PCO2

Key takeaways

PCO2 belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect PCO2 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of PCO2 from memory before moving on to harder problems.

Reference excerpt

pCO2, pCO2, or P CO 2 {\displaystyle P_{{\ce {CO2}}}} is the partial pressure of carbon dioxide (CO2), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO2 as a function of its concentration in gas or dissolved phases. The units of pCO2 are mmHg, atm, torr, Pa, or any other standard unit of atmospheric pressure.

Medicine

In medicine, the partial pressure of carbon dioxide in arterial blood is called P a CO 2 {\displaystyle P_{a_{{\ce {CO2}}}}} or PaCO2. Measurement of P a CO 2 {\displaystyle P_{a_{{\ce {CO2}}}}} in the systemic circulation indicates the effectiveness of ventilation at the lungs' alveoli, given the diffusing capacity of the gas. It is a good indicator of respiratory function and the closely related factor of acid–base homeostasis, reflecting the amount of acid in the blood (without lactic acid). Normal values for humans are in the range of 35–45 mmHg. Values less than this may indicate hyperventilation and (if blood pH is greater than 7.45) respiratory alkalosis. Values greater than 45 mmHg may indicate hypoventilation, and (if blood pH is less than 7.35) respiratory acidosis.

Aquatic sciences

Oceanographers and limnologists use pCO2 to measure the amount of carbon dioxide dissolved in water, as well as to parameterize its flux into (influx) and out of (efflux) the atmosphere. Carbon dioxide reacts with water to form bicarbonate and carbonate ions, such that the relative solubility of carbon dioxide in water is greater than that of other unreactive gasses (e.g. Helium). As more carbon dioxide dissolves in water, its pCO2 rises until it equals the pCO2 of the overlying atmosphere. Conversely, a body of water with a pCO2 greater than that of the atmosphere effluxes carbon dioxide. pCO2 is additionally affected by water temperature and salinity. Carbon dioxide is less soluble in warmer water than cooler water, so hot water will exhibit a larger pCO2 than cold water with the same concentration of carbon dioxide. pCO2 can be used to describe the inorganic carbon system of a body of water, together with other parameters such as pH, dissolved inorganic carbon, and alkalinity. Together, these parameters describe the concentration and speciation of inorganic carbon species (CO2(aq), HCO3−, CO32-) in water. Biological processes such as respiration and photosynthesis affect and can be affected by aquatic pCO2. Respiration degrades organic matter, releasing CO2 into the water column and increasing pCO2. Photosynthesis assimilates inorganic carbon, thereby decreasing aquatic pCO2.

See also Acidosis Alkalosis Arterial blood gas Blood gas tension Chemical equilibrium Hypercapnia pH Carbon Cycle xCO2

References

This article incorporates text by Glynda Rees Doyle and Jodie Anita McCutcheon available under the CC BY 4.0 license.

Illustrations

Worked examples

Example 1 — a first encounter with PCO2

Start with the simplest possible case. Write down what PCO2 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to PCO2 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about PCO2 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of PCO2

In research: PCO2 appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses PCO2 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: PCO2 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acid–base chemistry, Analytical chemistry, Chemical oceanography, so understanding it makes those chapters shorter.
In everyday life: Look for PCO2 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study PCO2 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what PCO2 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain PCO2 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is PCO2 in simple terms?

pCO2, pCO2, or P CO 2 {\displaystyle P_{{\ce {CO2}}}} is the partial pressure of carbon dioxide (CO2), often used in reference to blood but also used in meteorology, climate science, oceanography, and limnology to describe the fractional pressure of CO2 as a function of its concentration in gas or…

Why does PCO2 matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study PCO2?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on PCO2.