration standards which can be used to generate
calibration curves and perform standard additions.
n
SAMPLE DILUTION TECHNIQUES
& VOLUMETRIC MEASUREMENTS
If a test result using the SMART gives an
OUT OF RANGE!
message then the
sample concentration could be over range or under range. If it is over range, the
sample must be diluted. Then the test should be repeated on the diluted sample
to obtain a reading which is in the concentration range for the test. (Note: This
is not true for colorimetric determination of pH.)
Example:
Measure 5 mL of the water sample into a graduated cylinder. Add demineralized
water until the cylinder is filled to the 10 mL line. The sample has been diluted
by one-half, and the dilution factor is therefore 2. Perform the test procedure,
then multiply the resulting concentration by 2 to obtain the test result.
The following table gives quick reference guidelines on dilutions of various
proportions. All dilutions are based on a 10 mL volume, so several dilutions will
require small volumes of the water sample. Graduated pipets should be used for
all dilutions.
Size of Sample
Deionized Water to Bring
Volume to 10 mL
Multiplication Factor
10 mL
0 mL
1
5 mL
5 mL
2
2.5 mL
7.5 mL
4
1 mL
9 mL
10
0.5 mL
9.5 mL
20
If the above glassware is not available, dilutions can be made with the
spectrophotometer tube. Fill the tube to the 10 mL line with the sample then
transfer it to another container. Add 10 mL volumes of demineralized water to
the container and mix. Transfer back 10 mL of the diluted sample to the tube
and follow the test procedure. Continue diluting and testing until a reading,
which is in the concentration range for the test, is obtained. Be sure to multiply
the concentration found by the dilution factor (the number of total 10 mL
volumes used).
Example:
10 mL of sample is diluted with three 10 mL volumes of demineralized water;
the dilution factor is four.
SMART SPECTRO SPECTROPHOTOMETER 05.04
17
