Hydrochloric Acid Plus Sodium Hydroxide

In this experiment students neutralise sodium hydroxide with hydrochloric acid to produce the soluble salt sodium chloride in solution. They then concentrate the solution and allow it to crystallise to produce sodium chloride crystals

Yous have to decide if this experiment is suitable to use with different classes, and look at the need for preliminary training in using techniques involved in titration (encounter Teaching notes). What follows here assumes that teachers have judged the course to exist capable of doing this experiment using a burette with reasonable expectation of success.

Assuming that the students have been given training, the applied work should, if possible, start with the apparatus ready at each work place in the laboratory. This is to avoid vulnerable and expensive glassware (the burette) being collected from an overcrowded central location.

Fourth dimension required

Stage 1

Filling the burette, measuring out the alkali into the flask, and titrating it until it is neutralised takes about 20 minutes, with false starts existence likely for many groups. In practise it does non matter if the stop-point is overshot, fifty-fifty by several cubic centimetres, but the aim is to observe the proportions for a roughly neutral solution.

Stage 2

Producing a neutral solution costless of indicator, should accept no more 10 minutes.

Stage iii

Evaporating the solution may have the rest of the lesson to the point at which the solution can be left to crystallise for the adjacent lesson. Watching solutions evaporate can be tedious for students, and they may need another task to keep them occupied – eg rinsing and draining the burettes with purified water.

Equipment

Appliance

Eye protection
Burette, xxx or 50 cm^three (note i)
Conical flask, 100 cm³
Beaker, 100 cm³
Pipette, 20 or 25 cmⁱⁱⁱ, with pipette filter
Stirring rod
Small-scale (filter) funnel, about four cm diameter
Burette stand up and clamp (notation ii)
White tile (optional; note 3)
Bunsen burner
Tripod
Pipeclay triangle (note 4)
Evaporating bowl, at least 50 cm³ capacity
Crystallising dish (note 5)
Microscope or mitt lens suitable for examining crystals in the crystallising dish

Apparatus notes

If your school still uses burettes with drinking glass stopcocks, consult the CLEAPSS Laboratory Handbook, department 10.ten.1, for their care and maintenance. This experiment will not exist successful if the burettes used take stiff, blocked or leaky stopcocks. Modern burettes with PTFE stopcocks are much easier to utilize, require no greasing, and do not get blocked. Burettes with pinchcocks of whatsoever blazon are not recommended; while cheap, they besides are prone to leakage, especially in the hands of pupil beginners.
Burette stands and clamps are designed to prevent crushing of the burette by over-tightening, which may happen if standard jaw clamps are used.
The optional white tile is to go under the titration flask, merely white paper can be used instead.
Ceramic gauzes can exist used instead of pipeclay triangles, merely the evaporation then takes longer.
The evaporation and crystallisation stages may be incomplete in the lesson fourth dimension. The crystallisation dishes need to be set bated for crystallisation to take identify slowly. Yet, the dishes should not be immune to dry out completely, as this spoils the quality of the crystals. With occasional checks, it should be possible to decide when to decant surplus solution from each dish to leave skillful crystals for the students to inspect in the post-obit.

Chemicals

Sodium hydroxide solution, 0.iv Grand (IRRITANT), about 100 cm³ in a labelled and stoppered canteen
Dilute muriatic acid, 0.4 G, nigh 100 cm³ in a labelled and stoppered bottle
Methyl orange indicator solution (or alternative) in minor dropper bottle

Health, safe and technical notes

Read our standard health and safety guidance.
Wearable centre protection throughout.
Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentration used) – run across CLEAPSS Hazcard HC091a and CLEAPSS Recipe Volume RB085. The concentration of the solution does non need to be fabricated upwards to a high degree of accurateness, merely should exist reasonably shut to the same concentration equally the dilute hydrochloric acid, and less than 0.v Chiliad.
Dilute hydrochloric acrid, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043. The concentration of the solution does not need to be made up to a loftier degree of accurateness, but should be reasonably close to the aforementioned concentration as the sodium hydroxide solution, and less than 0.five Thousand.
Methyl orange indicator solution (the solid is TOXIC merely not the solution) – meet CLEAPSS Hazcard HC032 and CLEAPSS Recipe Book RB000.

Procedure

Stage 1

Using a small funnel, pour a few cubic centimetres of 0.4 One thousand hydrochloric acid into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, shut the tap and add together more solution upward to the nil marking. (Practise not reuse the acrid in the beaker – this should exist rinsed down the sink.)
Use a pipette with pipette filler to transfer 25 (or 20) cm^three of 0.4 One thousand sodium hydroxide solution to the conical flask, and add ii drops of methyl orange indicator. Swirl gently to mix. Place the flask on a white tile or piece of clean white paper under the burette tap.
Add the hydrochloric acid to the sodium hydroxide solution in minor volumes, swirling gently after each addition. Continue until the solution just turns from yellowish-orange to ruddy and tape the reading on the burette at this point. This coloured solution should now be rinsed down the sink.

Stage 2

Refill the burette to the zero marker. Carefully add the same volume of fresh hydrochloric acrid as you lot used in phase 1, stride 3, to another 25 (or 20) cm³ of sodium hydroxide solution, to produce a neutral solution, only this time without any indicator.

Stage 3

Pour this solution into an evaporating basin. Reduce the volume of the solution to about half by heating on a pipeclay triangle or ceramic gauze over a low to medium Bunsen burner flame. The solution spits about the terminate and you get fewer crystals. Practise not boil dry. Y'all may need to evaporate the solution in, say, twenty cmⁱⁱⁱ portions to avoid overfilling the evaporating basin. Do not attempt to elevator the hot basin off the tripod – permit to absurd first, and then pour into a crystallising dish.
Leave the full-bodied solution to evaporate further in the crystallising dish. This should produce a white crystalline solid in 1 or two days.
Examine the crystals under a microscope.

Looking for an alternative method?

Cheque out our practical video on preparing a table salt for a safer method for evaporating the solution, forth with technician notes, instructions and a run a risk assessment activity for learners.

Teaching notes

Titration using a burette, to measure volumes of solution accurately, requires careful and organised methods of working, manipulative skills allied to mental concentration, and attention to item. All of these are of course desirable traits to exist developed in students, but there has to be some degree of bones competence and reliability before using a burette with a course. The experiment is most likely to be suited to 14–xvi year old students. This is discussed further below, but what follows here assumes that you take judged the class to be capable of doing this experiment using a burette with reasonable expectation of success.

Students demand training in using burettes correctly, including how to clamp them securely and fill them safely. You should consider demonstrating burette technique, and give students the opportunity to practise this. In this experiment a pipette is not necessary, as the aim is to neutralise whatever volume of alkali is used, and that tin exist measured roughly using a measuring cylinder.

It is non the intention hither to exercise quantitative measurements leading to calculations. The aim is to innovate students to the titration technique only to produce a neutral solution.

Alternative indicators you tin utilise include screened methyl orange (green in brine, violet in acid) and phenolphthalein (pinkish in alkali, colourless in acid).

Leaving the concentrated solutions to crystallise slowly should assistance to produce larger crystals. The solubility of sodium chloride does non change much with temperature, so just cooling the solution is unlikely to form crystals.

Under the microscope (if possible, a stereomicroscope is all-time) you lot can come across the cubic nature of the crystals. If crystallisation has occurred in shallow solution, with the crystals only partly submerged, 'hopper-shaped' crystals may be seen. In these crystals, each cube face becomes a hollow, stepped pyramid shape.

Student questions

Stage ane

What substances have been formed in this reaction? Write a word equation and a symbol equation.

Stage 2

Why must you employ some other 25 cm³ of sodium hydroxide solution, rather than making your crystals from the solution in stage ane?

Phase iii

What shape are the crystals?