Thermodynamics

Experimental Setup – Binary Solutions

The experimental setup for analysis of the vapor pressure of binary mixture is pictured in Figure 1.

Main components (the numbers correspond to the labels in Figure 1):

1. Vacuum pump (1) — a rotary vacuum pump that evacuates the chamber and the manifold prior to and between measurements.
2. Pressure gauge (2) — a Thyracont VSC43MA4 piezoresistive vacuum transducer mounted at the top of the manifold, used to measure the absolute pressure inside the chamber. See the “Pressure Gauge” section below for details.
3. Experimental chamber (3) — the glass assembly in which the binary solution is held and its vapor pressure is measured. A close-up is shown in Figure 3.
4. Pressure controller / display (4) — a Thyracont VD6 panel-mount unit that supplies power to the gauge, reads its 4–20 mA output, and displays the pressure on its front-panel LED display.

1 2 3 4

Vacuum Pump

The experimental chamber is connected to a rotary vacuum pump that can provide a minimal pressure of 10^-2–10^-3 Torr. More information on rotary vacuum pumps can be found by reading the section in Experiment 2 “Vacuum Techniques” here. Additionally, general information on other types of vacuum pumps may be found here.

Pressure Gauge

The pressure in the manifold and chamber is measured by a Thyracont VSC43MA4 piezoresistive vacuum transducer (Figure 2). The sensing element is a temperature-compensated Al₂O₃ ceramic membrane sealed against the vacuum side with FKM (Viton); the deflection of this membrane is converted into an electrical signal by a piezoresistive bridge embedded on its back. Because the readout is purely mechanical (membrane deflection), the gauge is independent of gas type — an important property for this experiment, where the composition of the gas above the solution changes as the solvent and solute evaporate.

Key specifications:

• Measurement range: 1400 mbar down to 1 mbar absolute pressure (i.e. from slightly above atmospheric pressure to a low vacuum). This range comfortably covers the vapor pressures of the binary solutions in this experiment, which typically lie between a few tens and a few hundreds of mbar at room temperature.
• Sensor: piezoresistive Al₂O₃ ceramic membrane — chemically resistant and gas-type independent.
• Accuracy: ±0.3 % of full scale (combined non-linearity, hysteresis, and repeatability), i.e. roughly ±4 mbar.
• Response time: up to 20 ms.
• Output signal: 4–20 mA two-wire current loop (4 mA at zero pressure, 20 mA at full scale, linear in between).
• Supply voltage: 9–30 V DC, supplied by the VD6 controller through the M12 connector visible at the top of the gauge.
• Vacuum connection: DN16 ISO-KF small flange (with G1/4 female internal thread).
• Operating temperature: 5–60 °C; protection class IP54.

The gauge is connected by a shielded two-wire cable to the Thyracont VD6 controller below the chamber (component 4 in Figure 1). The VD6 supplies the 9–30 V DC excitation, reads the returning 4–20 mA current, converts it to a pressure in mbar, and displays the value on its front panel. No further calibration is required for everyday use; a brief zero/atmosphere check at the beginning of each session is sufficient.

Experimental Chamber

A close-up of the experimental chamber area is shown in Figure 3.

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In order to measure the vapor pressure of a solution:

1. Make sure that plug 3 is open and plugs 4 and 5 are sealed.
2. Open the valve to the vacuum pump, pump down the pressure in the chamber, and close the valve. In practice the lowest pressure reached on this system is around 4–6 mbar (roughly 3–5 Torr) — not a deep vacuum, but sufficient for the measurement.
3. Close plug 3, open plug 4, and insert the solution. Make sure that no air bubbles are trapped at the bottom of the chamber.
4. Close plug 4, and open plug 3 slightly to allow the solution to fill the experiment chamber to between one-half and one-third full.
5. Close plug 3, allow the pressure to stabilize and take a reading.

In order to clean the chamber:

1. Open plug 4.
2. Slowly retract plug 3 until it is completely open.
3. Place a “waste” cup under the exit funnel. Open plug 5 partially — do not unscrew it all the way — and let the liquid drain out in a controlled stream. Close it again once the chamber is empty.
4. Close plugs 4 and 5. Open the valve to the vacuum pump, pump down the chamber, and close the valve.
5. If the pressure is steady at the system’s base value (around 4–6 mbar, i.e. 3–5 Torr), the chamber is clean. Otherwise, repeat this sequence.

Experimental Setup – Sublimation of Iodine

Two independent setups are available for this experiment. Both stations measure exactly the same iodine sample (described in the shared section below); they differ only in the spectrometer and light-source assembly used. Read the shared description below first, then continue with the section corresponding to the station assigned to your group.

Iodine Sample & Heating — Common to Both Setups

The iodine sample, the heating method, and the temperature measurement are identical for Setup A and Setup B. The components described in this section are present at every station and operate the same way regardless of which spectrometer is used.

The iodine itself is sealed inside a glass tube placed inside a copper tube cuvette (Figure 4). A small halogen lamp is mounted inside the cuvette and serves as the heating element — when current flows through it, the lamp filament heats up and warms the surrounding copper body. The high thermal conductivity of copper produces a uniform temperature along the cell, and that heat is transferred to the inner glass tube, raising the temperature of the iodine and increasing its equilibrium vapor pressure. Optical windows at both ends of the assembly let the spectrometer beam pass through the iodine vapor inside the glass tube.

The temperature of the copper body is monitored using a thermocouple inserted through a hole at the bottom of the copper cuvette, so that its tip is in direct contact with the copper and reports the actual body temperature rather than the air temperature inside the cell. A thermocouple is a temperature sensor made of two dissimilar metals joined at one end — a temperature difference between this junction and a reference point produces a small voltage that can be measured and converted to temperature. In this experiment we use a type-K (Chromel/Alumel) thermocouple. Convert the measured voltage to temperature using this conversion table; remember that the thermocouple measures the difference between the tip and the reference (room temperature), so add room temperature to obtain the absolute sample temperature.

The thermocouple voltage is read by a Keithley 2100 6½-Digit Multimeter. On the 100 mV DC range it has a resolution of about 100 nV — well below the type-K thermocouple sensitivity (~41 µV/°C), giving easily sub-0.01 °C resolution. Set the function to DCV, choose the 100 mVDC range, and connect the thermocouple leads to the front-panel INPUT terminals (red = +, black = −).

A bench DC power supply is used to drive the heating lamp inside the cuvette. The exact model varies between stations — you can use whichever DC supply is available at your station. The supply lets you adjust the output voltage and current compliance: increasing the voltage increases the current through the lamp filament, which heats the lamp, then the copper body, and ultimately the iodine glass tube inside. Begin at low voltage and increase gradually while watching the thermocouple reading. When the experiment is complete, return the voltage to 0, switch off the supply, and keep the cuvette upright while it cools so that the iodine resublimates onto the bottom of the glass tube rather than coating the optical windows.

Setup A — Custom Halogen Source & Avantes AvaSpec-VRS2048CL-EVO

Setup A — Optical & Spectrometer Components

For Setup A, light is produced by a custom halogen source and analysed by the Avantes AvaSpec-VRS2048CL-EVO spectrometer. The full optical assembly is shown in Figures 5–7. Components (1)–(5) are labeled directly on the side and top photos (Figures 5 and 7); item (5) (iodine sample) and items (6) and (7) from the device list (heating power source and Keithley 2100 thermocouple voltmeter) are the shared components described in the “Iodine Sample & Heating” section above.

1. Avantes AvaSpec-VRS2048CL-EVO spectrometer:
   1. GY6.35 halogen lamp light source (50 W, 12 V) housed in a copper enclosure (1).
   2. Collimating and focusing lenses to direct the light beam (2).
   3. Optical fiber (SMA connector) that collects the focused beam and delivers it as a point source (3).
   4. Second focusing lens that directs the beam through the iodine cell (4).
   5. Detector: CMOS linear image sensor (Hamamatsu S11639-01, 2048 pixels) preceded by a 300 lines/mm diffraction grating.
   6. AvaSoft 8 software.
2. Iodine sample (5): shared copper-cuvette assembly — see “Iodine Sample & Heating” section above.
3. Power source for the heating lamp (6).
4. Thermocouple voltmeter, Keithley 2100 (7).

Light Source

The light source is a GY6.35 bi-pin halogen lamp (TAIYALOO model GY6.35-HB-50W, 12 V, 50 W). This is a tungsten halogen bulb with a quartz envelope, operated at 12 V DC/AC. The lamp produces a broad, continuous emission spectrum in the visible and near-infrared range, with an approximate color temperature of 2800–3000 K, making it well suited for absorption spectroscopy across the 400–1000 nm range.

The lamp is mounted inside a custom-built copper housing, which serves as both a mechanical mount and a heat sink. The copper enclosure holds the bulb at a fixed position relative to the optical axis and includes a small aperture at the front from which the light exits and enters the first collimating lens.

Key lamp specifications:

• Bulb type: GY6.35 bi-pin tungsten halogen
• Power: 50 W
• Operating voltage: 12 V (AC/DC)
• Approximate color temperature: 2800–3000 K
• Spectral output: continuous broadband, ~350–2500 nm

The lamp is powered by a dedicated DC power supply. Do not touch the quartz envelope with bare fingers, as skin oils can cause hot spots that shorten the bulb’s lifetime. Always allow the lamp to cool completely before handling.

Optical Path

The light from the halogen lamp is directed through the iodine sample using a two-stage optical system designed to create a well-collimated, spatially defined beam. The sequence is as follows:

1. Halogen lamp in copper housing → The lamp emits diverging broadband light from a small filament.
2. First collimating/focusing lens → A converging lens placed close to the lamp exit aperture collimates and focuses the beam into the entrance of an SMA optical fiber.
3. Optical fiber (SMA-SMA) → The fiber acts as a spatial filter and point source, scrambling the beam profile and producing a clean, uniform, diverging cone of light at its output tip.
4. Second collimating lens → A second converging lens re-collimates the diverging output of the fiber and focuses a parallel beam through the iodine cell.
5. Iodine cell → The copper tube cuvette containing the iodine sample sits in the beam path. The transmitted light exits the far end of the cell.
6. Collection fiber to spectrometer → The transmitted beam is collected by a second SMA optical fiber and routed to the AvaSpec-VRS2048CL-EVO spectrometer.

This optical arrangement ensures that the beam is spatially uniform across the iodine cell aperture and that only light that has passed through the iodine vapor reaches the detector.

Spectrometer

The Avantes AvaSpec-VRS2048CL-EVO is a compact, high-performance fiber-optic spectrometer from the VARIUS™ series. It is based on a symmetrical Czerny–Turner optical bench with a 75 mm focal length, delivering very low stray light (0.1–1%) across its full operating range. The spectrometer connects to the computer via high-speed USB 3.0 or Gigabit Ethernet, enabling scan rates as fast as 0.38 ms per spectrum.

The key technical specifications relevant to this experiment are:

• Wavelength range: 300–1100 nm
• Grating: 300 lines/mm, blaze wavelength 500 nm (type VA)
• Slit width: 50 μm (replaceable)
• Spectral resolution: ~2.5 nm FWHM
• Detector: Hamamatsu S11639-01 CMOS linear image sensor, 2048 pixels (14 × 200 μm)
• AD converter: 16-bit (dynamic range: 0–65,535 counts)
• Integration time: 9 μs – 59 s
• Signal-to-noise ratio: 300:1
• Sensitivity: 375,000 counts/μW per ms integration time

Detector

The detector in the spectrometer consists of two main elements:

1. Diffraction Grating. A ruled grating with 300 lines/mm and a blaze wavelength of 500 nm (Avantes type VA) is used to disperse the polychromatic light into its constituent wavelengths. You can read more about the principle of operation of diffraction gratings in Experiment 1 “Spectrum of The Hydrogen Atom”, sections 3.2.1 and 3.2.2 (here).

2. CMOS Linear Image Sensor. A Hamamatsu S11639-01 CMOS sensor with 2048 pixels (pixel size 14 × 200 μm) records the dispersed light. Unlike CCD detectors, CMOS sensors feature simultaneous charge integration across all pixels and a built-in electronic shutter, enabling very short integration times and high-speed readout. You can learn more about CMOS imaging sensors here.

It is important to note that the spectral efficiencies of the grating and the CMOS sensor both affect the measured signal, so that:

where I(λ) is the measured signal, G(λ) is the spectral efficiency of the diffraction grating, S(λ) is the spectral sensitivity of the CMOS sensor, and ρ(λ,T) is the actual light intensity that reached the detector. The efficiency and sensitivity curves are shown in Figures 8 and 9.

Software

The spectrometer is controlled by AvaSoft 8 (Figure 10), Avantes’ professional software for instrument control and data acquisition. AvaSoft 8 continuously reads spectra from the CMOS detector and displays them in real time. You can adjust three key parameters of the detection process:

• Integration time: The time, in milliseconds or microseconds, during which the CMOS sensor collects photons — analogous to the shutter speed in a camera. The AvaSpec-VRS2048CL-EVO supports integration times from 9 μs to 59 s. Adjust this value so that the brightest part of the spectrum reaches approximately 50,000–55,000 counts (about 80% of the 16-bit full-scale value of 65,535 counts). Avoid saturation (a flat-topped peak at 65,535 counts), which causes irreversible loss of spectral information.
• Averaging: The number of individual spectra averaged per displayed scan. Higher averaging improves the signal-to-noise ratio (S/N) by a factor of √N, where N is the number of scans averaged. This reduces time resolution but does not affect sensitivity.
• Boxcar smoothing: A spectral smoothing function that averages each pixel with its nearest neighbors. A value of n averages each data point with n pixels to its left and right, improving S/N by √(2n+1) but reducing spectral resolution. Keep this value small (0–3) to preserve the fine structure of the iodine absorption bands.

Before starting each measurement series, collect a dark spectrum (lamp off) and a reference spectrum (lamp on, no iodine) using the dedicated buttons. AvaSoft 8 will automatically subtract the dark and divide by the reference to display the transmittance or absorbance spectrum in real time. Data are saved as ASCII files and can be exported to Excel for further analysis.

Setup B — Ocean Optics LS-1 Light Source & StellarNet BLUE-Wave VIS-25

Experimental Setup B – Sublimation of Iodine

In Setup B we will use the following devices, shown in Figure B1:

1. StellarNet BLUE-Wave VIS-25 spectrometer:
   1. LS-1 Tungsten Halogen Light Source (1).
   2. Optical fiber (SMA-905), which collects transmitted light and transfers it to the detector (2).
   3. Detector composed of a 600 g/mm diffraction grating and a 2048-pixel CCD array.
   4. SpectraWiz software.
2. Iodine sample (3):
   1. Copper cuvette containing solid iodine.
   2. Resistive heating lamp.
3. Power source for the heating lamp — Thurlby Thandar PL154, 15 V / 4 A (4).
4. Thermocouple and voltmeter — Keithley 2100 6½-digit multimeter (5).

The power source (4) powers the LS-1 tungsten halogen light source (1), which generates broadband white light transmitted through the iodine cuvette (3) to the spectrometer detector (2). The iodine chamber is fitted with a thermocouple connected to voltmeter (5).

Light Source (Setup B)

The light source in Setup B is an Ocean Optics LS-1 Tungsten Halogen lamp. This lamp produces a continuous, broadband emission spectrum from approximately 360 nm to 2000 nm, with a color temperature of ~3100 K, making it ideal for visible-range absorption spectroscopy. The lamp is fiber-coupled via an SMA-905 connector directly to the iodine cuvette. More information about the LS-1 can be found here and here.

Spectrometer (Setup B)

The spectrometer used in Setup B is the StellarNet BLUE-Wave VIS-25 (Figure B2), a compact, rugged, fiber-optic miniature spectrometer optimized for visible-range measurements. The specific unit in the lab has the following identification data visible on its label:

• Model: BLUE-Wave VIS-25
• Serial number: 20081109
• Grating: 600 g/mm holographic
• Slit: 25 μm
• Calibration coefficients: C1 = 0.69789  |  C2 = 0.0001218  |  C3 = 336.31

The BLUE-Wave is a fiber-optically coupled instrument based on a symmetrical Czerny–Turner spectrograph (f/4, SymX geometry). Key specifications relevant to this experiment are:

• Wavelength range: 350–1150 nm (VIS model)
• Grating: 600 g/mm holographic
• Slit width: 25 μm
• Spectral resolution: ~1.0 nm FWHM (with 25 μm slit)
• Detector: 2048-pixel CCD array, pixel size 14 × 200 μm
• AD converter: 16-bit (dynamic range: 0–65,535 counts)
• Integration time: 1 ms to 65 s
• Signal-to-noise ratio: 1000:1
• Dynamic range: 2000:1 (with 6 decades)
• Stray light: <0.1% at 435 nm; <0.05% at 600 nm
• Interface: USB-2 (bus-powered, <100 mA)
• Fiber input: SMA-905, 0.22 NA
• Order sorting filter: integrated high-pass filter
• Dimensions: 1″ × 3″ × 5″ (25 × 75 × 125 mm)
• Weight: 14 oz (397 g)
• Software: SpectraWiz (included)

The spectrometer connects to the PC via a standard USB-2 cable and draws all its power from the USB port — no external power supply is needed. The modular, vibration-tolerant optical design has no moving parts, making the instrument suitable for portable and process applications as well as the teaching lab.

Detector (Setup B)

The detector in the BLUE-Wave VIS-25 consists of two elements:

1. Diffraction Grating. A 600 g/mm holographic grating disperses the polychromatic light into its constituent wavelengths. With 600 g/mm the grating provides twice the angular dispersion of a 300 g/mm grating, yielding finer spectral resolution (~1.0 nm with the 25 μm slit). You can read more about diffraction gratings in Experiment 1 “Spectrum of The Hydrogen Atom”, sections 3.2.1 and 3.2.2 (here).

2. CCD Array. A 2048-pixel CCD with 14 × 200 μm pixels records the dispersed spectrum. The CCD integrates charge simultaneously over all pixels during the integration period and then transfers the full spectrum to the 16-bit ADC. You can learn more about how a CCD works here and here.

As in Setup A, the spectral efficiencies of the grating and CCD both affect the measured signal:

where I(λ) is the measured signal, G(λ) is the spectral efficiency of the diffraction grating, S(λ) is the spectral sensitivity of the CCD, and ρ(λ,T) is the actual light intensity that reached the detector. The efficiency curves for the 600 g/mm grating and the CCD of the CHEM2000/BLUE-Wave family are shown in Figures B3 and B4.

Software — SpectraWiz (Setup B)

Setup B uses SpectraWiz, StellarNet’s proprietary spectrometer control and data acquisition software, which is included free with the BLUE-Wave. SpectraWiz continuously reads spectra from the CCD and displays them in real time. The adjustable parameters are identical in function to those of AvaSoft 8:

• Integration period (1): The time, in milliseconds, during which the CCD collects light — analogous to camera shutter speed. With the 16-bit ADC, the full-scale count value is 65,535. Adjust the integration time so that the brightest part of the spectrum reaches approximately 50,000–55,000 counts (≈80% of full scale). Avoid saturation (a flat-topped peak at 65,535 counts), which causes loss of spectral information. For reference, the original CHEM2000 (12-bit ADC) had a full scale of 4,096 counts; the BLUE-Wave’s 16-bit ADC gives ~16× finer resolution.
• Average period (2): The number of spectra averaged per displayed scan. S/N improves by √N with no effect on sensitivity. Increasing this reduces time resolution.
• Boxcar smoothing (3): Averages each spectral pixel with its n nearest neighbors on each side. S/N improves by √(2n+1). Keep this low (0–3) to preserve the fine structure of the iodine absorption bands.

To save results, enter your group number (5) and a file name (6), then press OK (7). Data are saved as tab-delimited ASCII files in D:\data\group number\file name and can be opened directly in Microsoft Excel for analysis.

Before recording the sample spectrum, collect a dark spectrum (light source blocked) and a reference spectrum (light source on, no iodine in path) using the dedicated toolbar buttons. SpectraWiz will use these to compute and display transmittance or absorbance spectra automatically.