LED Spectrophotometer to Determine Solution PH With Household Equipment

We built a spectrophotometer with an LED and a photodiode, which can be used with the help of red cabbage to determine the pH of solutions. This device uses light to measure the pH of a solution, and turns on a light if the solution is sufficiently basic. A light blue LED shines light through a cuvette. The light then hits a photodiode (BPW46), generating a current. That current is turned into a voltage with an op amp (MCP601), and the output of that voltage is fed through a unity gain buffer, and then to a red indicator LED.

The amount of current generated by the photodiode depends on the amount of light that reaches it. Light travels from the blue LED through the cuvette, which holds the sample. The sample is colored, and its color changes based on pH due to the use of red cabbage dye as a pH indicator. More basic solutions are blue-green, while more acidic solutions are red. The sample absorbs some of the blue light from the LED, and the amount it absorbs is based on its color. So, the amount of current the photodiode generates is based on the sample color, which is ultimately based on the sample's pH.

Our device was made as a project for E84 at Harvey Mudd College in the Fall 2022 semester.

-James Nicholson and Zoë Gomez-So

Acknowledgements:

-Harvey Mudd engineering department profs and staff

For electronics:

Light blue LED

Red LED

Photodiode (we used BPW46)

Wire

330Ohm resistor

470Ohm resistor

1MOhm resistor

2 MCP601 op amps

2 power supplies

Oscilloscope

Breadboard

For chemistry:

Cuvettes

Cuvette holder (obtained from engineering department)

Beaker

Hot plate

Water

Red cabbage

Ammonia

Baking soda

Sprite

White vinegar

pH paper (optional)

Pipettes

Something to hold liquids (tubes, beakers)

Optional:

Hot glue gun & hot glue sticks

Soldering equipment

Construct the circuit as shown in the circuit diagram above, perhaps on a breadboard. Optionally, solder some things (or everything) together.

We made our red cabbage dye solution by putting about 210g of red cabbage in a 1L beaker filled with 800mL of water, setting it on a hotplate, and heated it for 40 minutes. But any variation of heating cabbage in water should work fine. Then, we filtered off the cabbage chunks and collected our dark purple solution.

Create solutions of different colors. We used a ratio of 5 mL of red cabbage solution to 20 drops of the thing we wanted to analyze. Put each cuvette in the cuvette holder and record the output voltage of the circuit (we used an oscilloscope to do this). First, use samples of known pHs to generate a calibration curve. Then, if you want to analyze a sample of unknown pH, you can add that to the indicator and read the output voltage, then use the calibration curve to get the pH from the output voltage.