Why do we have to do chemical equations and calculations?

Fe & Sulfur reaction

(A look at a workplace exposure limits found in MSDS sheets)

Introduction

Why do chemical equations and calculations? Well, so we can grade students in tests in exams. Calculations and chemical equations are easy to mark as you are either right or wrong. If I ask you questions requiring written answers, I have to wade through lots of written explanations and the marking is sometimes subjective.

Editor: “No, that is too cynical”.

“Alright, well there is useful information in section 8 of a (Material) Safety Data Sheet (MSDS) that teachers can use and shows how a knowledge of chemical equations and calculations helps protect the health of their students and themselves and helps to assure their employers and safety officers that teachers and lecturers are responsible and professional users of chemicals.

Editor: “Much better. “OK, so here goes”.

What is section 8 of (Material) Safety Data Sheets?

This section is headed exposure controls and personal protection. The aim is to provide the employer and employee with information which will protect workers from being exposed to a chemical. (The has dropped the word “Material” from Material Safety Data Sheet but it still seems to be prevalent in the publications from many suppliers.)

Fig 1 shows section 8 of the MSDS sheets for methanol, from an American supplier; Fig 2 shows section 8 from a UK supplier. You will notice that the values are similar. There is no hard and fast rule on these Exposure controls and each country is allowed to make its own judgement, even in closely-connected countries as in the European Union, there are differences (even though Europe does have guidance, called Indicative Occupational Exposure Level Values or IOELV).

 


 

I will not go into the acronyms too deeply but TWA means a Time Weighted Average over 8 hours (useful for storage those working in factories or research laboratories) and STEL means Short-Term Exposure Limit averaged over 15 minutes (useful for school experiments in the lab). Seeing these values often means that safety officers immediately say “fume hoods or fume cupboards must be used”. And yet Americans pour methanol into their cars in garages, people all over the world, paint their nails with chemicals with an Occupational or Workplace Exposure Level (OEL and WEL). Err, there is a toxic chemical (with an OEL) are emitted from orifices in our bodies but in a lab, the safety officer would be up in arms, if they detected that gas (by odour) from a chemical reaction.

The Units

The unit, parts per million (ppm) or mg/m-3 represents 1cm3 of the chemical or 1 mg of the quoted gas in 1m3 of air. They are related to each other via the molar mass of the substance and the volume of a mole of gas at room temperature and pressure (I will use 24dm3 in the calculations). Able students could work out this relationship for themselves.

You will need to measure the size of the room that you work in, using meters as the units. I know some are going to say we can open the windows to increase ventilation and we are not taking into consideration diffusion; these are two very good discussion points and I have seen calculations that take them into account. However, we have to start somewhere and when I am confronted with providing advice and risk assessments to teachers on our CLEAPSS helpline, a few simple calculations are extremely useful.

The Experiment

Boiling a liquid In a room, 240m3, sealed for 15 minutes, I could boil 250 x 240 mg or 60g (76ml) of methanol without contravening the rules (naturally I would not, I would use a fume hood). When I worked this out for the first time, I was surprised that, in school terms, this is quite a large volume of liquid.

However, this makes pouring 10ml of methanol from a bottle into a flask or spirit burner a low risk action in terms of exposure. As recent events have shown, with regard to risk, fire is a much more serious issue.

It is useful to add that the use of a Liebig condenser is very efficient in stopping the organic chemicals filling a room but one needs to work on a case by case assessment.

Handling a gas jar of chlorine The volume of my measuring cylinders is 400ml. What if I should release 400ml of chlorine into the room? The STEL value for chlorine I found is 1 ppm. So I would have 400/240 or 1.7ppm of chlorine spread out into the room and I would be in contravention of the legal limit. (I am using a Canadian government website and SDSs from American chemical suppliers for information here; I am finding National US websites quite confusing.) In addition, this gas is slow to diffuse, so local concentration effects would be quite severe. So, gas jars of chlorine should always be handled in a fume hood.

Heating a solid The OEL value for lead vapour is 0.05mgm-3 which means that 12mg of lead would be the limit in the room. Lead has a boiling point of 1750C so heating 1 g of lead in a crucible to a melting point of 327C (alloying with tin) is hardly going to produce much vapour. (Evidence: weigh the solid lead in a crucible, heat it to melting, let it cool and weigh again; I have never detected any significant mass loss. Mercury is more of an issue because its boiling point is 357C so it does have a significant vapour pressure). The use of molten lead is a different issue in a metal workshop when handling large volumes of molten lead for several hours a day and efficient fume extraction needs to be installed.


 Photo 1

If sulfur is heated in a test tube on its own or with iron, it can, if done carelessly, combust at the opening of tube and burn forming sulfur dioxide. With a STEL of 0.25ppm or 0.67mgm-3, I am only allowed 60cm3 or 0.61g of the gas in the room. This can be achieved by burning 0.08g of sulfur. Now we can use engineering controls here by simply, limiting the bulk volume in a test tube and placing a plug of mineral wool in the mouth of the test tube which stops any hot sulfur vapour leaving the test tube and combusting with oxygen. So CLEAPSS in the UK advises schools that the iron/sulfur reaction can be carried out using no more than 2g of the mixture in a plugged test tube for a demonstration and 0.2g of the mixture in plugged ignition tubes for the students (Photo 1). You can see this in the CLEAPSS video. Of course no text books contain this information and many have poor diagrams with test tubes half full of the iron/sulfur mixture. This experiment has been a problem for teachers for over 100 years and students in the UK have been sent to hospital with breathing difficulties because it has been so poorly carried out. Interestingly, the UK Health and Safety Executive will not give a safe OEL value for sulfur dioxide so we use IOELV STEL value of 1ppm to help us in our calculations for schools. (Note this is a higher value than the USA value I have used here.) 

Electrolysis A very interesting issue is raised with electrolysis of aqueous chloride solutions. If we have 10 sets of equipment running in a 240m3 room, at the same time ,using a current 0.5A. How long would it take to reach an average concentration of 1ppm, ie 240cm3 of chlorine in the room?  

This is 0.01 mole of chlorine and would use 1950Coulombs of electricity in the preparation. With a current of 0.5A, this is 3850 seconds or 64 minutes. With 10 sets, the level is reached in 6.4 minutes. This experiment had been set as a practical exam question in the UK with disastrous results amongst less experienced teachers. Students were going to hospital with breathing difficulties. It is for that reason that I developed the microelectrolysis equipment. You can also view a related video. (see photo 2) There is only about 6cm3 of gas is produced, so with a warning to stop once the changes have been recorded and warnings not to inhale the gas directly, this procedure works really well. 

The Stink (Odour level)

Chemistry stinks! Well so does perfume! Our nose is a very sensitive organ although the sensitivity of some other mammals, such as Bloodhounds and Bassett Hounds is millions of times greater. In section 9 of a MSDS, suppliers can insert Odour and Odour threshold information. Suppliers seldom do. One of smelliest chemicals is hydrogen sulfide.

Fig 3 shows data for hydrogen sulfide from Scotland. Access the PDF document.


  

 Compare the odour threshold with the Occupational Exposure limit of 5ppm.

So if 10 sets of students add 1 ml of 1M hydrochloric acid to iron(II) sulfide, 120cm3 of gas will be produced in our 240m3 room at an average concentration of 0.5ppm. So we do not break any rules but the stink is there and students will comment (unfavourably), as well as your colleagues working next door! Using a microscale technique as in the video, the smell is significantly reduced. I hope you see that using 1 ml of 1M hydrochloric acid, nobody is going to be poisoned.

It is often the case that detecting the odour does not mean the Occupational Exposure limit is exceeded. In fact it is only with haloalkanes that this is an issue and detection of the odour means the OEL is exceeded. But, these chemicals are seldom used now because of environmental effects to the ozone layer. This is an area where safety officers, architects, and employers can become very worried about activities in the laboratory. One school in the UK thought they had to install banks of fume cupboards in a lab, as in Universities, but the legally required-annual inspection costs, lack of regular use etc finally meant they have been removed. This was a considerable waste of money due to an overreaction.

Ventilation So you can open the windows. UK law demands that fresh air is supplied into a room and my organisation recommends that mechanical ventilation should be increased to at least 5 changes an hour for laboratories. This means that over the 15 minutes time span of our OES, the volume of the room has expanded from 240 to 300 m3, not a great amount.

This was different in old laboratories with high ceilings and large windows (Photo 3).


 Photo 3

Chemistry Lab, Clifton College around 1900 (Note the teacher is wearing a mortarboard hat.

Ventilation is a vexed issue as it can affect the energy considerations of a building and with air conditioning, chemicals (and smells) can invade other rooms. I have found teachers of other subjects do not always get on well with chemistry teachers over this issue. The worst event we came across in this area was when a teacher exploded rubber balloons and the latex particles moved, via the air con, to another room where the teacher had a latex allergy.

Measuring levels There are instruments for measuring levels ranging from tens of dollars for “sniffer” and diffusion tubes to sophisticated electronic meters costing thousands of dollars. I think that teachers have enough common sense to avoid this excessive monitoring for the experiments. The only time they are recommended is for fume cupboard filter testing, measuring mercury levels and odours from drains, neighbouring factories etc.

Conclusion It is only by making use of chemical equations and calculations that arguments about whether a procedure is safe or not can be judged. These are a part of the process we go though at CLEAPSS when we advise teachers, technicians, exam bodies and authors about chemical activities in the labs. We treat these OELS as a maximum (ceiling) and because students have a lower body weight and so are more affected by chemicals, we never work up to these limits, but significantly below them. If a procedure produces levels too high for a class experiment then a demonstration can be done but if this produces a risk to the health of a teacher, a fume hood is required.

PS I hope this second exploration of the MSDS sheet is useful for you. All being well I hope to come to CHEMED 2015 so if you want to discuss this further, catch me. I am old, have a strange English accent and wear long trousers (no shorts) and I hope to have some workshops running with some of the procedures in the videos, live in the lab. There are more in www.microchemuk.weebly.com.

Cheers, Bob Worley