(17-01-2014, 07:35 )Cencio Ha scritto: Bene è tornato disponibile, anche se leggendo qua e là su internet vedo pareri discordanti...
Infatti....io l'ho ordinato perchè sembrava la quadratura del cerchio, poi dopo, gogolando un po' mi sembra di aver capito questo: il prodotto è costituito da un acido e una base, il massimo buffer lo avrebbero al ph 2.1 o 7.2, in quanto a 2.1 si avrebbe la stesa quantità di ioni di acido fosforico e di fosfato monobasico, a 7,2 la stessa quantità di ioni dibasici e monobasici.
Quindi in realtà valori come 5.2 o 5.3 sono abbastanza lontani dal range ottimale e l'effetto tampone dovrebbe essere scarso.
In pratica non sembra essere quello che sogniamo, qualcosa che butti dentro e automaticamente ti porta il ph a 5.2 fissandolo.
Ciao,
Andrea
Per chi conosce l'inglese copio da homebrewtalking:
"Let me start by pointing out that this has been discussed at length on this forum and a search of the archives should turn up a lot of posts.
This product is a combination of mono and dibasic sodium phosphate. It is mostly the former. The mix forms a buffer at pH 5.9 (that's not a typo). Remember that malt contains a lot of inorganic phosphate. I am guessing that the idea is to have the sum of the malt monobasic phosphate and the product's monobasic phosphate be such that its ratio to the dibasic (from the product) is such as to have it buffer at 5.2. That may or may not be how they designed it but, as I have stated here and elsewhere dozens of times, you do not design a phosphate buffer for pH 5.2 because that value is too far from either of the spanning pK's of phosphoric acid and, as a consequence, there will be little buffering capacity at pH 5.2. That is why this product comes no where near 'locking on' your pH at 5.2 at normal or even several time the normal dose. The theory says this; experiments in the lab confirm it.
As to the acid base nature of it: at pH 5.2 phosphoric acid is a fairly 'strong' acid. At pH 5.2 dibasic phosphate is a fairly 'strong' base of about equal strength. Thus they tend to neutralize one another with the result being the salt monobasic phosphate. At pH 5.2 there is relatively little phosphoric and relatively little dibasic phophate. The ratio of monobasic ion to phosphoric is pretty high as is the ratio of monobasic to dibasic. There isn't much buffering. At pH 2.1, OTOH, there are equal amounts of phosphoric acid and monobasic ion and at pH 7.2 there are equal amounts of monobasic and dibasic ions. These are the values at which the system will have maximum buffering capacity. The point being that a buffer consists of a mixture of an acid and a base. Keep in mind that the monobasic ion is an acid becase it can give up protons thus lowering pH. "
