We all know that “oil and water don’t mix,” that like dissolves like. We’ve heard statements that solvents (organic solvents) work for non-polar soils while water works for salts. Critical product cleaning, like life, is complex; and the range of soils even for a given manufacturing situation are more varied. While water is not an effective cleaning agent for oily residue, aqueous cleaning processes have often been found to be very effective. The reason is that aqueous cleaning agents are mixtures of chemicals. While organic solvent cleaning is based primarily on solvency, aqueous cleaning processes involve chemical reactions.
Let’s consider one aspect of aqueous cleaners, acid or base. Aqueous cleaning agents (or aqueous degreasers) to remove organic residues are typically alkaline. Examples of such organic soils include metalworking fluids that may contain oils, esters, and amines. Most aqueous cleaning agents for removing metal oxides are acids. While we are parsing out the impact of acids or bases, we do not recommend simply adding acid or base to water and attempting to clean parts. Effective aqueous cleaning agents are a blend of chemicals designed by skilled, experienced formulators. A good formulator is like a master chef. Great cooking involves chemistry; so does aqueous cleaning.
Whether an aqueous solution is an acid or base depends on the balance of protons (hydrogen ions), which lack an electron and are positively charged and hydroxide ions, that have an extra electron and are negatively charged. An acid has an excess of hydrogen ions and is a proton donor or an electron acceptor. A base has an excess of hydroxide ions and is a proton acceptor or an electron donor.
The most common measure of acidity or alkalinity is pH. Achieving the correct pH of wash and rinse tanks is essential for the cleaning process to work effectively. So what does pH mean? Why is low pH (acid) better for removing oxides and high pH (alkaline or base) better for removing grease? Read on!
What is pH?
The amount of acid of base in a solution is measured as the pH. The pH is the negative log of the hydrogen ion concentration. Liquid water, which has equal amounts of H+ and OH- ions, is neutral. The concentration of hydrogen ions in neutral water is 10-7 grams/liter. Mathematically, the exponent, -7, is the logarithm base 10 of that concentration. The negative of the exponent, -( -7) = 7, is called the pH of water. Acids have pH < 7, bases or alkalis have pH > 7. If the concentration of H+ is increased 10-fold, then the exponent is -6, so the solution has a pH of 6. Each 10-fold increase in concentration decreases the pH by 1. Likewise, a 10-fold decrease in H+ concentration increases the pH by 1. The common pH scale ranges from 0 to 14. Lemon juice, with a pH of 2 has 100 times the hydrogen ion concentration than tomato juice, with a pH of 4. Likewise, ammonia, with a pH of 11 is 100 times more basic than baking soda, with a pH of 9.
Why are alkaline cleaners (high pH) effective for oils?
It’s chemistry! Basically (pun intended), alkaline solutions react with complex organic molecules and convert them into other compounds that are more readily removed from the product. These chemical reactions involve hydrolysis. In hydrolysis, the excess of hydroxide ions donate electrons and cause an ester to split into an acid and an alcohol. These molecules are more easily dissolved into water and removed. Saponification of fats (soap formation) is an example of hydrolysis of certain fats.
Why are acidic agents (low pH) effective for removing oxides?
Acidic cleaning agents are sometimes referred to as brighteners. They are particularly effective in removing scale, rust, and tarnish. The excess of hydrogen ions in acids react to the oxygen in metal oxides through the process called protonation. The result is the creation of additional water, from the hydrogen ion in the acid and the oxygen in the oxide, plus a soluble salt.
The importance of rinsing
Rinsing provides a great example of the logarithmic (power of 10) nature of acids and bases. We’ve seen many examples where carry over from an alkaline wash tank solution causes the rinse tank to become alkaline. In one instance, there was a wash tank followed by three sequential rinse tanks. We measured the pH of the wash tank; it was 9. The pH of the first rinse tank was also 9 as was the second rinse tank. Can you hazard a guess about the pH of the third rinse tank? Even if 99% of an alkaline cleaning agent with a pH of 12 is removed in a single rinse, the resulting solution still has a pH of 10, still quite alkaline. Sequential rinsing is important but sequential rinsing alone is not enough. Reverse cascade, where fresh water is introduced into the final rinse and is carried upstream to the earlier rinses, is helpful. In some processes, the parts are spray rinsed over the wash tank; this prevents carryover of cleaning agent to the rinse tank. It also serves to minimize flash drying, where soils and cleaning agent residue might be burned onto the hardware being cleaned.
A few more thoughts
People and many other life forms favor an environment with a neutral pH. The process of removing soils is facilitated by moving farther up or down the pH scale. However, because highly acid or basic solutions require engineering controls and personal protective equipment to prevent damage to workers, near-neutral cleaning solutions have become favored by many companies. Some metals, such as aluminum, may be damaged by very high or very low pH cleaning agents. Near neutral cleaning agents, which have become particularly popular for removal of organics, are formulated with other additives to achieve cleaning. They may have their own issues including materials compatibility and rinseability. In addition, near neutral cleaners may turn into a food source for microscopic critters – so process monitoring is a must.
In contrast with solvent-based processes, aqueous cleaning is far more complex than solubility parameters. Aqueous cleaning involves complex chemical reactions to alter soils so that they can be more readily removed. Fortunately, the practical aspects of this chemistry can be readily understood. Understanding how acid cleaners and basic aqueous degreaser work can help in designing effective cleaning for the soils in question. The role of acid and base is only part of the story. Many more ingredients go into an effective aqueous cleaner.
Historically, the meaning of the letter ‘p’ in pH is not entirely clear. It has been referred to as the “power of Hydrogen,” partially because it is a mathematical exponent or power of a number. It has also been referred to as the “potential of Hydrogen”. The term was introduced in the early 1900s by the Danish biochemist S.P.L. Sørensen to represent the hydrogen ion concentration. Words for ‘power’ or ‘potential’ start with the letter p in the languages Sørensen published in, French, German, and Danish.
We thank our colleague, Darren Williams, Professor of Physical Chemistry at Sam Houston State University, for his helpful discussions. However, we are responsible for any errors or faux pas.