• The drop counter consists of a dispensing reservoir that is positioned above an optical-path slot. An infrared source sends a beam of radiation down the slot where it is detected by a photosensitive sensor. When a drop of solution from the dispenser blocks the beam of infrared radiation, the data collection program (Logger Pro) records the presence of a single drop as a digital event. When another probe is connected to the LabPro interface, this digital event signals Logger Pro to record the probe's current value. By calibrating the drop counter the number of drops collected can be converted into a volume.

Auto ID Capable?

• No. Logger Pro must be informed that the drop counter is connected using the LabPro icon.

Calibration

• In the absence of a calibration, volume data is recorded in terms of the number of drops counted instead of a volume. There are two options for calibration:
  • Manually enter a drops/mL ratio based on an earlier calibration or use an approximate value, such as 20 drops/mL (which is usually accurate to within ±10%)
  • Perform a new calibration using the following steps:
    • Fill the reservoir with deionized water.
    • Open the both valves so that deionized water exits from the reservoir and adjust the top valve so that a drop is dispensed every couple of seconds; the time between drops is not critical, provided that each drop passes through the drop counter before the succeeding drop. Do not adjust the top valve once it has been set.
    • Close the bottom valve and add additional deionized water to the reservoir if needed.
    • Weigh a small beaker and position it so that it can catch the drops of deionized water after they pass through the Drop Counter
    • Select Experiment:Calibrate:Drop Counter from the main menu. Begin the calibration by selecting Calibrate Now; the calibration will begin when the Drop Counter first detects a drop.
    • Open the bottom valve and allow the calibration software to gather data until approximately 10 mL of deionized water has been dispensed.
    • Determine the mass of water collected. Measure the temperature of the water and use its density to calculate the volume of water collected. Enter this value in the appropriate space on the software's Calibration page to complete the calibration.

Preparing Drop Counter for a pH Titration ( this procedure assumes use of a calibrated pH probe and a calibrated drop counter)

• Fill the reservoir with your titrant.
• Open both valves and adjust the top valve until you have a drop rate of one drop every couple of seconds; the time between drops is not critical, provided that each drop passes through the drop counter before the succeeding drop. Do not adjust the top valve once it has been set.
• Close the bottom valve and add additional titrant to the reservoir if needed. While using the drop counter, continue to refill the reservoir as needed.
• Place a stir motor below the drop counter.
• Place the sample in a suitable container (usually a small beaker) and place it on the stir motor. Insert the pH probe and add enough deionized water so that the tip of the pH probe is completely immersed.
• Add either a small stir bar to the solution or attach the microstirer to the tip of the pH probe. Adjust the stir rate so that the solution is mixed well.
• Set up your LoggerPro for data collection, choosing the option for digital collection and for manually stopping data collection.
• To begin the experiment, click on LoggerPro's Collect icon. Data collection will begin once the drop counter senses the presence of the first drop.
• Completely open the lower valve to allow the titrant to drop into the sample. Data collection will begin and the resulting graph of pH vs. volume of titrant displayed. Allow the titration to continue until you are well past the equivalence point.
• When the titration is complete, click on LoggerPro's Stop icon to end data collection. Close the lower valve on the reservoir to stop the flow of titrant to the sample.

Maintenance and Storage

• Rinse the reservoir with deionized water before storing.

Specifications

• response time is less than 70 ns
• maximum number of drop is 1504 (approximately 75 mL)