The concentration of a liquid extract is usually calculated as mass per volume (e.g., mg/mL or % w/v) by dividing the weight of the dried extract equivalent by the final extract volume. For example, if 2.5 g of dried material is contained in 50 mL of extract, the concentration is 50 mg/mL (5% w/v). If the obtained extract volume is less than the target, the difference is made up with distilled water (e.g., for a target of 100 mL and an actual volume of 82 mL, add 18 mL distilled water). Any dilution will proportionally reduce the concentration, so recalculation after volume adjustment is essential for accurate reporting.

Great questions—here’s a simple, defensible way to report the concentration of a liquid plant extract (after ethanol extraction + rotavap) and how to adjust its volume with water without ruining solubility.

1) What does “concentration” mean here?

For crude extracts, the most useful and reproducible definition is: Concentration = mass of dry extract solids per volume of liquid (mg/mL).

Because your rotavap concentrate still contains solvent and volatiles, you first determine the dry solids content of that liquid. Then you can express results as mg dry extract per mL (sometimes called “equivalent dry extract”, EDE).

Quick gravimetric assay (15–60 min)

Example: If 1.000 mL leaves 0.120 g dry solids → 120 mg/mL.

Tip: Do it in duplicate/triplicate and average. If you prefer %, you can also compute % (w/v) = (mg/mL)/10.

Alternative (if you know density ρ)

Measure density of the liquid extract (g/mL), and measure % dry residue (w/w) on a weighed aliquot. Then: Concentration (mg/mL) = ρ (g/mL) × (% dry residue / 100) × 1000.

2) “Yield” from plant material (optional but useful)

If you started with mₚ (g) dry peel and obtained mₑ (g) dry extract (after fully drying an aliquot), then: Yield (%) = 100 × mₑ / mₚ. This lets you report both extract concentration (mg/mL) and overall yield from plant.

3) How to adjust the volume with distilled water (without precipitation)

Many ethanol extracts contain hydrophobic/medium-polarity compounds that precipitate if you dilute straight into pure water.

Safe approaches:

• Reconstitute to a stock with a small fraction of ethanol (e.g., 10–30% v/v EtOH in water). This maintains solubility and is accepted in most bioassays.
• If you need 100% water: first dry the extract completely, then re-dissolve gradually in warm water (room temp–40 °C) with sonication. If turbidity appears, either: keep a minimal co-solvent (5–10% EtOH), or allow precipitate to settle and filter—but note you are then changing composition (losing hydrophobics).

Using the dilution equation

Once you know the stock concentration C₁ (mg/mL), to prepare C₂ at final volume V₂:

C1V1=C2V2⇒V1=C2V2C1C_1 V_1 = C_2 V_2 \quad \Rightarrow \quad V_1 = \frac{C_2 V_2}{C_1}

• Measure V₁ of stock; bring to V₂ with water or water/EtOH (the same solvent system used for stock).
• Mix gently; check for clarity. If clouding occurs, raise the ethanol fraction slightly (e.g., from 10% → 15%) until clear.

Example: Stock = 120 mg/mL (in 20% EtOH). Want 12 mg/mL, 100 mL final. V1=(12×100)/120=10V_1 = (12 × 100)/120 = 10 mL stock + 90 mL of 20% EtOH in water.

4) Quality checks

• Clarity/turbidity after dilution (visual or 600 nm absorbance).
• pH (some extracts need mild pH adjustment for solubility).
• Reproducibility: repeat the dry-residue assay monthly if you store the stock, because ethanol can evaporate and concentration will drift.

5) Reporting (what to write in your method)

“Crude ethanol extract was concentrated by rotary evaporation. Extract concentration was determined gravimetrically as dry residue (mg/mL) by drying 1.00 mL aliquots to constant mass at 60 °C under vacuum. Stocks were reconstituted at X mg/mL in 20% ethanol–water and diluted to working concentrations with the same vehicle.”