Introduction

Citric acid is a user-friendly, inexpensive, water-soluble, crystalline heavy carboxylic acid. In compare to making use of hydro-chloric mountain solutions, making use of solid citric acid and its services in water is safer and an ext convenient. Because that inexperienced student its titrations against strong base are less complicated to perform. This write-up presents a neutralization titration that a citric acid systems by sodium hydroxide equipment in a format suitable for beginning of the person titrators. A second article will imply applications of the very same experiment the are an ideal for competent titrators.

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Citric acid

Citric acid1 is produced by the fermentation of sugars (> 1 M tonne per year) for usage in beverages and also foods (70%); in detergents (20%); and in cosmetics, pharmaceuticals and also other chemicals (10%). It is developed as a crystalline solid, either anhydrous, or together a monohydrate, and also is easily accessible in either form at low cost. The hard monohydrate loses water below 100 °C when heated, forming the anhydrous solid, i m sorry melts at 156 °C, and also decomposes at 175 °C. Both solids are stable and remain free-flowing in the party after plenty of years of storage. That is really soluble in water. Citric acid have the right to be purchased at low price as a reagent chemistry of either form at assorted purity levels,2 or at also lower expense as a consumer substance that unspecified purity and also usually unspecified form.3

Titrating citric mountain against solid base

Citric Acid

(a triprotic acid)

192.1 g / mol

H3C6H5O7

*

pKa Values (1)

3.14

4.75

6.40

Citric acid has actually three carboxylic mountain groups, 3 ionizable, acidic hydrogen atoms and three Ka/pKa values. The neutralization reaction with sodium hydroxide has actually 3 to 1 stoichiometry as depicted by the well balanced complete neutralization equation. The reaction goes come completion and also is suitable for analysis titrations:

3 NaOH (aq) + H3C6H5O7 (aq) --> Na2C6H5O7 (aq) + 3 H2O

When titrated by a solid base such as 0.1 M NaOH solution, a equipment of citric acid traverses a buffer an ar during which the pH of the systems climbs slowly then an ext steeply. From about one added drop that the 0.1 M NaOH solution prior to the equivalence allude of the titration to around one added drop after the equivalence allude the pH the the titration equipment climbs incredibly steeply native slightly listed below 7 to above 9.A intuitive acid-base indicator favored for the titration must adjust from its acid colour come its base colour in the range 7 come 9. Phenolphthalein indicator is terrific choice because that this titration, changing from colourless come pink to red. In comparison come titrations v a solid acid such together hydrochloric acid, the diluted weak acid “mops up” the basic in the autumn of added titrant more slowly. The result is that as the equivalence point is approached, also with swirling, the pink-red colour appears where the drops of titrant go into the titration flask and persists, at first for fractions of a second, then longer and longer, until a pass out pink colour have the right to be seen throughout the systems that persists for at the very least one minute (see Question 3). This phenomenon renders these titrations an extremely easy to perform, and also therefore very suitable for novices.

Safety, disposal of waste, and also storage

Citric mountain is a fairly strong weak acid, but no unique precautions are compelled for that use. Citric acid powder is marketed for residence use v no restrictions. The pH that 0.033 M citric mountain is around 2.2, which is slightly greater than the of lemon juice.4 The 0.1 M sodium hydroxide and the phenolphthalein indicator are more hazardous. Handle and clean up solid citric acid as you would solid sodium hydroxide.

Titrated solutions and excess that reagent solutions may be safely disposed the in a sink. In ~ Mohawk college we store the dropper party containing the sodium hydroxide systems for long periods through no evident ill effects. Warehouse of citric mountain solution may not be advisable, due to the fact that it may well support microbiological life.

Experiment description

Here is a simple, fast and inexpensive an approach for performing titrations the citric acid solution with salt hydroxide solution. That is assumed that the titrations room being carry out gravimetrically using inexpensive, unbreakable, 60-mL regulated drop-dispensing polymer to express bottles.5a

Preparation, apparatus and also supplies

2-place top-loading digital balanceSufficient volume the 0.1 M NaOH (4.0 g/L)Sufficient volume of 0.033 M citric acid (6.4 g/L)Sufficient volume the phenolphthalein indicator in small labeled dropper bottles60-mL controlled drop-dispensing polymer to express bottles, labeled ‘NaOH’125-mL Erlenmeyer flasks10-mL i graduated cylinders*small beakersdropper pipetsdistilled water in squeeze bottleswaste containers

*A smaller sized bore diameter is much better for precision.

Make up the 2 reagents in approximate fashion, such the your breakthrough trial titrations indicate that a 5-mL sample the the citric acid equipment is neutralized by approximately 5 g that the salt hydroxide solution. As a sample calculation, label one or various other with a “fictitious” worth for concentration and have the students determine the concentration of the other solution by titration and also calculation. (See calculations below.)

The full technique and a sample titration should be demonstrated for novices. Point out a precision default of successful completion. A reasonable traditional would it is in “three titration masses within ± 3% of the average mass”. If using 50-mL burets, boost the volume that the citric acid samples from 5 mL to 10 mL come have roughly the same precision in the results.

*

Gravimetric titration through a polymer regulated drop-dispensing squeeze-bottle and also a 2-place digital balanceGravimetric titration with a polymer controlled drop-dispensing squeeze-bottle and a 2-place digital balance

Student instructions

Transfer around 10 mL the the citric acid solution right into a small beaker (100 or 250 mL). Usage this part of solution to wash the inner surface ar of the tiny beaker, the 10-mL i graduated cylinder and also the dropper pipet. This rinse part is waste. Repeat the rinsing double more. DO no RINSE the 125-mL Erlenmeyer flask, it may be left wet, but only v distilled water. (See Question 1.)Transfer around 50 mL of the citric mountain solution into the rinsed little beaker. This amount will certainly be needed for the three trials. For trial 1 transfer some of the equipment as carefully as you deserve to into the 10 mL i graduated cylinder, as much as the 5.0 mL line, making use of the dropper pipet to change the bottom of the meniscus to the line.Transfer the 5.0-mL sample that citric acid equipment for psychological 1 from the cylinder into a 125-mL Erlenmeyer flask. North the cylinder totally, by waiting till the last drops fall. Include distilled water from the squeeze-bottle so the the full volume in the flask is between 20 mL and 30 mL. Include 4 to 5 drops of phenolphthalein indicator solution come the flask. Swirl the flask gently to fully mix the contents.Press the zero/tare switch on the 2-place digital balance. Ar the drop-dispensing bottle include NaOH systems on the balance pan. The balance pan, the external of the dropper bottle and also your fingers need to be retained dry at every times. (See Question 2.)Record the fixed of the bottle and also its contents.Titrate the systems in the Erlenmeyer flask by adding drops of solution from the drop-dispensing bottle. Host the party upside under over the mouth the the flask. Count drops. Swirl the flask gently. The end suggest is got to when, on adding one fall of the base, the solution in the flask alters from colourless come pink or red and also the colour stays for at least a minute.

If, after a many drop enhancement of base the equipment is deep red, the end-point may have been passed, and the titration an outcome must not be supplied for calculations. If you “lose” a drop throughout a titration, the titration must not be used for calculations.

Repeat actions 4 and 5. Record the brand-new mass the the bottle and also its contents. Subtract your mass values to acquire the titration mass of the 0.1 M NaOH solution.Repeat the titration procedure. The fall count deserve to serve together a overview to speed up the repeat titrations. Proceed until the default of effective completion is achieved.

Calculations

The thickness of a 0.125 M NaOH systems at 20 °C is 1.0039 g/mL.6 For student calculations the density of a 0.1 M NaOH systems is for this reason close to unity in g/mL units that the mass values of titrations in g units have the right to be provided as volumes in mL devices without significant error.

My preferred method for introductory students is a 3-part calculation. Suppose that repeat titrations of 5-mL samples of citric acid solution produced a median titration result of 4.87 g the 0.0989 M NaOH (= 4.87 mL of 0.0989 M NaOH):

1.

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Calculate the moles of the recognized reagent (NaOH):

4.82 × 10-4 mol NaOH

2.

Calculate the moles of the unknown reagent (citric acid) using the balanced equation: