Lignosulfonates (LS) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin. Sulfonated lignin (SL) refers to other forms of lignin by-product, such as those derived from the much more popular Kraft process, that have been processed to add sulfonic acid groups. The two have similar uses and are commonly confused with each other, with SL being much cheaper. LS and SL both appear as free-flowing powders; the former is light brown while the latter is dark brown. Lignosulfonates have very broad ranges of molecular mass (they are very polydisperse). A range of from 1,000 to 140,000 Da has been reported for softwood lignosulfonates with lower values reported for hardwoods. Sulfonated Kraft lignin tends to have smaller molecules at 2,000–3,000 Da. SL and LS are non-toxic, non-corrosive, and biodegradable. A range of further modifications may be applied to LS and SL, including oxidation, hydroxymethylation, sulfomethylation, and a combination thereof. Preparation Lignosulfonates Lignosulfonates are recovered from the spent pulping liquids (red or brown liquor) from sulfite pulping. Ultrafiltration can also be used to separate lignosulfonates from the spent pulping liquid. AA list of CAS numbers for the various metal salts of lignosulfonate is available. The electrophilic carbocations produced during ether cleavage react with bisulfite ions (HSO3−) to give sulfonates. R-O-R' + H+ → R+ + R'OH R+ + HSO3− → R-SO3H The primary site for ether cleavage is the α-carbon (carbon atom attached to the aromatic ring) of the propyl (linear three carbon) side chain. The following structures do not specify the structure since lignin and its derivatives are complex mixtures: the purpose is to give a general idea of the structure of lignosulfonates. The groups R1 and R2 can be a wide variety of groups found in the structure of lignin. Sulfonation occurs on the side chains, not on the aromatic ring like in p-toluenesulfonic acid. Sulfonated Kraft lignin Kraft lignin from black liquor, which is produced in much higher amounts, may be processed into sulfonated lignin. The lignin is first precipitated by acidifying the liquor with then washed (other methods for isolation exist). Reaction with sodium sulfite or sodium bisulfite and an aldehyde under a basic environment completes sulfonation. Here the sulfonic acid groups end up on the aromatic ring instead of the