Crystalization -- Purifying organic chemicals

In lab last week we purified some organic chemicals using a technique known as crystallization  It's fairly simple and something you could easily do at your home or in your DIY lab.

There is a seven step process to recrystallizing a substance and removing impurities.

1. Choose a solvent
2. Dissolve the solute
3. De-color the solution
4. Filter suspended solids
5. Crystallize the solute
6. Collect and wash the crystals
7. Drying

1. Choosing a solvent.
The way crystalization works is, you place some impurity into a solution that will dissolve it. However, you don't want it to dissolve it very well at room temperature. it needs to be insoluble at room temperature, but very soluble at boiling point. For example, benzoic acid, when placed in water at 10 Celsius, only  2.1 grams will dissolve in 1 liter of water. When the water is brought to boiling, however, it will dissolve 68 grams in one liter.  

 - The essence is, find a solvent that will dissolve your solid while boiling, but is very poor at dissolving it while cool. 

- Some solutes (the solid you are trying to dissolve) are way too soluble in one solvent and not soluble enough in another solvent. To dissolve this problem, you simply heat up and dissolve as much solute as you can in the solution that is very soluble. Then you use the solvent that is not very soluble, and mix it together so that the over-all solution is now not very soluble. 

2. So, once you've picked the proper solvent for your crystallization, you need to dissolve your substance in that solvent. There are two ways this can be accomplished. 

You want to make sure you don't add too much solute because then re-crystallization will not give you the maximum possible yield. So, to make sure you use "just enough" solvent, you can either pick a desired amount of solvent and then heat it to a boil, then continue to add solute until it stops being dissolved.

- Another way to do this is to pick the amount of solute you need to purify, and place it in a beaker and heat the beaker, then slowly add boiling solvent until finally the last bit of solute is dissolved.
 - - This would be my preferred method because you will typically know how much solute you need to purify  before you start.

- Another way is to find the solubility product for each solute and solvent mixture at boiling and at another known temperature. Calculate the amount you can dissolve in that and use that theoretical amount as the amount you measure out for both the solute and the solvent. Then it can be adjusted by a few drops after it's all mixed together.

3. De-color the solution. -- you can use activated charcoal to remove colored impurities from your solution if they exist. To do this, make sure you have a good straining method to remove the charcoal from you solution when it is decolored. Decoloring should be done while the solution is still warm.

4. Using a filter (anything as simple as a coffee filter should work in most cases), you can take out any solids that were not able to be dissolved. These solids may be impurities or they may be undissolved particles of your solute (because you didn't have enough solvent)

5. To crystallize the solution, you simply let it cool. As it cools it should start creating crystals, but in the case that it doesn't do this by itself, this is called "supersaturation" and typically any disturbance will rapidly start the crystallization process.
You don't want your crystals to form too fast because if they do, they may re-trap some of the impurities that you were trying to remove. So, to prevent your solution from becoming super-saturated, you can simply add a very small seed of the solute (something for the crystals to adhere to) and it will move along from there.
If you don't have anything to re-seed it with, sometimes just scratching the inside of the glass will cause crystallization to occur (or even just tapping the glass against something lightly).

6. You want to dump out your leftover liquid (because it contains the impurities). Therefore, if you boiled out all of the liquid, you will have to re-do the entire experiment. You want that liquid to carry away the impurities when it leaves. So, you dump the liquid, then add more liquid to it (preferrably cold so that it doesn't dissolve all of your crystals), and then you dump it again. This is called Washing.

7. Finally, remove your crystals and let them dry. You can dry them by heating them slightly or by placing a vacuum hose over your glassware. This will decrease the vapor pressure and allow the liquid to evaporate at a higher rate.

I tried to be as non-technical as I possibly could I challenge you to pick a step and try to explain it in a more non-technical way. The only thing I can think of is to not use the words: Solute, Solvent, and Solution. What can you think of?

Lastly, can anyone think of a good "at home" recrystallization that could be added for practice? I know sugar and water are too soluble. I'm wondering how soluble flour, or sugar are in alcohol or acetone. I may try a few things in the kitchen this weekend and let you know how it goes.

Resonance and lewis dots: Organic Chemistry

Resonance is often confused with isomers.

Isomers are when two molecules have exactly the same atoms, but those atoms are put together in different ways

Resonance occurs when one particular molecule has multiple ways in which the electrons will fit into the compound. For example:





In this compound, the Nitrogen can either be double bounded with the oxygen on the right or the oxygen on the left. This is not a slight of hand. I am not flipping the molecule over like a pancake and turning the front side into the back side or the left into the right. The oxygen on the left in the first picture is the same oxygen on the left in the second picture. This is true because the octet rule for lewis structures makes both recordings equally as accurate.

In these situations, where multiple lewis structures are applicable, the thought is that electrons want to be in two different places equally as much. And therefore these electrons become delocalized. It is believed that these electrons are not simply moving back and forth between different atoms, but are actually in some kind of hybrid state.

Take the ozone molecule O3 for example:

image obtained from wikipedia

Because the lewis dot and kekule structures cannot accurately depict the resonance molecule, the hybrid is generated.
Each lewis structure is considered a contributor to the hybrid, but the hybrid is the most accurate depiction of the reality of how electrons are being used to bond atoms together.


What's the most famous resonance hybrid structure? I'm thinking Benzene. What are your thoughts?

We're Making Whiskey! Organic Chemistry

First Organic Chemistry lab was today.

The teacher has us making Rum! The purpose of todays lab was to teach us the process of distillation in a hands-on way. Distillation is one of several ways to purify organic compounds (crystallization was another method we discussed).

To do this experiment we took the following steps:


 1. We added 200 ml of diluted molasses (diluted with water and sugar) to a large Erlenmeyer flask,

Note: the instructor did not tell us how much he diluted the molasses, but that won't matter as you'll see how to calibrate your molasses and water using a hydrometer.
I recommend adding 1 ounce of molasses, 1.5 ounces of sugar (about one tenth of a pound), and 200 ml of distilled (bottled) water as a base solution.


2. Then we added 50 ml of yeast nutrient (to help the yeast grow and maintain).














3. We used a device called a Hydrometer at this point to see what our percent alcohol would be. This is a very simple device that works off of gravity.

  - To use one, you fill up a graduated cylinder (or any very skinny and tall container) with the solution.
  - You drop the hydrometer in and it will float. Wait for it to settle down and then you read where the water mark is on the hydrometer. Where the water mark is, is your percent alcohol potential.
  - This device works based on how much the device should float for every unit/volume of molasses or sugar added.

We had to ensure that our expected percent alcohol was between 10 and 16 percent.

4. Next, in a separate container, we soaked 5 grams of brewer's yeast in about 50 ml of tap water for 15 minutes.


5. After the soak time we dumped the yeast into the flask, put a rubber stopper in it, and shook everything up vigorously.

Note: The Rubber stopper had a hole in the middle.


 The hole is designed for an Airlock to be inserted into it, which keeps oxygen out (yeast make alcohol under anaerobic conditions). It not only keeps oxygen out, but it also allows carbon dioxide to escape (so too much pressure doesn't build up). Yeast produce carbon dioxide as a waist product while fermenting the sugars. The airlock looked like this one

NOTE: a poor boy airlock can be made by putting the mouth of a balloon over your bottle. The balloon needs to be able to expand quite a bit without bursting or releasing from the bottle.

7. We filled the airlock slightly over half way with water and then inserted the airlock into the rubber stopper

8. Now, we wait for about two weeks.

A few notes:
A. Light kills yeast so keep the yeast closed up until you're ready to use it
B. Place the bottle in a cold, dark, dry area
C. The instructor told us that our 16% alcohol would be distilled down to 50%. it would have a volume of about "1 shot," he told us. And we would then distill it again to about half of that volume and it would be approximately 190 proof (95%).
D. The lab stated that making the solution just "slightly" acidic would help prevent bacterial growth. The bottom line is, use clean equipment. And if you're on the cautious side, a few drops of lemon juice, orange juice, or vinegar will not hurt at all.
-- A few drops does not mean teaspoons. It means drops

E. I'll post another blog later for poor boy distilling as well as instructions on how we actually go about distilling our brew.

This blog has an excellent idea for a homemade still!

Qualitative Analysis Step-By-Step simple instructions

These instructions are boiled down and simplified from the lab book in class CHE 120. These instructions should be used hand-in-hand with the Data Sheet

I've found that when I go into lab with these condensed instructions, I tend to finish faster than trying to go directly from the lab book. Hope it helps you too!

Group 1

1.       Step 1:

a.       Cool test tube in ice bath

b.      5 drops 6M HCL (stir with glass rod)

c.       Use cold water to wash down solution that adheres to test tube

d.      Centrifuge

e.      Add additional 6M HCL to ensure that the solution is completely precipitated

f.        Centrifuge

g.       Decant

                                                               i.      Decant contains Group II & III (save for other tests)

2.       Step 2 (Separation of Pb ²⁺(aq) from Group I Chlorides)

a.       Add 15-20 drops of hot distilled water

b.      Stir well with glass stirring rod until all precipitate is colloidally suspended

c.       Boiling hot water bath for 1-2 minutes

d.      Centrifuge while still hot

e.      Decant

                                                               i.      PPT contains possibly AgCl(s) and HgCl2 (s) (step four)

                                                             ii.      Decant contains Pb²⁺ (used in step three)

3.       Confirmation of Pb²⁺

a.       Cool decantate

b.      Divide into two equal portions

c.       Add 1 drop of 0.2 M K₂CrO₄ to one part (Yellow PPT proves lead)

d.      Add 1 drop of 2 M H₂SO₄.

e.      Cool the Pb/H₂SO_4­ Solution (White PPT further confirms lead – may take time)

4.       Separation of Hg²⁺ and Ag⁺

a.       To PPT – add 4 drops of 15 M NH₃ (aq)

b.      Mix thoroughly

c.       Centrifuge

d.      Decant

                                                               i.      PPT: A gray-black residue (the mixture of Hg and HgNH₂Cl) proves mercury ion

                                                             ii.      Decant: add 16 M HNO­_3­ drop-wise to decantate and mix until slightly acidic

1.       Check with litmus paper

                                                            iii.      Formation of white PPT (AgCl) proves silver ion

Group 2

5.       Separation of Group II Cations

a.       15 drops of unknown solution into small casserole

b.      Add 6 drops of 3% H₂O₂

c.       Add 6 drops of 2 M HCl

d.      Boil down the volume on a hot plate to about 2 – 3 drops

e.      Allow to cool

f.        Add 6 drops of 6 M HCl

g.       Boil contents down to pasty mass (2-3 drops of liquid left)

                                                               i.      DO NOT BAKE THE RESIDUE

                                                             ii.      If too much is boiled off then add 2 drops DI water

h.      Cool

i.         Add 5 drops of 2 M HCl

j.        Swirl acid until entire residue dissolves

                                                               i.      If required, warm slightly and stir

k.       Transfer solution to test tube

l.         Add 8 drops of thioacetamide (CH₃CSNH₂) and mix thoroughly

m.    Add 8 drops of Hot distilled water

n.      Add 8 more drops of 1 M thioacetamide

o.      Add 1 drop 1 M Ammonium acetate (NH_4­C₂H₃O­_2­) mix well

p.      Heat in boiling water bath for 4 minutes

q.      Centrifuge

                                                               i.      PPT contains Group II (PbS Black, Bi₂S₃ Brown, CuS Black, and CdS Yellow)

                                                             ii.      Decante contains Group III and IV

1.       NOTE: CH₃CSNH₂ + 2H₂O -à CH₃COOH + NH₃ + H₂S

6.       Separation of lead from other Group II

a.       Put PPT from #5 into casserole

b.      Add 10 drops of 16 M HNO₃ to a casserole containing residue from #5

c.       Heat until all sulfides have dissolved (no more PPT) – tiny yellow crystal

d.      Add additional 16 M HNO₃ if needed

e.      Discard yellow sulfur particles

f.        Add 4 drops of 18 M H₂SO₄ to casserole

g.       Evaporate carefully under hood (1 – 5 drops remaining – dense white fumes)

h.      Add 15 drops of cold water

i.         Stir contents until all material in casserole is dissolved or suspended;

j.        transfer quickly to test tube

k.       wash casserole dish with four drops of cold water and transfer washing to same test tube

l.         Cool in ice bath

m.    Centrifuge

n.      Decant

o.      Wash PPT twice with 10 drops of cold water (discard decant of washing)

PPT contains PbSO_4­

Decant contains Bi, Cu, Cd

p.      Add to PPT, 4 drops of 1M NH₄C₂H₃O_2­

q.      Stir 30 seconds

r.        Add 2 drops  0.2 M K₂CrO₄ (yellow confirms lead)

7.       Separation of Bismuth

a.       To decant from step 6 add 15 M NH₃ drop wise until alkaline (litmus paper)

b.      Stir 2 minutes

c.       Centrifuge and decant – save decant for step 8

                                                               i.      PPT is bismuth

                                                             ii.      Decant is either Cd or Cu

d.      Wash the PPT twice with 15 drops of hot water

e.      Add 3 drops of 8 M NaOH and 2 drops of 0.2 M SnCl₂

f.        Stir – jet black PPT proves Bi

8.       Detection of Copper

a.       If decant from 7 is blue then copper is present (sometimes present when colorless)

b.      Add 5 drops of decantate into a different test tube

c.       Add 5 M acetic acid until decolorized

d.      Add 2 drops of 0.2 M Potassium hexacyanoferrate (K₄Fe(CN)₆)

                                                               i.      Red PPT confirms Cu

9.       9 Detection of Cadmium

a.       IF Copper is ABSENT

                                                               i.      Treat colorless solution with 2 – 3 drops of Ammonium Sulfide

                                                             ii.      Mix thoroughly

                                                            iii.      Let stand for 1 minute

                                                           iv.      Yellow PPT confirms CdS

b.      If copper is PRESENT

                                                               i.      Add 0.2M KCN (this is poisonous) dropwise, to a 10 drop portion of blue decantate until color disappears

                                                             ii.      Treat solution with 3 drops of Ammonium sulfide solution

                                                            iii.      Mix thoroughly

                                                           iv.      Let stand 1 minute

                                                             v.      Yellow PPT confirms CdS

Group III

10.   Place decant from step 5 into casserole and evaporate down to volume of 8 – 10 drops

a.       Transfer to test tube

b.      Centrifuge

c.       Decant to another test tube

d.      To decant:

                                                               i.      Add 4 drops 2 M NH₄Cl

                                                             ii.      Mix thoroughly

                                                            iii.      Add 15 M Aqueous NH₃ dropwise (stir) until alkaline

e.      To alkaline mixture add 9 drops of ammonium sulfide solution

f.        Centrifuge (BUT DO NOT DECANT)

g.       Note the color

h.      Heat in boiling water bath

i.         Centrifuge

j.        Add one more (NH₄)₂S to ensure it is complete

k.       Wash down sides of tube with hot water

l.         Centrifuge again

m.    Note color of supernatant liquid and decant

                                                               i.      Decant contains Group IV ions

n.      To PPT: wash three times with 20-drops of solution

                                                               i.      Solution is equal volume of water and 1 M ammonium acetate

11.   Further process Group III PPT for analysis

a.       Add 10 drops of 12 M HCL

b.      Mix thoroughly

c.       Transfer to casserole

d.      Boil gently 30 seconds

                                                               i.      If PPT is not dissolved, add 3 drops 16 M HNO₃

                                                             ii.      Mix thoroughly

                                                            iii.      Boil until solution is obtained (add more HNO₃ if needed)

e.      Add 10 drops of cold water

f.        Transfer to test tube

g.       Centrifuge to remove sulfur

h.      Decant into casserole

i.         Make strongly alkaline by adding 8 M NaOH and mix

                                                               i.      If ppt is still mushy/non-fluid then add 10-20 drops of water

j.        Add 2 drops of 3% H_2­O₂

k.       Stir for 1 minute

l.         Boil for 2 minutes

m.    Replinish lost water

n.      Transfer to test tube before PPT can settle

o.      Centrifuge

p.      Decant

                                                               i.      PPT : wash three times with hot water and analyze in step 12

                                                             ii.      Decant is discarded

12.   Detection of Iron

a.       To PPT, add 20 drops of 2 M H₂SO₄ and mix thoroughly

                                                               i.      Transfer to casserole

                                                             ii.      Boil gently 1 minute

                                                            iii.      Add a drop of 3% H₂O₂

                                                           iv.      Boil for 1 minute after PPT is completely dissolved

                                                             v.      Add 10 drops of water

                                                           vi.      Cool

                                                          vii.      Divide into four equal portions

b.      To one portion, add  1 or 2 drops of 0.2 M KSCN

                                                               i.      Blood red proves iron

13.   Detection of Cobalt

a.       To a second portion, Add 10 drops of 1 M NaF

b.      Mix well

c.       Add 10-20 drops of saturated solution of NH₄SCN (ammonium thiocyanate) in ethyl alcohol

d.      Formation of blue solution proves Co²⁺

14.   Detection of nickel

a.       To third solution, add enough 6 M NH₃ to make it basic

                                                               i.      If PPT shows up (Fe(OH)₃ or Mn(OH)₂) then centrifuge and use the decant

b.      To clear decant, add 2-4 drops of DMG (dimethylgloxime)

c.       Mix thoroughly

d.      Stand for 1 minute

e.      Strawberry red proves Nickel

15.   Detection of Manganese

a.       Dilute fourth portion with equal volume of water

b.      Add 2 drops of 3 M HNO₃

c.       Mix thoroughly

d.      Add a few grains of solid sodium bismuthate

e.      Mix thoroughly

f.        Let stand 1 minute

g.       Centrifuge

h.      A pink to reddish purple solution proves Mn

Lab 7 - 9 Qualitative Analysis Data Sheet

 In order to make the qualitative analysis table fit on this blog, I had to post it in "paint" and make it a picture.
If you would like to access the Text version of this, please click here

To see a full list of Chemistry 120 Lab help, click here

If you would like help with Chemistry Homework, I will create a screencasted video
teaching you how to solve any Chemistry Question (Step-by-step).
You can either choose from questions posted on this blog or you can submit your own questions.
The cost is 5.50 for each submission and the video will be created using Educreactions iPad app.
You will have lifetime access to the video I create. 

Your question:

 

Procedure #	Liquid or Solid Used	Reagent	Observation	Ppt contains	Centrifugate Contains	Applicable Equations
1	Group I, II, III, IV	HCl	White & Yellow	AgCl, Hg2Cl2,   PbCl2	Group 2, 3, and 4	Ag+ + HCl  à AgCl + H+ 2Hg+ + 2HCl à Hg2Cl2 + 2H+ Pb2+ + 2HCl à PbCl2 + 2H+
2	PPT From #1	Hot H2O	PPT Still Present	AgCl, HgCl2	Pb2+	PbCl­2 + H2­O + Heat à Pb2+ (aq) + Cl-(aq)
3	Half of decant from #2	K2CrO2	Yellow Precipitate	PbCrO4	N/A	Pb2+ (aq) + K2CrO4 à PbCrO4 + 2K+
	Other half of decant	H2SO4	White Precipitate	PbSO4	N/A	H2SO4 + Pb2+ à PbSO4 + 2H+
4a	PPT From #2	NH3	Black/Gray PPT	Hg + HgNH2Cl	Ag(NH3­)2+	AgCl + 2 NH3 à Ag(NH3­)2+ + Cl- Hg2Cl2 + NH3 à Hg(l) + HgNH2Cl(s) + NH4+ + Cl-
4b	Decantate	HNO3	White Precipitate	AgCl	N/A	Ag(NH3)2+(aq) + 2H+(aq) →  Ag+(aq) + 2 NH4+(aq) Ag+(aq) + Cl-(aq) →  AgCl(s)
5	Decant from #1	H2O2, (NH4)2S, NH4C2H3, H2O, Heat	Dark/Hunter Green	PbS, Bi2S3, CuS, CdS	Group III and IV	Pb2+ + S2- à PbS 2Bi3+ + 3S2- à Bi2S3 Cu2+ + S2- à CuS Cd2+ + S2- à CdS
6a	PPT From #5	HNO3, H2SO4, Heat	Yellow Crystal Forms, ions   dissolve, and White Smoke Appears	Sulfur	Group II ions	PbS + 2 NO3- +4H+ à Pb2+ + S +2NO2 + 2H2O CuS + 2 NO3- +4H+ à Cu2+ + S +2NO2 + 2H2O CdS + 2 NO3- +4H+ à Cd2+ + S +2NO2 + 2H2O Bi2S3 + 6 NO3- +12H+ à 2Bi2+ + 3S +6NO2 + 6H2O
6b	Decant from 6a	Ice, NH4, HC2H3O2, KCr2O4	White Precipitate, Then Dissolve, Then Yellow PPT	PbSO4	Bi3+, Cu2+, Cd2+	Pb2+ + SO42- + ICE à PbSO4 + Heat PbSO4 + 3NH4 + C2H3O2 à Pb(C2H3O2­­­)3- + NH4 + SO42- Pb(C2H3O2­­­)3- + KCrO4 à PbCrO4 + 3(C2H3O2)- + K+
7a	Decant from #6	NH3	Blue	Bi(OH)3(s)	Cu(NH3)42+,   Cd(NH3)42+	Bi3 + 3NH3 +3H2O à Bi(OH)3 +3NH4+ Cu2+ + 4NH3 à Cu(NH3)42+ (blue) Cd2+ + 4NH3 à Cd(NH3)42+
		NaOH, SnCl2
7b	PPT From #7a	H2O, NaOH, SnCl2	Jet Black	Bi(S)	N/A	SnCl2 +3NaOH à Sn(OH)3- +3Na+ +2Cl- BiOH + SnOH à SN(OH)62- + Bi0(S)
8	5drops Decant from #7	HC2H3O2	Lost Color	N/A	Cu2Fe(CN)6	Cu(NH3)42+ + 4HCH3CO2­ à Cu2++ 4NH4 + 4CH3CO­2 2Cu2+ + K4Fe(CN)6 à CuFe(CN)6 4K-
		K4Fe(CN)6	Turned Red
9	Decant from #7	KCN, (NH4)2S	Yellow	CdS	N/A	Cd(NH3)42+ + 4CN- àCd(CN)42-+4(NH3) Cd(CN)42- ßà Cd2+ + 4CN- Cd2+ + S2- à CdS
10a	Decant from #5	NH4Cl NH3	Blackish	NiS (Black) FeS (Black) CoS (Brown) MnS (Pink)	Group IV	Fe2+ + S2- à FeS Co2+ + S2- à CoS Ni2+ + S2- à NiS Mn2+ + S2- à MnS
10b	Precipitate  from 10a	(NH4)2S Heat NH4C2H3O2		NiS
11	PPT from #10	HCl, Heat, HNO3 H2O,  NaOH H2O2	PPT Dissolves Sulfur PPT	Sulfur	N/A	Fe3+ + 3(OH)- à Fe(OH)3 (red) Ni2+ + 2(OH)- à Ni(OH)2 (Green) Mn2+ + 2(OH)- à Mn(OH)2 (part 1) Mn(OH)2 + 2(OH)- à MnO2 +2(H2O)  (Part 2) MnO2 + H2O2  à MnO +H2O + O2  (Blackish brown) 2Co(OH)3 + H2O2 à 2Co(OH)2 + 2H2O + O2 (pink to blue) (Note: the mechanism for Co and Mn are similar – I abbreviated the Co formula here)
			Darkened	Fe(OH)3 Ni(OH)2 MnO Co(OH)2
12	¼ of PPT from #11	KSCN	Red	Fe(SCN)63-		Fe3+ + 6SCN- à Fe(SCN)63-
13	¼ of PPT from #11	NaF NaSCN	Blue hue	Co(SCN)43+		Co2+ + 4(SCN)- à Co(SCN)42-
14	¼ of PPT from #11	NH3DMG (dimethylgloxime)	Strawberry Red	NiC8H4N4O4		Ni2+ + 6NH3 à Ni(NH3)62+
						Ni(NH3)62+ + 2(CH3)2C2(NOH)2 à NiC8H4N4O4 + 2NH4++4NH3
15	¼ of PPT from #11	H2O HNO3 NaBiO3	Purple (pink to reddish purple)	MnO4-		2Mn2+ + 5HBiO3 + 9H+ à 2MnO4 + 5Bi3+ + 7H2O