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BET surface area of a titania reference material

Run a static manometric sorption analyser by the protocol in the NIST SRM 1898 certificate: weigh, outgas, measure the free space with helium, immerse in liquid nitrogen, dose nitrogen between 0.05 and 0.30, and compare the BET surface area with the certified value.

Difficulty ●●○ ≈ 30 min

Aim Measure the multipoint and single-point BET surface areas of nanoscale titania, check that the fit meets the protocol, and explain why the single-point value is low.
The NIST SRM 1898 protocol · static manometric method · N₂ at 77.3 K

Gas sorption analyser: the surface area of a titania

Static manometric sorption analyser
N₂Hevacuum pumppressure0.00 kPa77.3 Ksamplep⁰ tube

Procedure

Isotherm
00.10.20.30.4p/p⁰05101520n / cm³(STP) g⁻¹
BET plot (points in 0.05–0.30)
00.10.20.3p/p⁰0123x/[n(1 − x)] / 10⁻² g cm⁻³
Data points
Targetp / kPap/p⁰Adsorbed / cm³(STP)n / cm³(STP) g⁻¹
No points yet.
The sample is a model: it obeys the BET equation exactly, with the certified multipoint area of NIST SRM 1898, 55.55 m²/g, and C = 73.9 from the certificate’s ratio of single-point to multipoint area; each dose settles within a 2 % scatter of its target pressure and carries a 0.3 % scatter in the amount (deterministic: the same actions give the same numbers). The outgassing loss is taken as 1.0 %, the certificate’s “of order 1 %”. p⁰ comes from the Clausius–Clapeyron equation with nitrogen’s normal boiling point, 77.355 K, and its enthalpy of vaporisation, 5.57 kJ/mol. The instrument is a sketch, and the approach to equilibrium after each dose is not computed with real time constants.

Procedure

  1. Set the powder mass between 0.2 and 0.6 g, as the certificate requires.
  2. Press “Outgas under vacuum”: 110 °C for 10 min, then 200 °C for 2 h. Note the mass loss.
  3. Measure the free space with helium, then raise the liquid-nitrogen Dewar and read p⁰.
  4. Dose nitrogen at least four times between 0.05 and 0.30 (or press “Run the routine six points”), waiting for equilibrium each time.
  5. Read the multipoint and single-point areas and compare them with the certified values; then untick “Divide by the outgassed mass” and see what changes.

What to notice

  • The six routine points give a multipoint area of about 55.8 m²/g (C ≈ 72, r² > 0.9999) and a single-point area of about 54.0 m²/g, both within the certificate’s uncertainty.
  • The single point is about 3 % below the multipoint value, just like the two certified values (55.55 and 53.85): the single-point method ignores the intercept of the BET plot.
  • Dividing by the mass before outgassing lowers both areas by 1 % (to about 55.2 and 53.5 m²/g) — still inside the uncertainty here, but a systematic error all the same.
  • Without outgassing, a free-space measurement or the liquid-nitrogen bath, the analyser refuses to dose and says why.

In a real lab

  • The health risks of nanoscale titania are not fully established, and NIOSH considers ultrafine titanium dioxide a potential occupational carcinogen: transfer small amounts of powder away from strong draughts, and avoid raising or breathing dust.
  • While outgassing, evacuate no faster than 0.667 kPa/s or the powder is sucked into the vacuum lines; without automatic control, use a slow ramp such as 1 °C/min.
  • Liquid nitrogen causes cold burns: wear a face shield and insulated gloves when pouring it; keep its level at least 50 mm above the sample and constant within 1 mm during the run.

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