1 M HCl & NaOH for pH Adjustment

Preparation of 1 M stock solutions of hydrochloric acid and sodium hydroxide for routine pH adjustment of media and buffers.

1 M HCl & NaOH for pH Adjustment, v1.0 Kathryn E. Caruso · 0009-0003-2436-1791 Foreman Lab · Center for Biofilm Engineering, Montana State University Updated March 2026

How to cite this protocol Caruso, K.E. (2026). 1 M HCl & NaOH for pH Adjustment, v1.0. Foreman Lab, Center for Biofilm Engineering, Montana State University. https://kathryncaruso.github.io/methods/1m-hcl-naoh-ph-adjustment/

Purpose

Prepare 1 M hydrochloric acid (HCl) and 1 M sodium hydroxide (NaOH) stock solutions for routine pH adjustment of media and buffers.

Key parameters:

Parameter Value
Target concentration 1 M (each)
Final volume 50 mL (each)
Storage Room temperature, 50 mL Falcon tubes
Shelf life ~3–6 months; remake as needed

Materials

Reagents:

  • Concentrated HCl (~37 %, ~12.1 M; ACS grade)
  • NaOH pellets (ACS grade, ≥97 %)
  • Deionized (DI) water

Equipment:

  • 50 mL Falcon tubes (polypropylene) ×2
  • 100 mL glass beaker ×2
  • Graduated cylinder (10 mL)
  • Analytical balance
  • Magnetic stir plate and small stir bar
  • Weighing boat or weigh paper
  • Lab spatula
  • PPE: lab coat, safety goggles, chemical-resistant gloves
  • Labels / lab marker

1 Preparation of 1 M HCl (50 mL)

1.1 Calculation

To prepare 50 mL of 1 M HCl from concentrated HCl (~12.1 M):

\[V_1 = \frac{C_2 \times V_2}{C_1} = \frac{1\;\text{M} \times 0.05\;\text{L}}{12.1\;\text{M}} \approx 4.13\;\text{mL}\]

Important

Always add acid to water. Never add water to concentrated acid. The exothermic reaction can cause violent boiling and splashing. Always add acid slowly to water.

1.2 Procedure

  1. Add approximately 40 mL of DI water to a 100 mL glass beaker.
  2. In a fume hood, measure ~4.1 mL of concentrated HCl using a graduated cylinder.
  3. Slowly add the concentrated HCl to the water while gently swirling.
  4. Allow the solution to cool to room temperature.
  5. Transfer to a 50 mL Falcon tube and bring to the 50 mL mark with DI water. Cap and invert to mix.
  6. Label the tube (see Labeling & Storage section).

2 Preparation of 1 M NaOH (50 mL)

2.1 Calculation

NaOH molecular weight = 40.00 g/mol

\[m = C \times V \times MW = 1\;\text{M} \times 0.05\;\text{L} \times 40.00\;\text{g/mol} = 2.00\;\text{g}\]

Important

Exothermic dissolution. Dissolving NaOH pellets is exothermic. Add pellets slowly while stirring. At this small scale the heat is manageable, but still add in portions.

2.2 Procedure

  1. Add approximately 40 mL of DI water to a 100 mL glass beaker with a small stir bar on a stir plate.
  2. Weigh 2.00 g of NaOH pellets on an analytical balance. Work quickly — NaOH is hygroscopic and absorbs moisture and CO₂ from the air.
  3. With stirring, add the NaOH pellets to the water. At this scale you can add them in one or two portions.
  4. Allow the solution to cool to room temperature.
  5. Transfer to a 50 mL Falcon tube and bring to the 50 mL mark with DI water. Cap and invert to mix.
  6. Label the tube (see Labeling & Storage section).

Labeling & Storage

  1. Label each Falcon tube clearly with:
    • Solution name and concentration (1 M HCl or 1 M NaOH)
    • Date prepared
    • Preparer’s initials
  2. Store at room temperature on the bench or in a tube rack.
  3. Remake as needed — at 50 mL these are quick to prepare fresh.

Expected Results

  • 1 M HCl: clear, colorless solution.
  • 1 M NaOH: clear, colorless solution; slightly viscous feel.
  • Both solutions should adjust media pH efficiently in small volumes (typically a few mL per liter of media).

Troubleshooting

Problem Possible Cause Solution
NaOH solution is cloudy Undissolved pellets or carbonate Stir longer; filter if carbonate precipitate formed
pH adjustment requires large volumes Stock may be below 1 M Re-prepare fresh
HCl fumes during preparation Normal — conc. HCl is fuming Always prepare in fume hood

Notes & Modifications

  • To scale up, multiply proportionally (e.g., for 100 mL: double all amounts).
  • If your concentrated HCl has a different assay (check the bottle label), recalculate the volume needed accordingly.
  • Polypropylene Falcon tubes are compatible with both 1 M HCl and 1 M NaOH.