Required Practical 1: Making a Volumetric Solution

Required Practical 1 has two connected parts. First you make a standard solution of sulfamic acid, H₂NSO₃H (also written H₃NSO₃): a solution whose concentration is known accurately because it was made from a weighed mass of a pure solid in a known volume.

Then you use that acid in an acid-base titration to find the concentration of a sodium hydroxide solution.

The chain of reasoning is:

The relative formula mass is needed at the first step, and students lose marks by copying a wrong value: Mr(H₂NSO₃H) = 3 × 1.0 + 14.0 + 32.1 + 3 × 16.0 = 97.1. Sulfamic acid is monoprotic, so one mole of acid neutralises one mole of sodium hydroxide:

H₂NSO₃H(aq) + NaOH(aq) → H₂NSO₃Na(aq) + H₂O(l)

A solid that can be weighed out to make a standard solution directly is a primary standard.

It must be available in high purity, stable in air (not hygroscopic or deliquescent, and not reacting with carbon dioxide), soluble in water, and have a reasonably high Mr so that weighing errors are a small fraction of the mass.

Sulfamic acid meets all of these, which is why it is chosen.

Sodium hydroxide fails the test: the pellets absorb water and carbon dioxide from the air while they sit on the balance, so their mass never gives their moles. Its concentration has to be found by titration against a primary standard instead.

The skills are examined in the written papers (at least 15% of the marks test practical skills) and the practical endorsement.

On this practical that means being able to describe weighing by difference, the transfer with washings, the volumetric flask technique, the titration technique, and the choice of indicator and its colour change.

It also means the treatment of titres, the calculation to 3 significant figures and the percentage uncertainty of each measurement.

The method has nine steps. Each one carries its reason, because the reasons are what the mark scheme rewards: “rinse the beaker” scores nothing on its own, “rinse the beaker so that all of the acid reaches the flask” scores the mark.

A volumetric flask has a single calibration line on its narrow neck. It is calibrated to contain 250.0 cm³ at 20 °C when the bottom of the meniscus sits exactly on that line.

The narrow neck is what makes it precise, because a small error in height is a very small error in volume.

A 250 cm³ measuring cylinder, by contrast, has an uncertainty of about ±1 cm³ or more, which is why it is never used to make a standard solution.

Fill to about 1 cm below the line with the wash bottle, then finish with a dropping pipette so that you cannot overshoot.

Read the mark with your eye level with the line: looking down on it makes the meniscus appear too high and causes parallax error.

If the level does go above the line the solution must be thrown away and the preparation started again, because there is no way of knowing how much extra water was added.

Never heat a volumetric flask or pour a warm solution into it. The reason is the liquid, not the glass.

A warm solution contracts as it cools, so a flask filled to the line while warm ends up below the line at 20 °C, the volume is less than 250.0 cm³ and the concentration is higher than the calculation says.

Glass expansion is a much smaller effect. Cool the beaker to room temperature before transferring.

Volumetric flask technique: calibrated to contain 250.0 cm³ at 20 °C, bottom of the meniscus on the line at eye level, cool the solution before filling, stopper and invert to mix.

The sulfamic acid solution goes in the burette; 25.0 cm³ of sodium hydroxide measured with a volumetric pipette goes in the conical flask with 2 to 3 drops of methyl orange.

Methyl orange is yellow in alkali and red in acid, so the flask starts yellow and the end point is the first permanent orange. Going on to red means the end point has been passed and the titre is too large.

Technique points that score: rinse the burette with the acid and the pipette with the alkali; fill the burette below eye level and remove the funnel before reading; make sure the jet below the tap is full of acid.

Read the bottom of the meniscus at eye level and record to 0.05 cm³; stand the flask on a white tile; run a rough titration first.

Near the end point add acid drop by drop with continuous swirling and rinse the sides of the flask with distilled water; repeat until two titres are concordant, within 0.10 cm³ of each other.

Water in the conical flask does not matter: the moles of sodium hydroxide were fixed when 25.0 cm³ was pipetted, and extra water only dilutes them without changing their number.

Water in the pipette or the burette does matter, because it dilutes a solution before its volume is measured.

The titration set-up: sulfamic acid in the burette read from the bottom of the meniscus with the funnel removed, 25.0 cm³ of sodium hydroxide and methyl orange in the flask on a white tile, and the yellow, orange and red colour sequence.

The mass of sulfamic acid is 2.41 g and the mean titre is 20.05 cm³. Every step is shown with its unit, and the unrounded value is carried from step to step.

The final answer is given to 3 significant figures because the mass (2.41 g) and the titres (20.05 cm³, 25.0 cm³) are known to 3 significant figures.

Two checks on the answer. First, the units: mol ÷ dm³ gives mol dm⁻³ at steps 2 and 5, and mol dm⁻³ × dm³ gives mol at step 3.

Second, the sense: the titre (20.05 cm³) is a little less than the pipette volume (25.0 cm³), so for a 1 : 1 reaction the alkali must be a little less concentrated than the acid, and 0.0796 is indeed a little less than 0.0993.

• Leaving a volume in cm³: divide by 1000 before using c = n ÷ V; an answer of 79.6 mol dm⁻³ for a dilute alkali should ring alarm bells.
• Averaging every titre, including the rough one, or treating only identical titres as concordant.
• Rounding too early: 0.0248 ÷ 0.250 gives 0.0992, while the unrounded 0.02482 gives 0.0993. Carry an extra figure and round once at the end to 3 s.f.
• Weighing the empty bottle first and assuming every grain was transferred; weigh the bottle with the acid, then with the residue.
• Reading the top of the meniscus, leaving the funnel in the burette, or starting with an air bubble in the jet.
• Not swirling, so the end point appears early in one part of the flask, or going on past orange to red.
• Too much indicator: 2 to 3 drops, because the indicator is itself a weak acid that reacts with some of the alkali.
• Drying the conical flask “for accuracy”: water in the flask changes nothing, since the moles of alkali were fixed by the pipette.
• Asserting the 1 : 1 ratio without writing the equation, or applying 1 : 1 to a diprotic acid or a carbonate.
• Quoting 0.079612 mol dm⁻³: five or six significant figures cannot be justified by 3 s.f. data.

Say

“Weigh the bottle with the acid, tip it out, reweigh the bottle with any residue: mass used = m₁ − m₂.”

“Rinse the beaker, rod and funnel and add the washings so that all of the acid reaches the flask.”

“Bottom of the meniscus on the line at eye level.” “Methyl orange turns from yellow to orange.”

“Titres concordant within 0.10 cm³ of each other; mean to 2 d.p.; answer to 3 s.f.”

Do not say

“Fill the flask up” (make up to the mark). “Concordant means the same” (within the stated limit). “Heating the flask expands the glass” (the liquid contracts on cooling). “Dry the conical flask” (water there does not matter). “Sulfamic acid : NaOH is 1 : 1” without the equation.

Watch for

Calculation questions that swap the primary standard for a carbonate or a diprotic acid: read the equation for the ratio.

Uncertainty questions: two readings for a mass by difference and for a titre, so double the instrument uncertainty before dividing.

Evaluation questions: say whether the alkali concentration comes out too high or too low, and why.

Related technique on Required Practical 1 (concentration of hcl by titration) and, for the same volumetric glassware used in gas work, Practical Skills: Gas Volumes (molar volume of a gas).