Measuring Concentration in Percent (%)

Many industrial pickling operations like to use percent as the “units” for analysis, reporting and
process control. In some ways, this makes sense; in other ways it can be fraught with pitfalls. In
regard to chemistry, percent has at least three different meanings and associated uses:

Volume / Volume Percent (% v/v)
This measurement is what we are all familiar with when cooking. It is the basis for recipes
around the world. Two cups of flour, two cups of milk, one egg, shortening and you are off
making pancakes. For acid pickling, it is often the basis for bath makeup: 75 gallons of water, 20
gallons of bulk nitric acid and 5 gallons of bulk hydrofluoric acid. Defined as a 20% v/v nitric, 5%
v/v hydrofluoric acid pickling solution. This can be very useful for bath makeup because it is
easy to explain and easy to measure. It is a very practical way to train operators and works with
a variety of available tools like drums, buckets, process tank level, etc.

Weight / Weight Percent (% w/w)
Instead of a weight of acid per volume as above, this is a weight of acid per weight of solution.
This is often used to define bulk chemical solutions because it can easily be measured during
production and transportation. It is very useful for pricing and shipping. Nitric acid is typically
supplied in bulk as 67% w/w. After on-site makeup of process solutions, it is less useful because
most of the measuring tools on-site are volume based. The mixed solution density needs to be
known in order to be useful and practical for process control in a pickling / etching application.

Weight / Volume Percent (% w/v)
Here the “w” represents the weight of some specific constituent, like nitric acid, in a volume of
mixed solution. For example, 20% w/v nitric acid means there is 200 grams of HNO 3 per liter of
acid solution. This version of percent is very useful for chemical calculations, mass balance
calculations and other process control measurements because it is unambiguous and easy to
define. For example, if I have a 1000 liter tank at 20% w/v HNO3, there is 200 g/l x 1000 liters /
1000 g/kg, or 200 kg of HNO 3 in that process tank.

These three different definitions of percent are often encountered in the supply-makeup-
process control chain. Almost as often, confusion reigns. Here is a typical situation:
– The facility receives 67% w/w bulk nitric acid and 49% w/w hydrofluoric acid from their
chemical supplier.
– The bath makeup instructions look something like this for a 10,000 liter pickling tank
with 13.5% HNO3 and 3.5% HF:
o Fill the process tank ½ full with water (~ 5,000 liters)
o Add 1,350 liters of 67% HNO3
o Add 350 liters of 49% HF.
o Top off with additional water until the tank reaches 10,000 liters

Note the v/v and w/w notation is purposely left off. Most sites don’t have that in their
instructions either to keep the instructions simple or because they just don’t understand
the meanings anyway.

-The process control charts use a basis of either grams per liter (g/l), ounces per gallon
(oz/gal) or percent (% w/v OR % w/w). % v/v is never used for process control because there
is no way to measure a % v/v for verification.
The above situation mixes all three definitions of percent and are often confused or misused.
The table below defines the etch solution as described above:

Note that all the percentages in the table have different values but describe a solution with
identical concentrations. Because volume is not conserved when mixing solutions, we cannot
directly equate the makeup percentage in % v/v to the mixed pickling solution’s % w/v, even if
we knew the mixed solution’s density. There is no way to “back calculate” a solutions % v/v
makeup without knowing the original bulk solution concentration and specific gravity.

To complicate the issue, when selecting “%” as the reporting units the Scanacon SA80 reports
percentages differently for different analyses. Acid and HF are in % wt/wt, while metals are in %
w/v. Because of the lack of understanding all these different percentages, we urge our
customers not to use % as the reporting units on our acid analyzers. The SA80 analyzer has
three other options that are much better defined: Grams per liter (g/l), ounces per gallon
(oz/gal) or moles per liter (M/l).


% v/v can still be used for bath makeup since that makes the instructions so simple, but when it
comes to process control of the etch bath itself, we recommend using one of the other unit
reporting options that are mass/volume based.