Seeking White Powder Alkali Indicator (that can take 900°C)

[some bloke]

Hello all!

I'm doing some research for a project I have and I need to try and source an indicator which is white/bland in colour when dry and is quite vivid in colour when in an alkali solution.

The solution in question is a ceramic slurry, which has a PH of between 9.5 and 10.

The aim is for the slurry to be coloured when wet, but to dry white - like that emulsion paint which goes on pink but dries white. The more vivid the colour, the better. The slurry is naturally an off-white colour.

I have access to a few indicators (methyl orange, iodine indicator, phenolphthalein) which might work, but I'm unsure as to what colour they will change to (if any) and whether they will remain that colour when dry (I know that the iodine comes in a powder form, which is white).

The ceramic is built up over several coats, and then is fired at 900°C. The aim is to be able to visually determine whether the slurry is dry between coats, but for the indicator not to give off toxic gases or anything like that when the ceramic is fired.

I know very little about chemistry, so I will appreciate any assistance or recommendations that people can suggest!

 

[Borek]

The aim is to be able to visually determine whether the slurry is dry between coats

Then look for something that changes color when wet vs when dry. It can be actually much easier.

None of the of the indicators you have listed will survive roasting, but even if, your idea of using pH indicator would be still wrong on many levels. Actually it is so off I have no idea where to start explanations

 

[some bloke]

Then look for something that changes color when wet vs when dry. It can be actually much easier.

None of the of the indicators you have listed will survive roasting, but even if, your idea of using pH indicator would be still wrong on many levels. Actually it is so off I have no idea where to start explanations

Haha, ok so I know less than I thought!

Can you recommend where to start with materials which change colour between being wet & dry?

My original thought was from the basic observation that the iodine indicator is white when dry and changes colour when immersed in our acid/oxidiser bath which it's used for testing. I assumed that an indicator would dry white again, as it would no longer have an acid/alkali solution to indicate.

I know so little about chemistry!

 

[Borek]

Can you recommend where to start with materials which change colour between being wet & dry?

Inorganic hydrated salts do that very often.

 

[Lord Jestocost]

One should always keep in mind that any indicator material which cannot completely burnt off during the sintering step contaminates the final ceramic layer and might influence the properties of the product.

 

[some bloke]

Inorganic hydrated salts do that very often.

I have just had a quick look at inorganic hydrated salts and found that they contain water molecules, and tend to crumble when heated as the water escapes - which may well compromise the ceramic I'm trying to produce. I'm wondering about cobalt Chloride, as it has a very notable reaction to water which would be a very good visual cue as to whether the coat is dry, but will need to do more research into its reactions to heating.

One should always keep in mind that any indicator material which cannot completely burnt off during the sintering step contaminates the final ceramic layer and might influence the properties of the product.

The final appearance of the ceramic is not of importance - it will be used as an investment shell for casting aluminium and then destroyed to reveal the metal. My biggest worries are whether the "indicator" will cause weakness as it burns off, and whether there will be any reaction between the "indicator" material and the molten aluminium, in such a way as to compromise the aluminium.

 

[mjc123]

Cobalt chloride would be poor, as the pink colour when wet is not very intense, while the blue colour when dry is much stronger, which is the reverse of what you want.

 

[some bloke]

Cobalt chloride would be poor, as the pink colour when wet is not very intense, while the blue colour when dry is much stronger, which is the reverse of what you want.

This would not be a problem, actually - the aim is for a visual cue that the coat is dry. If it goes from bland to an intense colour, that would be as good (if not better) than going from an intense colour to bland.

I will have to try and source some cobalt chloride and do some tests.