Crystals in a Gel
Purpose:
A dilute solution of sodium metasilicate and a weak acid are used to make a gel. Diffusion of a metal solution will react with the substrate chemical to produce the crystals over a period of several days.
Chemicals for all gel recipes:
- Acetic acid
Mix 57 ml of concentrated glacial acetic acid per liter of solution to make a 1 M solution. You can also use full strength 5% vinegar. Put 25 ml of the 1.0 M acetic acid into a 3 x 20 cm test tube. Measure the solution with aqueous ions listed below and add it to the acid, mixing well.
Disposal of extra acetic acid should be done by diluting slowly by adding it to at least a 20 fold excess of water and then neutralized. This solution goes down the drain with excess water.
SAFETY:
Concentrated glacial acetic acid is corrosive to skin and tissue and toxic by ingestion. It has a moderate fire risk with a flash point of 39oC.
- Saturated sodium metasilicate Na2Si3O7 otherwise known as waterglass
Dilute one part of waterglass with four parts of water. If you have a hydrometer it should read 1.06 g/ml. You will use 25 ml of the waterglass solution and add it to the acid mixture. Mix well and cover with a stopper or parafilm. NOTE: Setting time will vary from minutes to days.
- Metals or ionic solutions as indicated. These will be added to the solidified gel.
- COPPER TREE
gel contains 1.0 M CuCl2 (13.5g/100ml solution). Add 5 ml to the acetic acid (vinegar).
develop: After the gel sets, gently push in an ungalvanized nail. Add about 5 ml of 1.0 M NaCl (5.9g/100ml solution) on top of the nail.
Safety and Disposal:
CuCl2 is toxic by ingestion and inhalation. It requires solid waste disposal in a landfill (Flinn #26a).
LEAD TREE
gel is 1.0 M Pb(NO3)2 (33.1g/100ml solution) Add 1.0 ml to the acetic acid.
develop: After the gel sets, gently insert a thin piece of zinc metal about 0.5 x 1.0 cm. Add 5 ml of water with a few drops of acetic acid in it to prevent dehydration. Try a small piece of iron or aluminium. (Any metal higher than lead on the electrochemical series should displace the lead ions.)
Safety and Disposal:
Pb(NO3)2 is toxic by ingestion and inhalation; strong oxidant; dangerous fire risk in contact with organic material. Disposal is done by dissolving the lead nitrate in water with 6 M HCl. Add a 3 fold molar excess of sodium sulfide or thioacetamide and stir for one hour. Adjust to neutral pH with 3 M sodium hydroxide. Filter, allow to dry and place the remaining metal sulfide in a plastic container and bury in a landfill (Flinn #27f). The remaining solution with S2- is to be added to iron (III) chloride, neutralized with Na2CO3, filtered, and disposed of in a landfill. The solution is suitable for drain disposal. Pb is placed in a plastic container and suspended in a bag of wet ready-mix concrete and then buried in a landfill when solidified (Flinn #27d).
LEAD IODIDE FERNS
gel is 1.0 M Pb(C2H3O2)2 (32.5g/100ml solution). Add 2.0 ml to the acetic acid.
develop: After the gel sets, add 10 ml of 2.0 M KI (33.2g/100ml solution). Expect feathery dendrites and free-growing hexagonal plates near the bottom.
Safety and Disposal:
Pb(C2H3O2)2 is a known animal carcinogen, and eye, skin and respiratory irritant, and it reacts violently with bromates. KI presents no safety problems. PbI2 disposal is the same as Pb(NO3)2 (Flinn #27f). KI may be rinsed down the drain with large amounts of water (Flinn #26b).
LEAD IODIDE
gel is 1.0 M KI (16.6g/100ml solution) Add 2.0 ml to the acetic acid.
develop: add 10 ml of 1.0 M Pb(C2H3O2)2 (32.5g/100ml solution).
Safety and Disposal:
See recipe #3.
SILVER ACETATE
gel is just a mix of waterglass with 25 ml of 1.0 M acetic acid.
develop: Add a few ml of 0.5 M AgNO3 (8.5g/100ml solution).
Safety and Disposal:
AgNO3 is a corrosive solid which causes burns. It is toxic. Avoid contact with eyes and skin. Silver acetate is severely toxic by inhalation and ingestion. Disposal: Silver compounds are expensive and can be reclaimed. Flinn has a recipe which involves dissolution in HNO3, purification from Cu contamination, treatment with a strong base, and conversion to nitrate. Landfill disposal involves addition of 50% molar excess of NaCl, filtration and drying of the AgCl. Drain disposal is suitable for the remaining solution (Flinn #11).
CALCIUM TARTRATE
gel is 1.0 M tartaric acid, HOOC(CHOH)2COOH (15g/100ml solution) (NO ACETIC ACID IN THIS ONE). The waterglass is added drop by drop to the tartaric acid in a large test tube with constant stirring. It takes 24 - 36 hours to gel and must be given the opportunity to set undisturbed for another 12 hours.
develop: 1.0 calcium chloride CaCl2.2H2O (14.7g/100ml solution). Add 10 ml very slowly with a pipette, or, at least pour very gently down the side of the tube. This will take 4 - 5 days to react.
Safety and Disposal:
Neither tartaric acid, calcium tartrate nor calcium chloride presents known safety problems. Disposal of tartaric acid is done by adding it to a 20-fold excess of water, neutralized with sodium carbonate or sodium hydroxide, and rinsed down the drain with excess water (Flinn #24a). Both calcium tartrate and calcium chloride are suitable for waste disposal in a landfill, or if in solution form, drain disposal (Flinn #26a or #26b).
COPPER (II) TARTRATE
gel is 3.0 M tartaric acid, HOOC(CHOH)2COOH (15g/100ml solution) (NO ACETIC ACID IN THIS ONE).
develop: Add 10 ml of saturated CuSO4 (31.6g/100ml solution).
Safety and Disposal:
Safety: see recipe #6 for tartaric acid. CuSO4 is a skin and respiratory irritant, toxic by ingestion and inhalation. Disposal of copper sulfate and copper tartrate can be done in a landfill if solid or drain disposal if in solution (Flinn #26a or #26b).
Clean up
To remove the gel use methanol.
Discussion
When the gel is examined under a microscope, it resembles a water-soaked sponge. The speed of the chemical reactions in silica gel is governed by the slow pace at which fluids flow through the porous mass. This is the reason to expect the crystals to grow slowly in the gel. The advantage, however is the perfection in form.
Silica gels are made by precipitating silicic acid from the sodium silicate solution. The acetic acid is added in excess to end up with a solution buffered by CH3COOH.CH3COOH-. Most acetates are soluble and a low pH inhibits the precipitation of basic salts or silicates. The standard gel is distinctly on the acid side in the buffer regiion of the acetic acid-acetate system. The more slowly the gels set, the clearer that they are likely to be.
Incorporating one reagent into the acetic acid before mixing with acetic acid avoids precipitation of silicates or hydroxides.
Commercial waterglass is not as acceptable as reagent grade sodium metasilicate because it is often contaminated with unwanted impurities and is of varying and inaccurate composition. The lower limit on the specific gravity is about 1.02 g/cm3. Instead, prepare a stock solution of sodium metasilicate by adding 500 ml of deionized or distilled water to 244 g of Na2SiO3.9H2O.
Slow addition of the surface reactant with a pipette is often desirable because it avoids a concentration of ions in a certain area possibly causing some premature gelling and turbidity.
The acidity and silicate concentration are very important for the gelation process. In actuality, there are two types of ions produced: the H3SiO4- and the H2SiO42-. Their amounts are dependent on the pH. If the pH is high, the H2SiO42- is predominant. If the pH is low the H3SiO4- is favored. The H3SiO4- is responsible for the long chain polymerization products. Cross links are formed between these chains and these contribute to the increase in viscosity that occurs as the gel is formed. Henisch explains that the very long chains, because of their lower mobility will cross link more slowly than short chains. At very low pH values, polymerization is not as favored and chain formation is slowed. A pH of 8 seems to be optimum.
Silica gels seem to favor crystals whereby organic gels like agar favor amorphous or microcrystalline deposits.
This page is from a Woodrow Wilson workshop.
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A dilute solution of sodium metasilicate and a weak acid are used to make a gel. Diffusion of a metal solution will react with the substrate chemical to produce the crystals over a period of several days.
Mix 57 ml of concentrated glacial acetic acid per liter of solution to make a 1 M solution. You can also use full strength 5% vinegar. Put 25 ml of the 1.0 M acetic acid into a 3 x 20 cm test tube. Measure the solution with aqueous ions listed below and add it to the acid, mixing well.
Disposal of extra acetic acid should be done by diluting slowly by adding it to at least a 20 fold excess of water and then neutralized. This solution goes down the drain with excess water.
SAFETY:
Concentrated glacial acetic acid is corrosive to skin and tissue and toxic by ingestion. It has a moderate fire risk with a flash point of 39oC.
Dilute one part of waterglass with four parts of water. If you have a hydrometer it should read 1.06 g/ml. You will use 25 ml of the waterglass solution and add it to the acid mixture. Mix well and cover with a stopper or parafilm. NOTE: Setting time will vary from minutes to days.
- COPPER TREE
gel contains 1.0 M CuCl2 (13.5g/100ml solution). Add 5 ml to the acetic acid (vinegar).
develop: After the gel sets, gently push in an ungalvanized nail. Add about 5 ml of 1.0 M NaCl (5.9g/100ml solution) on top of the nail.
Safety and Disposal:
gel is 1.0 M Pb(NO3)2 (33.1g/100ml solution) Add 1.0 ml to the acetic acid.
develop: After the gel sets, gently insert a thin piece of zinc metal about 0.5 x 1.0 cm. Add 5 ml of water with a few drops of acetic acid in it to prevent dehydration. Try a small piece of iron or aluminium. (Any metal higher than lead on the electrochemical series should displace the lead ions.)
Safety and Disposal:
gel is 1.0 M Pb(C2H3O2)2 (32.5g/100ml solution). Add 2.0 ml to the acetic acid.
develop: After the gel sets, add 10 ml of 2.0 M KI (33.2g/100ml solution). Expect feathery dendrites and free-growing hexagonal plates near the bottom.
Safety and Disposal:
gel is 1.0 M KI (16.6g/100ml solution) Add 2.0 ml to the acetic acid.
develop: add 10 ml of 1.0 M Pb(C2H3O2)2 (32.5g/100ml solution).
Safety and Disposal:
gel is just a mix of waterglass with 25 ml of 1.0 M acetic acid.
develop: Add a few ml of 0.5 M AgNO3 (8.5g/100ml solution).
Safety and Disposal:
gel is 1.0 M tartaric acid, HOOC(CHOH)2COOH (15g/100ml solution) (NO ACETIC ACID IN THIS ONE). The waterglass is added drop by drop to the tartaric acid in a large test tube with constant stirring. It takes 24 - 36 hours to gel and must be given the opportunity to set undisturbed for another 12 hours.
develop: 1.0 calcium chloride CaCl2.2H2O (14.7g/100ml solution). Add 10 ml very slowly with a pipette, or, at least pour very gently down the side of the tube. This will take 4 - 5 days to react.
Safety and Disposal:
gel is 3.0 M tartaric acid, HOOC(CHOH)2COOH (15g/100ml solution) (NO ACETIC ACID IN THIS ONE).
develop: Add 10 ml of saturated CuSO4 (31.6g/100ml solution).
Safety and Disposal:
To remove the gel use methanol.
Discussion
When the gel is examined under a microscope, it resembles a water-soaked sponge. The speed of the chemical reactions in silica gel is governed by the slow pace at which fluids flow through the porous mass. This is the reason to expect the crystals to grow slowly in the gel. The advantage, however is the perfection in form.
Silica gels are made by precipitating silicic acid from the sodium silicate solution. The acetic acid is added in excess to end up with a solution buffered by CH3COOH.CH3COOH-. Most acetates are soluble and a low pH inhibits the precipitation of basic salts or silicates. The standard gel is distinctly on the acid side in the buffer regiion of the acetic acid-acetate system. The more slowly the gels set, the clearer that they are likely to be.
Incorporating one reagent into the acetic acid before mixing with acetic acid avoids precipitation of silicates or hydroxides.
Commercial waterglass is not as acceptable as reagent grade sodium metasilicate because it is often contaminated with unwanted impurities and is of varying and inaccurate composition. The lower limit on the specific gravity is about 1.02 g/cm3. Instead, prepare a stock solution of sodium metasilicate by adding 500 ml of deionized or distilled water to 244 g of Na2SiO3.9H2O.
Slow addition of the surface reactant with a pipette is often desirable because it avoids a concentration of ions in a certain area possibly causing some premature gelling and turbidity.
The acidity and silicate concentration are very important for the gelation process. In actuality, there are two types of ions produced: the H3SiO4- and the H2SiO42-. Their amounts are dependent on the pH. If the pH is high, the H2SiO42- is predominant. If the pH is low the H3SiO4- is favored. The H3SiO4- is responsible for the long chain polymerization products. Cross links are formed between these chains and these contribute to the increase in viscosity that occurs as the gel is formed. Henisch explains that the very long chains, because of their lower mobility will cross link more slowly than short chains. At very low pH values, polymerization is not as favored and chain formation is slowed. A pH of 8 seems to be optimum.
Silica gels seem to favor crystals whereby organic gels like agar favor amorphous or microcrystalline deposits.
This page is from a Woodrow Wilson workshop.
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