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Sample Pretreatment

  • Page ID
    218829
  • Sample preservation and preparation

    Purpose: The purpose of this assignment is to address questions related to sample preservation and preparation for analysis.

    Learning Outcomes:

    At the end of this assignment you will be able to
    1. Suggest a method to preserve a sample for analysis
    2. Understand the time frame within which a sample needs to be analyzed

    Assignment:

    Once a water sample is collected and possibly filtered if particulate matter is present, it is important to ensure that its chemical composition is not altered prior to analysis. Samples can only be stored for a limited period of time.

    In many cases water samples can be stabilized for long term storage by controlling the pH, cooling, limiting light exposure or adding a chemical preservative.

    Consult the literature in regards to water sample preservation and maximum holding times (refer to section 7.3 in Harvey, D. Chapter 7, Implementing the Sampling Plan).

    The table below provides guidelines on preservation methods and maximum holding times for some analytes in natural waters and wastewaters.

    Preservation Methods and Maximum Holding Times for Selected Analytes in Natural Waters and Wastewaters

    Analyte

    Preservation Method

    Maximum Holding Time

    ammonia

    cool to 4 ºC; add H2SO4 to pH < 2

    28 days

    chloride

    none required

    28 days

    metals—Cr(VI)

    cool to 4 ºC

    24 hours

    metals—Hg

    HNO3 to pH < 2

    28 days

    metals—all others

    HNO3 to pH < 2

    6 months

    nitrate

    none required

    48 hours

    organochlorine pesticides

    1 1mL of 10 mg/mL HgCl2 or immediate extraction with a suitable non-aqueous solvent

    7 days without extraction

    40 days with extraction

    pH

    none required

    analyze immediately

    Q1. Under what conditions should the water samples be stored and within what time frame should the samples be analyzed?

    Q2. What changes could take place that would cause sample degradation?

    Q3. No preservation method is required for nitrates. However, a water sample can only be stored for up to 48 hours. What possible degradation could occur over an extended period of time? What chemical preservative could be added to prolong the life of the sample? How could you devise an experiment to test whether compositional changes for nitrate occur over time?

    Q4. Cr(VI) must be analyzed within 24 hours. What might happen to compromise the analysis of Cr(VI) if the samples were held only longer before completing the analysis?

    Q5. Preservation of water samples for metals analysis requires acidification with nitric acid below pH 2. Why is this procedure required for analysis of calcium and magnesium?

    Consider the Ksp for Ca(OH)2 and Mg(OH)2.

    Ksp Ca(OH)2 = 6.5 x 10-6

    Ksp Mg(OH)2 = 2 x 10-13

    Q6. Compare the solubility of Ca(OH)2 and Mg(OH)2 at pH 2 and pH 8. What could happen to the Ca2+ and Mg2+ ions stored over time if the pH was not adjusted to an acidic value?

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