The simple synthesis of sodium citrate from sodium hydrogencarbonate and citric acid, for use in making Benedict’s reagent. This post also serves as a test for a new writing layout as I move slightly closer to a scientific paper format, and also includes a test video on my new YouTube channel which, for now, will produce a video now and then to accompany a chosen post here on WordPress.
Sodium citrate (Na3C6H5O7) can be easily and simply prepared by the acid-base reaction of sodium hydrogencarbonate (NaHCO3) and citric acid (C6H8O7):
3NaHCO3 (aq) + C6H8O7 (aq) → Na3C6H5O7 (aq) + 3H2O (l) + 3CO2 (g)
The solution will then be heated to the boiling point of water until enough water boils off that a precipitate forms, where from the solution will then be cooled to allow the sodium citrate product to crystallise from the reaction solution. The product will then be filtered, washed, dried, and stored in a container. I am aiming to form 47 grams of sodium citrate pentahydrate.
(I originally was aiming for 40 grams, although my calculations were based off sodium citrate being a dihydrate when in fact I discovered – half way through the reaction – that I would actually be forming the pentahydrate when searching for the solubility of sodium citrate  so excuse the funny numbers)
I plan to use the sodium citrate for use in making Benedict’s reagent, the reagent used to test a substance for the presence of reducing sugars.
|Sodium hydrogencarbonate||Low Hazard
It liberate carbon dioxide on gentle heating (or with acids). Sodium hydrogencarbonate is an approved food additive, E500, and is used as baking soda. 
Causes serious eye and skin irritation and may cause respiratory irritation.
|Sodium citrate||Low Hazard
Sodium citrate is an approved food additive, E331. 
Highly flammable liquid & vapour.
Sam’s personal safety notes:
- Gloves and goggles should always be worn when handling chemicals to protect your sensitive eyes and skin.
- Acetone readily evaporates at room temperature so it is suggested that you work in a well ventilated area due to the potentially harmful vapour. Also open flames should be avoided due to acetone being highly flammable. Acetone can also dissolve some plastics so avoid plastic reaction vessels.
- Citric acid can make you skin and eyes slightly sore, but as long as your eyes are washed pretty quickly afterward and your skin washed before touching other objects, it is extremely safe to work with.
My hotplate stirrer was set up with a 250ml glass beaker on top. 100ml of distilled water was added to the beaker, along with a magnetic stir-bar.
I turned stirring on and up to medium power, while I dumped a pre-weighed 29 grams of citric acid straight into the beaker (28.58 grams to be precise but my large scales can only measure to 0 decimal places). I left the solution to stir until all of the citric acid had dissolved.
Once all dissolved, I added small portions of sodium hydrogencarbonate by spatula from a pre-weighed container of 38 grams of the compound. Carbon dioxide gas is a byproduct of this reaction so I must add the sodium hydrogencarbonate slowly to the citric acid solution or else risking extreme fizzing which could cause the solution to bubble over and out of the container, hence the larger than needed beaker.
Just 34.27 grams were required, but, due to the lack of accuracy of the scales I was using, small portions of the 38 grams were added until fizzing stopped and pH showed neutral, therefore negating the inaccuracy of the scales and showing when all of the citric acid had been neutralised.
I ended up adding the full 38 grams of sodium hydrogencarbonate to cause the pH paper to finally produced the green colour than indicates a pH of 7 – neutral.
I placed a thermometer in the solution, held in place by the clamp seen on the right in figure. 5, and I turned the heating on and up to medium in order to get the solution boiling quickly. Once achieved, just below 100ºC was maintained by lowering the heating accordingly.
At around 80ml of liquid left, I transferred the solution to a smaller 100ml beaker to allow faster boiling.
Once down to ~50ml of solution left, a white precipitate quickly formed; this is seen from Figure. 7 to Figure. 8.
I immediately took the beaker off the hotplate and allowed to cool until room temperature with a lid on top to prevent the entering of dust. I ended up leaving the beaker for around 2 days in total, but even a few hours should suffice.
I set up for gravity filtration (although vacuum filtration can be used if you own the equipment, mine arrived just after I completed this synthesis slightly annoyingly). I poured any liquid into the filter paper, followed by the solid crystals that had formed – these crystals were very hard and required quite some persuasion from a few sharp metal objects to break up.
The filter paper ended up being too small so I eventually swapped it out for a larger piece of filter paper.
I then washed the filter paper with 2 portions of 5ml of acetone to dry the product, although I would recommend washing first with a similarly small portion of ice-cold water. I found it difficult to find the solubility of sodium citrate in acetone online, but did find that it was only slightly soluble in ethanol , so I decided it should have a low enough solubility to keep any lost product to a minimum.
I then took the filter paper out and placed it on top of some tissue paper in a glass dish, and left it to dry under a lamp for a day.
The final product was very lumpy so I enlisted the help of my mortar and pestle, reducing the particle size to a more easy to work with powder.
I was mostly satisfied with the new consistency of the sodium citrate so I placed it into a pre-weighed glass storage jar. The mass of sodium citrate pentahydrate came out as 43 grams.
My theoretical yield was 47 grams, while the actual yield of sodium citrate pentahydrate was 43 grams; therefore:
(43 ÷ 47) × 100 = 91.489
My percentage yield, when rounded to the appropriate 0 decimal places, produces a value of 91%. Although:
100 − ((43 − 2(0.5)) ÷ 43) × 100 = 2.33
100 − ((29 − 2(0.5)) ÷ 29) × 100 = 3.45
2.33 + 3.45 = 5.78
My percentage yield comes with a percentage error of just under 6%, so could be anywhere just below or above 97% or 85% respectively.
Either way, it is a decent percentage yield for me as I didn’t require all that much sodium citrate and just wanted to attempt a larger scale synthesis than needed as the reactants are cheap and easy to get hold of.
My yield was most likely lower than it could have been potentially due to a combination of not all of the citric acid reacting; the switching of the filter papers, as product will be lost in the discarded filter paper; the inaccuracy of the scales; and that when crystallising the sodium citrate, I did not put the solution in the fridge/freezer to reduce the temperature to gain more product – not done as the crystals already formed were a solid block and were hard enough to break up.
This is a nice easy preparation to get me back into chemistry and making posts this summer as I am currently enjoying a fat holiday before university. My upcoming posts might be on a weird schedule as I have recently become interested in Myrmecology and keeping ants as pets (of which I might make another blog to write about, so stay tuned!)
Thanks for reading!
(Please check out my video below. It is a new idea, aiming at an audience who are less familiar with chemistry than the audience this blog is aimed at. The quality is still a work in progress though, and I hope to improve, especially with the audio, in future videos)
Synthesis of sodium citrate
18thTimeLucky – Amateur Experimentalism
 “CRC Handbook of Chemistry and Physics, 97th Edition”. p. 4-85
(Sodium citrate pentahydrate soluble in water 92g per 100ml at 25 celcius)
 “Student Safety Sheets, 2nd Edition, 2018”. CLEAPSS. p. 25
(Safety information on sodium hydrogencarbonate)
 “Student Safety Sheets, 2nd Edition, 2018”. CLEAPSS. p. 33
(Safety information on citric acid)
 “Material Safety Data Sheet – Sodium Citrate”. Colonial Chemical Solutions, Inc.
(Safety information on sodium citrate)
 “Student Safety Sheets, 2nd Edition, 2018”. CLEAPSS. p. 61
(Safety information on acetone)
 “Lotus Illustrated Dictionary of Organic Chemistry”. p. 172.
(sodium citrate slightly soluble in alcohol)