Technical Tips

Serial Dilutions

Calibration problems often occur when you try to pipet too small of a volume. As an extreme example, several times we have seen a technician try to make a 1 ppm standard by pipetting 0.1 ml of 1000 ppm stand into a 100 ml flask. The math works out, however, the relative error in this method can easily be 25%!

For diluting standards, you should never pipet less than 5 ml. In addition, if you are using 3 or more standards for an ISE calibration, each standard should be made from the next most concentrated standard.

For example, let’s say you have 1000 ppm Ammonia Standard and you want to make standards of 20.0 ppm, 2.0 ppm, and 0.2 ppm.

  1. Use clean, dry volumetric Class A pipet(s) and clean, dry volumetric Class A flask(s). Do not use the same pipet or flask more than once unless you thoroughly wash and dry it between uses.
  2. Fill a 1 liter flask about 1/2 full with deionized water. Pipet 20 ml of 1000 ppm standard into this flask. Mix thoroughly by slowly inverting at least 5 times. Fill to the mark with deionized water. Again, mix thoroughly by slowing inverting at least 5 times. This is your 20.0 ppm standard.
  3. Fill a 250 ml flask about 1/2 full with deionized water. Pipet 25 ml of the newly prepared 20.0 ppm standard into this flask. Mix thoroughly by slowly inverting at least 5 times. Fill to the mark with deionized water. Again, mix thoroughly by slowly inverting at least 5 times. This is your 2.0 ppm standard.
  4. Fill a 100 ml flask about 1/2 full with deionized water. Pipet 10 ml of the newly prepared 2.0 ppm standard into this flask. Mix thoroughly by slowly inverting at least 5 times. Fill to the mark with deionized water. Again, mix thoroughly by slowly inverting at least 5 times. This is your 0.2 ppm standard.

Obviously you can change the volumes as long as you are pipetting at least 5 ml. With the above volumes, you have enough 2.0 ppm standard for one more calibration and enough 20.0 ppm standard for about 8 more times. These concentrations should be stable enough to use for at least one month.

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