1. Getting on the Same Page: Calibration

    Investigation

    Goal

    Calibrate a pH–measuring device, correcting its readings to agree with the pH value of known standard solutions.

    Research question

    The “heart” of every laboratory pH-meter or every digital probe over time may develop certain accuracies in different parts of the pH–scale, that can be determined (and corrected) by comparing pH–readings of the instrument with known pH–values of a specially prepared solutions.

    Equipment

    Digital pH probe

    Chemicals for the preparation of standard solutions:
    Distilled water
    Sodium acetate trihydrate
    Glacial acetic acid
    Disodium hydrogen phosphate
    Citric acid
    Potassium dihydrogen phosphate
    Sodium hydroxide

    Laboratory glassware:
    Three chemical glasses or bulbs with tight lids, 100 ml in volume used as storage containers.
    Six glasses or plastic cups for the preparation of solutions 100-200 ml volume.

    Investigation Protocol

    Preparation of standard buffering solutions with known pH.

    Prepare three standardbuffering solutions: one in the acidic range, pH = 4; one in neutral, pH = 7; and one – in alkaline, pH = 8.

    1. Standard bufferingsolution, pH = 4,0
      • Prepare two solutions:
        Sodium acetate (CH3COONa) concentration of 0.2 mol / l, for which 2.72 g of sodium acetate trihydrate should be dissolved in 100 ml of distilled water;Acetic acid (CH3COOH) - 1.15 ml ice (m. e., 100 percent) of acetic acid poured into a volumetric flask of 100 ml volume and bring the solution to 100 ml with distilled water.
      • Mix 18.0 ml of Sodium acetate with 82.0 ml Acetic acid.
    2. Standard neutral solution, pH = 7,0
      • Prepare two solutions: Hydrogen phosphate (HNa2O4P) by dissolving 7.15 g hydrogen phosphate in 100 ml of distilled water Citric acid (C6H8O7) by dissolving 2.1 g citric acid in 100 ml of distilled water.
      • To prepare neutral buffering solution (pH 7.0) mix 82.4 ml of disodium hydrogen phosphate with 17.6 ml of citric acid.
    3. Standard alkaline solution, pH = 8,0
      • Prepare two solutions: Potassium dihydrogen phosphate (KH2PO4) by dissolving 2.72 g of KH2PO4 in 100 ml of distilled water Sodium hydroxide (NaOH) by dissolving 0.8 g NaOH in 100ml distilled water.
      • To prepare alkaline buffering solution (pH 8.0) mix 50.0 ml of potassium dihydrogen phosphate with 46.8 ml of sodium hydroxide and add water to bring total volume to 200.0 ml.
    4. Prepared calibration solutions should be stored in flasks or beakers with lapped stoppers. Stored in such way, standard-buffering solutions can be used for 1 month. At the end of the month they need to be discarded and replaced with the newly prepared. The date of preparation and pH value of the solution should be written on each flask (glass), to prevent any confusion or mistake.

    Probe Calibration Procedure

    A pH-meter calibration is performed by measuring a series of reference standards, so-called pH buffers that have known, accurate pH values.

    Before calibration

    If the electrode was stored in the storage solution, rinse it in distilled water thoroughly, and gently blot it dry with a lint-free tissue. Avoid rubbing or wiping the electrode bulb.

    Be sure the sensor and the buffer solution are at the same temperature. Often buffering solutions are kept refrigerated. If it is the case, allow time for temperature equilibration to the room temperature.

    The main steps for a pH calibration procedure:

    1. Turn the pH–meter on, allowing adequate time to warm up (check the operating manual).
    2. Pour the necessary amounts of buffer solutions into three individual glass beakers and mark them clearly (pH=4; pH=7; pH=9); close the buffer containers promptly to avoid carbon dioxide absorption. Note: buffer solutions will remain stable in a glass beaker for a maximum of two hours. Discard used buffer. Do not pour it back into the storage container.
    3. Place the electrode into the first buffer. Make the first, second and third measurements. Between each measurement rinse the measuring electrode thoroughly with distilled water and gently blot it dry with a lint-free tissue. Do not rub or wipe the electrode bulb.
    4. Enter all three results for the probe readings of the standard solutionsinto the project Report Form. Then calculate the average value for each of the readings.
    5. Using the data, graph the actual readings of the instrument against the values of pH standard solutions. See an example of such a graph below:

    Measurement results

    • Calibration data allows for necessary amendments to the actual pH–readings made by pH-meter in every project. To do so, use the chart (the correction value is highlighted in red).
    • It is sufficient to conduct a calibration procedure once before the start of the project. But it is even better to repeat the procedure every month to make sure that no deviation from previous calibration results has occurred.

    Safety tips

    Be careful while working with chemicals. Avoid contact with skin.

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    • Project was published on:November 17, 2014
    Report Form Before filling in the Report Form, please read the Investigation Protocol