Yes, most of the wet-end cationic starches for papermaking can be spotted using I2/KI solutions and will form the I3- helix complex IF they contain amylose.

BUT this depends on the amount : wet-end starch is used in small amounts so you might get better results if you extract the paper with hot water, reduce the extracts down to a few drops and addone or two drops of I2/KI in them : color will turn to violet if amylose containing starch has been extracted.

Especially if the starch has been cross-linked.

Remember just that this traditional test depends entirely on the presence of amylose in the starch structure and that contrast will depend on the amylose/amylopectin proportion.

Note by the way that newer starches (mostly for surface treatment) are sometimes degraded down to dextrins and will NOT react with I2/KI.

This is quite a complicated problem for at least 2 reasons:

1. More than 1 type of starch may be present in paper: (a) cationic starch for internal sizing and filler retention (= wet end application); (b) oxidized starch, converted (= slightly hydrolysed) starch, and starch ethers for surface sizing; (c) starch ethers for paper coating. [See e.g. J. BeMiller, R. Whistler (eds) Starch, Chemistry and Technology, 3rd ed., Academic Press, 2009, pp. 657-713.]

2. The starch-iodine interaction depends on (a) amylose (Am) content, (b) starch molecular weight (MW), (c) amount of modifying groups (ether, ester). [Amount of modification = Degree of substitution = DS.]

Ad 1: Starch content in paper may widely vary with paper type and quality but is about 2 % on average. Most is used in surface sizing; wet end application seldom covers more than 0.5 % (based on paper). This makes it fairly difficult to analyse for cationic starch.

Ad 2: Starch-iodine (I) complex formation decreases with (a) decreasing Am, (b) decreasing MW, (c) increasing DS. This means that all starch types applied in papermaking have a similar effect: a decrease in starch-I binding as compared to unmodified (= native) starch.

Pierre Caulet states that only amylose-containing starches are able to complex with I. This is true in the case that the amount of Am-I complex is measured by potentiometric or amperometric titration. However, although no true I-complex is formed with amylopectin (Ap), there is a certain interaction between Ap and I as shown by a shift in absorption maximum: from 620 nm for Am-containing starch to 530-580 nm for Am-free starches. This trend is expected not only for case (a) but also for cases (b) and (c). It follows that staining with I is not a unique indicator for the presence of cationic starch but only proves the presence of some type of starch.

With regard to the solution state of starch in water, 3 states may be discerned: (1) intact = granular form and crystallinity preserved, (2) gelatinized = granular identity preserved, crystallinity lost, swellable in water, (3) molecularly dissolved = dispersed into constituting molecules. Only starch in states 1 and 2 can be detected by microscopy, eventually assisted by iodine staining. State 3 might be detected from I-colour, if not too dilute.

The literature (see the reference above), suggests that starch additives require complete molecular dispersion for effective functionality, i.e. state 3. This can be checked in real paper samples by microscopy. Cationic starch is in state 1 as produced but readily dissolves into state 3 when cooked.

Information on the internet suggests that there are no N-containing constituents in paper other than cationic starch (except perhaps in some pigments).

Based on the above information I recommend to follow Pierre Caulet ’s advice and make an extract of finely shredded paper in water (120 °C, pressure cooker, in order to solubilize recrystallized (= retrograded) starch) and after drying analyse the residue for starch and %N. %N in starch can be compared to %N of a commercial cationic starch. From that the amount of cationic starch and other (N-free) starches in paper can be estimated.

I tried to be synthetic in my answer and stayed very practical, and simplified things probably too much. Peter A. M. Steeneken gives a very complete answer to your problem and I full recommend it.

Fyi, we generally only do spot tests with I2/KI when testing for starch presence, whatever the type and the amount, but I confirm this method has loopholes...

Hello,

What color you get? Native starch form very dark-navt color inclusion complexes with helices of amylose. Cationic starch do not orginize like that, moreover ionic groups repel each other (helices in unmodified starches are formed via OH groups directed inside the helix), however, cationic groups can bonds with iodine. Did you obtain dark violet/purple color? So you probably have some "shorter" complexes.

30-40 years ago we investigated the reactivity of amylose (Am) and amylopectin (Ap) in potato starch by fractionating modified starch (cationic or methyl-substituted) and studying degree of substitution (DS) and iodine binding (IBC, by amperometric titration) of Am and Ap. Fractionation was achieved either by complexation with amyl alcohol (AmA, practicable up to DS 0.05) or MgSO4 (higher DS). λmax were not measured. Some conclusions relevant to the issue here:

- For cationic starch DS ≤ 0.01, time for complexing with AmA increased from 1 day (unmodified starch) to 6 days for cationic starch. IBC of Am fraction showed only a minor decrease by substitution.

- For methyl starch DS 0.06, time for complexing with AmA increased to 6 months. IBC of Am fraction decreased from 20 to 17.4 %. Methyl starch at DS 0.15 could only be fractionated by MgSO4. IBC of Am fraction decreased to ca. 10 %.

See the attached files.