Usually, two different methods are used for preparation of aluminium hydroxide. First one is the interaction of a solution of an aluminium salt with an alkali. The second method is the reaction of a solution of alkali metal aluminate and a strong acid.

In this experiment these methods are combined together (in one chemical reaction).

Two aluminium compounds (K[Al(OH)4] and Al2(SO4)3) react with each other in aqueous solution to form aluminum hydroxide Al(OH)3. The solution of K[Al(OH)4] is alkaline, the solution of Al2(SO4)3) is acidic. Together they give Al(OH)3 and a neutral salt - K2SO4.

A solution of potassium aluminate K[Al(OH)4] was added to a solution of aluminium sulphate Al2(SO4)3. At first a white precipitate of aluminium hydroxide Al(OH)3 was formed. Then the precipitate was dissolved by new portions of the potassium aluminate solution because it contained excess of potassium hydroxide KOH also.

More of an open discussion than a question, but still decided to post it here. The story begins with me and my labmates trying to synthesize some aluminum acetylacetonate (the tris(acetylacetonato)aluminum complex) in our lab, mostly for educational purposes. We came up with our own procedure (based on some works found on the internet) which is as follows:

0,062 mol of Al3+ (aluminum sulfate octadecahydrate),

0,1922 mol of acetylacetone (3,1 eq.)

0,1984 mol of anhydrous sodium acetate (3,2 eq.)

all substrates were added together in a 500ml beaker from separate flasks. The inorganic salts were pre-dissolved in small amounts of water and freshly filtered before the addition. Oddly enough, when we poured acetylacetone into the solution, it slowly (over the course of 20 seconds) turned yellow (at that time more like orange-oily-yellowish, unfortunately no photo), but even with some stirring still stayed a clear liquid. We were all surprised because we expected a precipitate to form. After a couple minutes, the contents of the beaker became cloudy, due to the complex gradually falling out of solution. We stirred the mixture a bit more, and what we were left with was an innocent banana slushie (picture 2). The lab session came to an end, so we left the beaker to sit undisturbed for a few days. After coming back, we vacuum filtered the solution and collected the precipitated yellow powder (pictures 1 and 3). Having made the desired product we started to wonder: "Why is it yellow though?". None of us could think of an answer. We all know chemistry well enough to be aware that the intense yellow color is something unusual for our compound. According to my own knowledge, the complex we've made should be just plain white, considering the aluminum ions as well as the acetylacetone ligand rarely contribute any color to their salts (when talking acac complexes with metals outside the d block). The rest of the ions: sodium, acetate, and sulfate are also colorless. So where did the color come from? In our case the purity of the substrates was out of the question, the inorganic salts were relatively high-grade and as I mentioned, they were dissolved and filtered before use. The acetylacetone was also freshly distilled (a day or two before the experiment) and its natural slight yellow tint appeared almost unnoticeable compared to the color that the solution took after we added it. In my opinion, there are only two possible explanations:

The yellow powder is in fact the pure product with its natural color (unlikely as it seems, some precipitates do not always derive their color based on the ions they're formed from, like lead iodide for example),

The color of the sample is a result of another product forming next to the desired one.

Assuming option 1 presents the correct explanation, it would still be unknown why exactly is the complex yellow. On the other hand, if option 2 describes the real scenario, it would still leave the question about the nature of that side product (we deducted from our observations that the color must be a result of a chemical reaction), namely: what is it? how did it form in said conditions?

Whatever the answer may be, I would very much like to know the opinion of you all, I hope together we will be able to solve this mystery.

Why is my aluminum acetylacetonate yellow?
https://www.reddit.com/r/chemistry/comments/13hr2u8/aluminum_acetylacetonate_mystery/

Decomposition of aluminate in a solution is used to precipitate aluminium hydroxide in the aluminium production technology. We have reproduced this process in the laboratory.

A potassium aluminate solution was added to a large amount of tap water. Potassium aluminate are decomposed to form a solution of potassium hydroxide and a precipitate of aluminium hydroxide.

[Al(OH)4]- <=> Al(OH)3 + OH-

At first aluminium hydroxide formed a suspension then the precipitate sedimented.

Tyndall scattering was shown for the aluminium hydroxide suspension.

Aluminium hydroxide precipitate was stayed under water for a month. The aging process took place. Then an excess of glacial acetic was added to a small amount of the precipitate. The precipitate was not dissolved. After a while the reaction mixture was filtered to separate an insoluble solid phase. An ammonia solution was added to the filtrate (to determine aluminium in the solution). There was no precipitate (aluminum was not found). Therefore, aluminium hydroxide had not dissolved in glacial acetic acid (even partly).

In the last experiment, we used aged aluminium hydroxide to react with glacial acetic acid. The hydroxide did not react. Therefore, we should prepare a freshly precipitated aluminium hydroxide and carry out this rection again. To obtain a freshly precipitated aluminium hydroxide the reaction of ammonium alum with an aqueous ammonia was used. The solution of ammonia was added to the solution of ammonium alum (ammonium aluminium sulfate). A gelatinous white precipitate of aluminium hydroxide was formed. The precipitate was filtered, then washed by distilled water and finally placed into a beaker.

In the next experiment the fresh aluminium hydroxide precipitate will be used for the reaction with acetic acid.

The freshly precipitated aluminium hydroxide was immediately washed by distilled water and placed into a beaker. An excess of glacial acetic acid was added to the precipitate. Initially, the precipitate was a gel, but after adding acetic acid and stirring the gel was destroyed. A white suspension was formed.

After an hour, most of the aluminium hydroxide was dissolved. The suspension became translucent.

The solid phase was filtered off. An aqueous solution of ammonia was added to the filtrate.

As a result, the white gelatinous precipitate of aluminium hydroxide was formed.

So, freshly precipitated aluminium hydroxide mainly dissolves in glacial acetic acid but aged aluminium hydroxide does not dissolve (see pt.1).

Initially, a freshly precipitated aluminium hydroxide was obtained by the reaction of an ammonium alum solution and a concentrated aqueous ammonia solution. The precipitate was filtered and washed by distilled water.

Then a 50% solution of citric acid (excess) was added to the aluminium hydroxide. The main quantity of the precipitate was dissolved immediately. The white suspension was formed. The rest of the solid phase was dissolved after 3 hours.

Therefore, a freshly precipitated aluminium hydroxide dissolves in a solution of citric acid to form a solution of aluminium citrate.

By the way, aluminum citrate probably causes Alzheimer's disease (toxicologists are now discussing the issue).

In the previous experiment a freshly precipitated aluminium hydroxide was dissolved in an excess of a citric acid solution. The solution of aluminium citrate was formed.

The purpose of this experiment is to destruct the obtained aluminium citrate. It is predicted that the aluminium citrate can be destructed by adding of ammonia or a strong alkali (sodium, potassium or lithium hydroxide). But another method was chosen - oxidation of citrate (and the excess of citric acid) by ammonium persulfate and hydrogen peroxide.

The decomposition of aluminium citrate in the solution leads to formation of a precipitate of aluminium hydroxide (if no strong acids were added).

The oxidizing agents, chosen for this reaction, must not form a precipitate as byproduct, therefore, we used ammonium persulfate and hydrogen peroxide and did not use, for example, potassium permanganate KMnO4, potassium dichromate K2CrO7 or calcium hypochlorite Ca(ClO)2.

The result of the experiment was: the addition of ammonium persulfate and hydrogen peroxide and boiling of the solution did not lead to the formation of any precipitate.

Therefore, aluminium citrate is not oxidized by ammonium persulfate and hydrogen peroxide in a boiling solution (at least, under the conditions of this experiment).