Sciencemadness Discussion Board - Chlorine - Illustrated Practical Guide

Chlorine - Illustrated Practical Guide

len1 - 27-12-2007 at 20:08

This is a first in a series of Illustrated Key Syntheses in Chemistry which I intend to post, with the purposes and guidelines outlined in the thread of the same name in the General Chemistry section.<br> I shall attampt to ensure that the first post in all syntheses contains all the relevant information. If something needs to be added later on, due to comments further down the thread for instance, I shall edit the first post.

Aim

It is the aim here to present an illustrated demonstration of a Cl2 generator, with calculated yields, impurities, etc. Although inspired by the the method in the Practical Chlorine post it is substantially simpler and will yield better Cl2.

Motivation

The fact that chlorine is a key element in many inorganic and organic syntheses, in the chlorination of organic compounds - a route to many organic chemicals, and in formation of such useful inorganic compounds as ZnCl2, FeCl3, ALCl3, etc, is without question. However, one might wonder if its synthesis is really necessary given the topic seems to be exhaustively covered in most elementary practical guides. In fact the vast majority of these present only three methods suitable for the laboratory. Reduction with permanganate, manganese dioxide, or the chromates.

The first,

KMnO4 + 8HCl -> Mn2+ + K+ + 5/2Cl2 +4H2O +3Cl-

is really overkill. KMnO4 is a much stronger oxidizing agent than is necessary to oxidize Cl-. Here are some more minuses

It is relativey expensive, and available only in speciality outlets.
It is wasteful, contributing no chlorine to the reaction and with 3/8 of Cl2 remaing in solution.
The product Cl2 is heavily contaminated with H2O and HCl due to the reaction being highly exothermic.
The exothermicity also means there is a great chance for thermal run-away - due to higher temperatures speeding up the reaction, which releases more heat etc.
As a result, means for dealing with spray produced, absorbing water and HCl, and cooling the reaction need to be provided - which all add to unnecessary hassle in controlling the chlorine generation , when you should be concentrating on what it is you are trying to chlorinate.

The other reactions suffer from similar problems, although oxidation with MnO2 is easier to control.

A much forgotten post by Organikum (practical method to chlorine, an outflow from the chlorine thread) suggested the use of Trichloroisocyanuric acid C3N3O3Cl3, a chemical ubiquosuly availabale as a pool chlorinator together with HCl as a means of producing Cl2 without the piggyback O2 impurity which the other common chlorinator, hypochlorite bleach, is purported to produce.

Findings

I have found that the TCCA reaction is almost ideal for producing chlorine

Enthalpy change almost zero - no heating up observed
No Cl2 wastage - all chlorine released
Reaction is almost stoichiometric, >93%
O2, H2O, HCl impurities minimal - <0.3% O2, <1% H2O + HCl
Easily controlled, stirring not required.
Dry chlorine means minimal attack of polyethylene tubing.

Use of hypochlorite instead of TCCA was found to:

generate substantial heat and HCl + H2O impurities, -tubing degraded almost instantly
require shaking to maintain
O2 impurity present in Cl2 at 1.6% by volume - possibly acceptable in many syntheses.

Findings regarding collecting Cl2 is gasometers

References suggest 0.1N HCl is optimal for reducing Cl2 solubility in the gasometer. I found a solubility in accord with 0.06mol/L quoted in some references, the 0.5 - 0.7 gm/L quoted in other references is much too low.
At higher HCl concentrations Cl2 solubility increases greatly - this means Cl2 gathered from the hypochlorite reaction is unsuitable for collection over water.
Solubility in 5 mol/L NaCl solution (about saturation) is minimal, being of the order of 0.003mol/L (with no HCl), a figure much lower than found in some references - and in accord with others. Saturated NaCl solution is ideal for storing Cl2 in a gasometer.

Theory

Cl2 from TCCA (a six-membered ring, C and N alternating)

C3N3O3Cl3 + 3HCl -> 3Cl2 + C3N3O3H3 (isocyanuric acid)

The TCCA reacts essentilly from solid phase as both it and the product acid are only slightly soluble.
Cl2 from calcium hypochlorite

Ca(OCl)2 + 4HCl - > Ca2+ + 2H2O + 2Cl2 + 2Cl-

Reaction from solid phase as Ca(OCl)2 slightly soluble.
O2 from calcium hypochlorite

Ca(OCl)2 -> CaCl2 + O2

The latter process is clearly spontaneous. As most reactions it is accelerated by heat, (especially since it involves gas evolution), as well as moisture due to the hydration energy of CaCl2.

Method

All reactions were carried out at an ambient temperature of 30C

Yield estimation for Cl2 using gasometers

The chlorine generator for TCCA is extremely simple. The TCCA, if in tablet form, is broken with a hammer inside a plastic bag so the pieces fit in a mortar, ground to a coarse powder, and deposited in a flask with a ground-glass neck. HCl is diluted to 15% wt/L and poured into a dropping funnel. The funnel tap is opened so the HCl drips on the powder at a rate of about 1 drop/sec. The Cl2 flow rate is about 1L/2 mins. Estimate the empty volume of the generator and discard twice that volume of the initial gas generated. The Cl2 being heavier, tends to displace the air ahead of it. There is no need to cool the HCl as the reaction generates no perceptible heat, and there is no need to stir the reagents - the reaction is instanteneous.

In addition, one does not gain anything from heating the reagents - raising the temperature to 60C was found to liberate only 3% extra Cl2 after the reagents had been mixed and allowed to equilibrate for 10mins. However the addition of HCl and H2O impurities to the generated chlorine at the higher temperature was substantial.

In the present experiment, 11.6gms TCCA (0.05 mol), figure 1, was placed at the bottom of a 250ml RB flask, and 36 ml of HCl consisting of a 50:50 30% wt/v HCl : H2O mixture (0.15 mol) was dripped-in in 10mins. 3443ml of gas was generated and collected in a gasometer over 5mol/L NaCl solution. This represents 93% yield of theoretical. The difference is assumed dissolved in the NaCl solution - representing a solubility of about 0.004 mol/L Cl2.

<IMG src="http://www.sciencemadness.org/scipics/Len1/Cl2_figs1_2.JPG">

Impurities

Water

An inverted U-tube, used to eliminate direct spray, leads from the RB flask to delivery tube, with 15.4gms of fused CaCl2 dessicant between two plugs of glass wool. While CaCl2 has a relatively high water vapor pressure, here it is used solely to gauge the amount of H2O in the generated gas, and the small amount it doesnt absorb is immaterial. The outlet from the tube bubbles into parafin to prevent moisture entering the tube from the outlet. The weight gain for 3.4L of Cl2 generated was 0.101gms. Since CaCl2 absorbs up to 0.32 H2O gm/gm, the amount of CaCl2 needed at 50% absorption is 0.19gms/L of Cl2 generated with this apparatus. There is so little H2O in the generated gas that the requisite CaCl2 can be placed between wool plugs in the connecting glassware. This should be followed by a CaSO4 plug for low final H2O partial pressure (CaSO4 dries better than CaCl2, but is capable of absorbing far less H2O).

Oxygen

After the generator had evolved the first two litres of Cl2, the chlorine was bubbled into a 1L RB flask until it filled it completely, figure 3. Since Cl2 is much heavier than air, almost all air originally in the generator had been purged at this stage. The openning of the filled flask was now placed in a 10% wt/L NaOH solution, whereupon the Cl2 was chemically combined in the following reaction:

2NaOH + Cl2 -> NaOCl + NaCl + H2O

The equilibrium constant for this reaction

[Cl-][ClO-]/[Cl2]aq[OH-]^2 ~ 10^20 mol/L

so the vapor pressure of Cl2 above such a solution is frations of an atmosphere, and almost all Cl2 dissolves.

The air/O2 is not absorbed see the small bubble in figure 4. This measured 4ml, as obtained by inverting the flask, and filling to overflow with the remnant NaOH solution. This represents 0.35% O2/air by volume. It is not claimed that all this impurity gas is generated in the reaction, such as small amount could represent remnant air in the apparatus, as well as air dissolved in the solution. Rather it gives a ballpark figure for all these components combined. It will be used as the control result when we come to evaluating the O2 impurity in the Ca(OCl)2 chlorine generator.

<IMG src="http://www.sciencemadness.org/scipics/Len1/Cl2_figs3_4.JPG">

The hypochlorite reaction

The amount of Ca(OCl)2 corresponding to the mole fraction of chlorine (0.15mol) of the previous experiment is 10.7gms. The available Cl2 from this reagent is 142/143 ~ 993gms/kg as evidenced by the reaction, however only 650gms/kg was quoted on the container. This was assumed to correspond to the purity of the hypochlorite, and hence to achieve the same amount of Cl2 evolved as in the TCCA experiment, where the yield was almost 100%, the amount of hypochlorite was increased by a corresponding fraction, figure 5. The setup can be seen in figure 6.

<IMG src="http://www.sciencemadness.org/scipics/Len1/Cl2_figs5_6.JPG">

The results were inferior to the TCCA reaction in almost every respect. Its best to summarise them pointwise.

When half the stoichiometric HCl was used up the reaction ceased - the Ca(OCl)2 available chlorine was less than half the quoted value. This reagent easily loses its chlorine on standing. The amount of Cl2 evolved was about 1.5L compared to the 3.4L of the TCCA.
The reagent mixture heated up, requiring cooling to prevent excessive effervescence and run-away.
H2O and HCl impurity evolution was excessive due to the heat of the reaction.
Wet chlorine is much more corrosive to the plastic tubing, as can be seen by comparing the tube in figure 2, through which about 6.8L of TCCA generated Cl2 had passed, with that in figure 7, after just 1.5L of Ca(OCl)2 generated Cl2.
The HCl generated increases the solubility of the Cl2 in concentrated brine, so the gasometer liquid in figure 7 contains much more Cl2 (yellower) than in figure 2.
The reagents would not react well even at 60C without shaking, when violent effervescence could begin.
The reaction is thermally unstable.
A much greater amount of O2 is evolved, compare the bubble in figure 8, with that in figure 3. The O2 volume over and above that in the TCCA experiment amounted to 20ml - in 1120ml of gas, representing 1.6% oxygen impurity in the chlorine.

<IMG src="http://www.sciencemadness.org/scipics/Len1/Cl2_figs7.JPG">

Conclusion

The TCCA method is a hassle free means of chlorine generation. Its thermal stability, uniform evolution of Cl2, and low impurities mean that the scrubing need be far less intensive, and attention can be dedicated to the chlorination process rather than looking after the gas generator.

[Edited on 13-1-2008 by len1]

chromium - 7-1-2008 at 15:20

Very good artidles (this one and about diethyl ether). Only criticism that comes to mind is that titles could be more descriptive. This text for example is not everything about chlorine but rather its production from TCCA and from Ca(ClO)2.

len1 - 7-1-2008 at 16:15

Well, it would be impossible in a single article to cover everything about chlorine - thats a huge undertaking for a specialised book, that I doubt anyone here would undertake.

I also wouldnt say its completely accurate to describe this as just an article about Cl2 production from TCCA and Ca(OHCl)2. The three most common methods discussed in the introduction, as well as others which are not discussed to keep the article short, such as electrolysis, were considered, I chose these two methods as the most suitable for amateurs, because of their simplicity, cheapness, reliance on easily accessible materials. In the end the TCCA method turned out to be so superior in my view, I could not see the need to describe any other methods for chlorine preparation, if the aim is just to prepare Cl2 and get on with the synthesis.

I take the point however that the title hints at a 'review of chlorine', which it isnt. Its an illustrated practical guide with analysis for fascile chlorine preparation in the laboratory. Maybe it should be titled such? I dont know, what do people think?

Arsole - 13-12-2012 at 23:23

I liked the title. I read it as "Chlorine, this is how you do it".

Oscilllator - 18-12-2012 at 21:51

You might want to explain that TCCA is Trichloroisocyanuric acid. I had to look it up.

hyfalcon - 19-12-2012 at 02:41

If you would read the entire post you wouldn't have had to look it up.

Organikum - 20-12-2012 at 00:09

For the sake of readers who want to repeat what you describe you could mention that PE is just not the right plastic for tubing in this case. Sinple PVC works better, of course it gets turbid and the softeners degrade so it gets pretty hard too, but there is no structural breakddown even in prolonged use. It is even superior to PTFE tubing as this suffers from quite some permeability. PVC not at all.

Good work though!

/ORG