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Equipment Construction: An Inexpensive Spectrophotometer

by Robert Stelter, Jo Ann Tunt and Caroline Virkhaus

* Purpose

To assembe a CdS spectrophotometer to use in the colorometric analysis of a colored solution. To prepare a calibration curve and to determine the concentration of an unknown solution. This CdS spectrophotometer may also be used in phosphate determinations and in kinetic studies of the reaction of crystal violet with NaOH.

* DESCRIPTION

The CdS spectrophotometer consists of a CdS photocell, a multimeter, and two electrical leads. All parts of the spectrophotometer are simple, inexpensive and yield high quality reproducible experimental results.

* TIME REQUIRED

40-45 minutes

* MATERIALS

Chemicals

Equipment

* SAFETY INFORMATION

No obvious hazards are present in the activity. Colored solutions will stain skin and clothing. Eye protection should be worn as directed.

* PROCEDURE

  1. Assembling the Spectrophotometer

    1. The device consists of a sensor, electrical leads, a readout device (multimeter) and a physical support. The sensor recommended is a CdS flat plate photocell available from Radio Shack (276-1657, assorted/no specification sheets, 5 for $1.99 without discount (Nov. 1989) or EG&G VacTek (VT-series 203 or 203H recommended, $2.83 ea or $2.03 ea/100 order (Nov. 1989)). Reproducible conditions must be assured to maintain quality results from measurements. Two possible designs are pictured.

      [DIAGRAM]

    2. Secure the photocell device to its mounting equipment.

    3. Attach the photocell leads to a multimeter. A digital multimeter (DMM) is recommended for reproducible results and higher resolution. Inexpensive devices may be purchased from Radio Shack or other suppliers for under $30.00 with no discount applied.

    4. Set the multimeter to the Ohms scale. Do not leave the multimeter in Ohms mode for long periods of time as the internal power source will be depleted.

  2. Calibrating the Spectrophotometer

    1. Make a serial dilution of a colored solution to 1/10 the original concentration in one drop increments (i.e., 1 drop water/9 drops color solution, 2/8, etc.)

    2. Place the spectrophotometer directly under an overhead light source to assure reproducible light intensity without shadows. Do not move the assembly until all measurements are finished!

    3. Using a Beral pipete, add distilled water to the face of the photocell to completely cover the cell in a single spherical drop. Wipe off. Repeat without removing the drop.

    4. Read and record the multimeter data.

    5. Remove the drop.

    6. Repeat the drop procedure using the colored solutions, starting with the most dilute and finishing with the most concentrated, in sequence. Make sure that the photocell face is cleaned and rinsed with the test solution once prior to taking data on that solution.

    7. Plot a graph of resistance (in Ohms) against concentration (in drops). This serves as a calibration curve for the photocell/multimeter device. This calibration may vary somewhat from other pair instruments. A linear regression (if the graph indicates a linear function) may be done on the data to generate an equation for the relationship between concentration and resistance.

    8. (Optional) If a commercial spectrophotometer is available, use macroscale quantities of your dilutions in cuvettes to generate absorbance data. Graph the absorbance data against the resistance data. What is the result?

  3. Determination of an Unknown

    1. Obtain an unknown solution of the same stock name as your serial dilutions. Generate a resistance reading for the unknown using several trials and taking an average reading.

    2. Determine the "concentration" of the unknown solution using your graphical curve or a regression analysis.

* DISCUSSION

A careful analysis of the interaction between light and solutions can yield both chemical structure and concentration information. The light absorbed and transmitted actually gives detailed data on electronic transitions within molecules (in the visible and UV range) which can be interpreted as information on structural relationships. In this experiment, another general law is considered. The Lambert/Bouger/Beer law (commonly known as Beer's Law) states that if monochromatic radiation is incident on a solution, the amount of light absorbed or transmitted is an exponential function of the concentration of the absorbing species, a property known as absorptivity, and the path length of the light through the material.

Mathematically, the law is stated as:

Equation #1: T = 10 -abc
or
Equation #2: -logT = abc

Where T is the transmittance, 'a' is a constant dependent on the substance involved, 'b' is the path length through the material, and 'c' is the molar concentration. The law is often expressed in linear form, substituting

A = - log T, where A is called the absorbance of the solution, as:

Equation #3: A = abc

One of the earliest and most useful applications of light absorption phenomena was in the technique of chemical analysis called colorimetry. In colorimetry, a set of colored solutions of known composition and preparation origin is placed in a set of containers, usually sealed. These materials are usually chemically reactive dyes or dyes which are carefully matched to colors of reaction products. An unknown concentration of a solution could be compared against the known set of "standards" and the concentration determined. Modern instrumentation allows for much higher resolution of "colorimetric" analysis and extends the range of analysis from the visible to the infrared and ultraviolet. In commercial spectrophotometers, the amount of absorption which occurs is carefully detected, processed, and corrected with sophisticated electronic and mechanical devices. Even small samples of 0.1mL can be measured in very expensive microspectrophotometers with great accuracy and precision. The instrument which has ben constructed contains all the "working parts" without the expense of a commercial instrument.

* DISPOSAL

Waste generated (depending on the dye selected) should be diluted with water and disposed of in the sink. Waste on wipers should be disposed of in the trash.

* TECHNICAL DATA

Spectral Response of VT-203 Series Diodes (VacTec)

Operating Conditions for VT-203 Diodes (VacTec)
Sensitive area: 0.07 Sq. In Max. Applied Voltage (AC or DC): 200V

Resistance at 10 Lux: 15K Ohms (40%) Max. Power Dissipation (25 C): 0.25W

Typical resistance, 100 Lux: 3 K Ohms Operating Temp. Range: -30 to 70°C
Resistance (dark) minimum: 500K Ohms

The VT-203H is a higher resistance model of the same base CdS cell material.

Radio Shack diodes must be characterized individually as most product bags are mixed components and no data sheets are supplied.

[GRAPH]

* REFERENCES

Staff and participants, 1989 Dreyfus Institute on Environmental Chemistry, Princeton University, Princeton, NJ.

Dr. Stephen Thompson, 1989 Dreyfus Institute on Environmental Chemistry, Princeton University

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