The basic idea

In common syntheses of aspirin, salicylic acid is treated with acetic anhydride, causing a chemical reaction that turns salicylic acid's hydroxyl group into an ester group. Because acetic anhydride is used for the synthesis of heroin by the diacetylation of morphine, acetic anhydride is listed as a U.S. DEA List II precursor, and therefore it is restricted in many other countries including mine, so I won't use it. Instead I'll follow a slightly different path as a starting point:

As acetic acid is less reactive than acetic anhydride, the reaction yield will be lower. Furthermore the purity level of the acetyl-salicylic acid will be lower.

Balanced equation:

$\color{White} \large C_7H_6O_3+C_2H_4O_2 \rightarrow C_9H_8O_4+H_2O$ Molar mass salicylic acid: 138.122 g/mol

Molar mass acetic acid: 60.052 g/mol

Molar mass acetyl-salicylic acid: 180.157 g/mol

Molar mass water: 18.015 g/mol

Theoretical yield:

138.122 g salicylic acid + 60.052 g acetic acid → 180.157 g acetyl-salicylic acid

But I expect the actual yield will be only half of the theoretical one.

Synthesis of aspirin at a laboratory scale

Measure 1 g of salicylic acid ( ± 0.01 g)
Place it under fume hood into 50-ml Erlenmeyer flask
Cover salicylic acid crystals with 2 - 2.5 ml glacial acetic acid
Swirl flask and add 2.5 drops of concentrated sulfuric acid
Heat flask gently in a 70°C hot water bath for 10 minutes

Remove flask from water bath, add 15 ml distilled ice water
Allow mixture to stand for 5 minutes
Chill solution in an ice bath until crystallization of aspirin occurs
Stir occasionally
If crystallization does not occur, scratch walls and bottom of flask with a glass stirring rod to promote crystallization
If an oil appears instead of a solid, re-heat flask in hot water bath until oil disappears and cool again

Set up a vacuum filtration apparatus with Hirsch funnel
Transfer solid aspirin onto filter paper
Add small amounts of distilled ice water and repeat until transfer of aspirin crystals is complete
Wash aspirin crystals on filter paper with 10 ml of distilled ice water
Pour filtrate in the sink

Transfer aspirin crystals from filter paper to a 100-ml beaker
Dissolve crystals in a minimum volume of 10 ml pure ethanol
Warm solution in a 60°C hot water bath
Pour 25 ml distilled water into solution after solution has reached 60°C
If a solid forms, continue warming until solid dissolves

Remove beaker from heat
Cover beaker with a watch-glass
Set beaker aside to cool slowly. Do not disturb solution
Needle-like crystals of pure aspirin should form overnight

Testing for Impurities

Iron(III) chloride can be used to determine the purity of the synthesized aspirin. Iron(III) chloride forms a purple complex with the phenol group. Salicylic acid is a phenol. It cannot be removed by using cold distilled water as its solubility in water is poor. So if salicylic acid is still present, the product will turn purple when Iron(III) chloride is added.

Synthesis of salicylic acid at a laboratory scale

Salicylic acid can be synthesized by a base-catalyzed hydrolysis of methyl salicylate, commonly known as oil of wintergreen or wintergreen oil:

Balanced equation:

$\color{White} \large C_8H_8O_3+2~NaOH+H_2SO_4 \rightarrow C_7H_6O_3 + Na_2SO_4 + CH_3OH + H_2O$ As we can see, side products are water, sodium sulfate and methanol. The hydrate of sodium sulfate is known as Glauber's Salt, a formerly used general purpose laxative. Methanol is the simplest alcohol and highly toxic.

Molar mass methyl salicylate: 152.15 g/mol

Molar mass sodium hydroxide: 39.9971 g/mol

Molar mass sulfuric acid: 98.079 g/mol

Molar mass sodium sulfate: 142.04 g/mol

Molar mass methanol: 32.04 g/mol

Therefore theoretically:

152.15 g methyl salicylate + 79.9942 g sodium hydroxide + 98.079 g sulfuric acid → 138.122 g salicylic acid + 142.04 g sodium sulfate + 32.04 g methanol + 18.015 g water

Lab synthesis procedure:

Set up water jacketed condenser apparatus with 50 ml round bottom flask
Pour 3.5 ml of distilled water into the flask
Add 0.448 g sodium hydroxide
Swirl until solid dissolved
Add 0.23 ml of methyl salicylate to the NaOH solution
Drop boiling stone to bottom of flask
Attach flask to water jacketed condenser
Heat mixture for 15 minutes under fume hood

After reflux, separate heat source, allow equipment to cool down to room temperature
Remove condenser
Add 3 M sulfuric acid in 0.5 ml increments till white precipitate forms and stays
Add another 0.5 ml sulfuric acid

Set up a vacuum filtration apparatus with Hirsch funnel
Filter mixture
Dissolve salicylic acid crystals by using a minimal amount of boiling hot distilled water
Let the solution cool to room temperature, then cool it in ice bath for 8 minutes
Vacuum filter again and let product dry

Aspirin lab-on-a-chip

A lab-on-a-chip (LOC) is a device that integrates one or several laboratory functions on a single chip of only millimeters to a few square centimeters in size.

The rendering below shows a jacketed condenser apparatus, put together in sandwich construction, milled out from two borosilicate glass plates. The heating element is just a 1.5 Ω 50W electrical resistor as it is used in hot beds of 3-D printer.

Another possibility would be to 3-D print structure of apparatus with fire-proof and chemical resistant silicone on carrier glass plates:

The chemputer

A chemputer is a kind of 3-D printer that 'uses a bathroom sealant as a material to print reaction chambers of precisely specified dimensions, connected with tubes of different lengths and diameters. After the bespoke miniature lab had set hard, the printer could then inject the system reactants, or "chemical inks", to create sequenced reactions'.